Awesome

NullDev JavaScript StyleGuide

✨ Now with the new ESLint Flat Config Format! ✨

<img height="150" width="auto" src="https://i.imgur.com/eCmJFjH.png" /> A mostly reasonable approach to JavaScript Or... Mainly NodeJS...

Setup :gear:

To integrate into your own project:

Make sure ESLint is installed. For VSCode, download the extension here
Navigate inside your project
Install Babel-ESLint*: $ npm install @babel/eslint-parser @babel/core eslint --save-dev
Get the config: $ wget https://raw.githubusercontent.com/NullDev/JavaScript-Styleguide/master/eslint.config.js
Done! :)

*) The rationale for using Babel-ESLint is that it supports the newest Stage-3 ECMAScript features.

Or use the NullDev Project Template ...

... with already preset config files and project structure

DOWNLOAD

$ npm i - and you're ready to go!

Table of Contents :bookmark_tabs:

:clipboard: Types
<details> <summary>View contents</summary>
- 1.1 Primitives
- 1.2 Complex
</details>
:link: References
<details> <summary>View contents</summary>
- 2.1 Prefer const
- 2.2 Disallow var
- 2.3 Block scope
</details>
:package: Objects
<details> <summary>View contents</summary>
- 3.1 Literals
- 3.2 Computed properties
- 3.3 Object shorthand
- 3.4 Object concise
- 3.5 Grouped shorthand
- 3.6 Quoted properties
- 3.7 Prototype Builtins
- 3.8 Rest spread
</details>
:bookmark_tabs: Arrays
<details> <summary>View contents</summary>
- 4.1 Literals
- 4.2 Push
- 4.3 Array spreads
- 4.4 spread for iterable
- 4.5 Array-like objects
- 4.6 Mapping
- 4.7 Callback return
- 4.8 Linebreaks
</details>
:hammer: Destructuring
<details> <summary>View contents</summary>
- 5.1 Object destructuring
- 5.2 Array destructuring
- 5.3 Object over array
</details>
:page_facing_up: Strings
<details> <summary>View contents</summary>
- 6.1 Quotes
- 6.2 Length
- 6.3 Template literals
- 6.4 Eval
- 6.5 Escaping
- 6.6 Regular Expressions (RegEx)
</details>
:pager: Functions
<details> <summary>View contents</summary>
- 7.1 Strict mode
- 7.2 Declarations
- 7.3 IIFE's
- 7.4 Non-function blocks
- 7.5 Blocks (Info)
- 7.6 Arguments shadow
- 7.7 Rest
- 7.8 Default parameters
- 7.9 Side effects
- 7.10 Defaults last
- 7.11 Function constructor
- 7.12 Signature spacing
- 7.13 Mutating parameters
- 7.14 Reassigning parameters
- 7.15 Spread
- 7.16 Signature invocation indentation
</details>
:arrow_right: Arrow Functions
<details> <summary>View contents</summary>
- 8.1 Usage
- 8.2 Implicit return
- 8.3 Wrap
- 8.4 Omitting parentheses
- 8.5 Confusing functions
- 8.6 Linebreaks
</details>
:triangular_ruler: Classes & Constructors
<details> <summary>View contents</summary>
- 9.1 Use class
- 9.2 Inheritance
- 9.3 Chaining
- 9.4 toString methods
- 9.5 No empty constructors
- 9.6 No duplicate members
- 9.7 this in Class-Methods
</details>
:postbox: Modules
<details> <summary>View contents</summary>
- 10.1 CommonJS
- 10.2 Duplicate imports
- 10.3 Mutable exports
- 10.4 Imports first
- 10.5 Imports extensions
- 10.6 Multiline imports
</details>
:arrows_clockwise: Iterators and Generators
<details> <summary>View contents</summary>
- 11.1 Higher-order functions
- 11.2 Generators
- 11.3 Spacing
</details>
:bookmark_tabs: Properties
<details> <summary>View contents</summary>
- 12.1 Dot notation
- 12.2 Bracket notation
- 12.3 Exponentiation operator
</details>
:floppy_disk: Variables
<details> <summary>View contents</summary>
- 13.1 Global namespace
- 13.2 Declaration per variable
- 13.3 Grouping
- 13.4 Placing
- 13.5 Chain assignment
- 13.6 Increment & decrement
- 13.7 Linebreaks
- 13.8 Unused variables
- 13.9 Bitwise floor
</details>
:arrow_heading_up: Hoisting
<details> <summary>View contents</summary>
- 14.1 About
- 14.2 Anonymous function expressions
- 14.3 Named function expressions
- 14.4 Function declarations
</details>
:left_right_arrow: Comparison Operators & Equality
<details> <summary>View contents</summary>
- 15.1 Strict comparison
- 15.2 Conditional statements
- 15.3 Shortcuts
- 15.4 Truth Equality
- 15.5 Switch blocks
- 15.6 Nested ternaries
- 15.7 Unneeded-ternaries
- 15.8 Mixed operators
</details>
:black_square_button: Blocks
<details> <summary>View contents</summary>
- 16.1 Braces
- 16.2 Cuddled elses
- 16.3 Returns
</details>
:wrench: Control Statements
<details> <summary>View contents</summary>
- 17.1 Length
- 17.2 Selection operators
</details>
:pencil: Comments
<details> <summary>View contents</summary>
- 18.1 Language
- 18.2 Multi-line
- 18.3 Single-line
- 18.4 Spaces
- 18.5 Prefixing / Action-items
- 18.6 FixMe
- 18.7 ToDo
</details>
:white_circle: Whitespace
<details> <summary>View contents</summary>
- 19.1 Soft tabs
- 19.2 Space Before blocks
- 19.3 Around keywords
- 19.4 Infix operators
- 19.5 LF Line-breaks
- 19.6 End of file
- 19.7 Chains
- 19.8 After blocks
- 19.9 Padded blocks
- 19.10 Inside parentheses
- 19.11 Inside brackets
- 19.12 Inside curly braces
- 19.13 Line length
- 19.14 Block spacing
- 19.15 Comma spacing
- 19.16 Computed properties
- 16.17 Function call spacing
- 19.18 Key spacing
- 19.19 No Trailing spaces
- 19.20 Multiple empty lines
</details>
:small_red_triangle_down: Commas
<details> <summary>View contents</summary>
- 20.1 Leading commas
- 20.2 Comma Dangling
</details>
:heavy_exclamation_mark: Semicolons
<details> <summary>View contents</summary>
- 21.1 Use semicolons
</details>
:twisted_rightwards_arrows: Type Casting & Coercion
<details> <summary>View contents</summary>
- 22.1 Explicit coercion
- 22.2 Strings
- 22.3 Numbers
- 22.4 Deviations
- 22.5 Bitwise
- 22.6 Booleans
- 22.7 Valid Typeof
</details>
:scroll: Naming Conventions
<details> <summary>View contents</summary>
- 23.0 Language
- 23.1 Descriptive
- 23.2 camelCase
- 23.3 PascalCase
- 23.4 Underscores
- 23.5 this
- 23.6 Filename
- 23.7 Default export
- 23.8 Singleton
- 23.9 Acronyms and initialisms
- 23.10 Uppercase
- 23.11 State-Booleans
</details>
:arrow_down: Accessors
<details> <summary>View contents</summary>
- 24.1 Not required accessors
- 24.2 Getters & Setters
- 24.3 Boolean prefix
- 24.4 Consistency
</details>
:high_brightness: Events
<details> <summary>View contents</summary>
- 25.1 Hash
</details>
:book: Standard Library
<details> <summary>View contents</summary>
- 26.1 isNaN
- 26.2 isFinite
</details>
:nut_and_bolt: jQuery
<details> <summary>View contents</summary>
- 27.1 Prefix
- 27.2 Cache
- 27.3 DOM queries
- 27.4 Find
- 27.5 Shorthands
- 27.6 Don't bloat .ready()
</details>
:arrows_counterclockwise: ECMAScript 5 Compatibility
<details> <summary>View contents</summary>
- 28.1 Compatibility table
</details>
:six: ECMAScript 6+ (ES 2015+) Styles
<details> <summary>View contents</summary>
- 29.1 ES6 Styles
- 29.2 Proposals
</details>
:electric_plug: Testing
<details> <summary>View contents</summary>
- 30.1 Tests
</details>
:chart_with_upwards_trend: Performance
<details> <summary>View contents</summary>
- 31.1 Performance
</details>
:books: Resources
<details> <summary>View contents</summary>
- 32.1 Learning ES6+
- 32.2 Read This
- 32.3 Tools
- 32.4 Further Reading
- 32.5 Books
- 32.6 Blogs
- 32.7 Podcasts
</details>
:copyright: Copyright
<details> <summary>View contents</summary>
- 33.1 Base
- 33.2 License
</details>
:recycle: Amendments
<details> <summary>View contents</summary>
- 34.1 Forking
</details>

Types

1.1 Primitives: When you access a primitive type you work directly on its value.
- string
- number
- boolean
- null
- undefined
- symbol
- bigint
```
const foo = 1;
let bar = foo;

bar = 9;

console.log(foo, bar); // => 1, 9
```
- Symbols and BigInts cannot be faithfully polyfilled, so they should not be used when targeting browsers/environments that don't support them natively.

1.2 Complex: When you access a complex type you work on a reference to its value.
- object
- array
- function
```
const foo = [1, 2];
const bar = foo;

bar[0] = 9;

console.log(foo[0], bar[0]); // => 9, 9
```

⬆ back to top

References

2.1 Use const for all of your references; avoid using var. eslint: prefer-const, no-const-assign

Why? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code.
```
// bad
var a = 1;
var b = 2;

// good
const a = 1;
const b = 2;
```

2.2 If you must reassign references, use let instead of var. eslint: no-var

Why? let is block-scoped rather than function-scoped like var.
```
// bad
var count = 1;
if (true){
    count += 1;
}

// good
let count = 1;
if (true){
    count += 1;
}
```

2.3 Note that both let and const are block-scoped.

// const and let only exist in the blocks they are defined in.
{
    let a = 1;
    const b = 1;
    var c = 1;
}
console.log(a); // ReferenceError: a is not defined
console.log(b); // ReferenceError: b is not defined
console.log(c); // 1

⬆ back to top

Objects

3.1 Use the literal syntax for object creation. eslint: no-new-object
```
// bad
const item = new Object();

// good
const item = {};
```

3.2 Use computed property names when creating objects with dynamic property names.

Why? They allow you to define all the properties of an object in one place.


function getKey(k){
    return `a key named ${k}`;
}

// bad
const obj = {
    id: 5,
    name: "foo",
};
obj[getKey("enabled")] = true;

// good
const obj = {
    id: 5,
    name: "foo",
    [getKey("enabled")]: true,
};

3.3 Use object method shorthand. eslint: object-shorthand

// bad
const foo = {
    value: 1,

    addValue: function (value) {
        return foo.value + value;
    }
};

// good
const foo = {
    value: 1,

    addValue(value) {
        return foo.value + value;
    }
};

3.4 Use property value shorthand. eslint: object-shorthand

Why? It is shorter and descriptive.

const foo = "bar";

// bad
const obj = {
    foo: foo,
};

// good
const obj = {
    foo,
};

3.5 Group your shorthand properties at the beginning of your object declaration.

Why? It’s easier to tell which properties are using the shorthand.

const foo = "bar";
const bar = "foo";

// bad
const obj = {
    test: 1,
    key: 2,
    foo,
    abc: 3,
    xyz: 4,
    bar,
};

// good
const obj = {
    foo,
    bar,
    test: 1,
    key: 2,
    abc: 3,
    xyz: 4,
};

3.6 Only quote properties that are invalid identifiers. eslint: quote-props

Why? In general it is subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.
```
// bad
const bad = {
    "foo": 3,
    "bar": 4,
    "data-foo": 5,
};

// good
const good = {
    foo: 3,
    bar: 4,
    "data-foo": 5,
};
```

3.7 Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf. eslint: no-prototype-builtins

Why? These methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).
```
// bad
object.hasOwnProperty(key);

// good
Object.prototype.hasOwnProperty.call(object, key);

// best - Stage 3 Proposal:
// https://github.com/tc39/proposal-accessible-object-hasownproperty
Object.hasOwn(object, key);
```

3.8 Prefer the object spread operator over Object.assign to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted.

// very bad
const original = { a: 1, b: 2 };
const copy = Object.assign(original, { c: 3 }); // this mutates `original`
delete copy.a; // so does this

// bad
const original = { a: 1, b: 2 };
const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }

// good
const original = { a: 1, b: 2 };
const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }

const { a, ...noA } = copy; // noA => { b: 2, c: 3 }

⬆ back to top

Arrays

4.1 Use the literal syntax for array creation. eslint: no-array-constructor
```
// bad
const items = new Array();

// good
const items = [];
```

4.2 Use Array#push instead of direct assignment to add items to an array.

const someStack = [];

// bad
someStack[someStack.length] = "foobar";

// good
someStack.push("foobar");

4.3 Use array spreads ... to copy arrays. (Read more...)

// bad
const len = items.length;
const itemsCopy = [];
let i;

for (i = 0; i < len; i += 1){
    itemsCopy[i] = items[i];
}

// good
const itemsCopy = [...items];

4.4 To convert an iterable object to an array, use spreads ... instead of Array.from.

const foo = document.querySelectorAll(".foo");

// good
const nodes = Array.from(foo);

// best
const nodes = [...foo];

4.5 Use Array.from for converting an array-like object to an array.

const arrLike = { 0: "foo", 1: "bar", 2: "baz", length: 3 };

// bad
const arr = Array.prototype.slice.call(arrLike);

// good
const arr = Array.from(arrLike);

4.6 Use Array.from instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.
```
// bad
const baz = [...foo].map(bar);

// good
const baz = Array.from(foo, bar);
```

4.7 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following 8.2. eslint: array-callback-return

// good
[1, 2, 3].map((x) => {
    const y = x + 1;
    return x * y;
});

// good
[1, 2, 3].map(x => x + 1);

// bad - no returned value means `acc` becomes undefined after the first iteration
[[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
    const flatten = acc.concat(item);
});

// good
[[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
    const flatten = acc.concat(item);
    return flatten;
});

// bad
inbox.filter((msg) => {
    const { subject, response } = msg;
    if (subject === "foo") return response === "bar";
    else return false;
});

// good
inbox.filter((msg) => {
    const { subject, response } = msg;
    if (subject === "foo") return response === "bar";
    return false;
});

4.8 Use line breaks after open and before close array brackets if an array has multiple lines

// bad
const arr = [[0, 1], [2, 3], [4, 5]];

const objectInArray = [{
    id: 1,
}, {
    id: 2,
}];

const numberInArray = [
    1, 2, ...
];

// good
const arr = [
    [0, 1], 
    [2, 3], 
    [4, 5],
    ...
];

const objectInArray = [
    {
        id: 1,
    },
    {
        id: 2,
    },
];

const numberInArray = [
    1,
    2,
    ...
];

⬆ back to top

Destructuring

5.1 Use object destructuring when accessing and using multiple properties of an object. eslint: prefer-destructuring

Why? Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.
```
// bad
function getFullName(user){
    const firstName = user.firstName;
    const lastName = user.lastName;

    return `${firstName} ${lastName}`;
}

// good
function getFullName(user){
    const { firstName, lastName } = user;
    return `${firstName} ${lastName}`;
}

// best
function getFullName({ firstName, lastName }){
    return `${firstName} ${lastName}`;
}
```

5.2 Use array destructuring. eslint: prefer-destructuring

const arr = [1, 2, 3, 4];

// bad
const first = arr[0];
const second = arr[1];

// good
const [first, second] = arr;

5.3 Use object destructuring for multiple return values, not array destructuring.

Why? You can add new properties over time or change the order of things without breaking call sites.

// bad
function processInput(input){
    // then a miracle occurs
    return [left, right, top, bottom];
}

// the caller needs to think about the order of return data
const [left, __, top] = processInput(input);

// good
function processInput(input){
    // then a miracle occurs
    return { left, right, top, bottom };
}

// the caller selects only the data they need
const { left, top } = processInput(input);

⬆ back to top

Strings

6.1 Use double quotes "" for strings instead of single quotes ''. eslint: quotes
Why? While other Styleguides may enforce single quotes, they mostly do it because of consistency (in favor of older projects). Here are some reasons for using double quotes: 
- Double quotes eliminate the need to escape apostrophes: "I'm" vs 'I\'m'.
- From a linguistic point of view, double quotes identify a passage of quoted text while single quotes are commonly used as a contraction.
- Double quotes are used to define strings in many other languages. Single quotes are used to define char's in some.
- JSON Strings are only valid with double quotes.
```
// bad
const name = 'foo bar';

// bad - template literals should contain interpolation or newlines
const name = `foo bar`;

// good
const name = "foo bar";
```

6.2 Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.

Why? Broken strings are painful to work with and make code less searchable.

// bad
const errorMessage = "This is a super long error - lorem ipsum dolor \
sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget \
dolor. Aenean massa. Cum sociis natoque \
penatibus et.";

// bad
const errorMessage = "This is a super long error - lorem ipsum dolor " +
    "sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget " +
    "dolor. Aenean massa. Cum sociis natoque " +
    "penatibus et.";

// good
const errorMessage = "This is a super long error - lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor. Aenean massa. Cum sociis natoque penatibus et.";

6.3 When programmatically building up strings, use template strings instead of concatenation. eslint: prefer-template template-curly-spacing

Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

// bad
function sayHi(name){
    return "How are you, " + name + "?";
}

// very bad
function sayHi(name){
    return ["How are you, ", name, "?"].join();
}

// bad as well
function sayHi(name){
    return `How are you, ${ name }?`;
}

// good
function sayHi(name){
    return `How are you, ${name}?`;
}

6.4 Never use eval() on a string, it opens too many vulnerabilities. eslint: no-eval

6.5 Do not unnecessarily escape characters in strings. eslint: no-useless-escape

Why? Backslashes harm readability, thus they should only be present when necessary.
```
// bad
const foo = '\'this\' \i\s \"quoted\"';

// good
const foo = "\"this\" is 'quoted'";
const foo = `my name is "${name}"`;
```

6.6 Do not split regular expressions, even if some parts are used multiple times. The only exception are computed RegEx'es.

Why? It has a great impact on readability and can lead to extremely confusing code

// bad
const baseSite = "http(?:s?):\/\/website\.com\/";
const topic = "(?:top|new|user\/\w+\/(?:uploads|likes))(?:(?:\/\w+)?)\/(\d+)";
const comment = "(?:(?::)comment(\d+))";

const uploadsRegex = new RegExp(baseSite + topic, "gi");
const commentRegex = new RegExp(baseSite + topic + comment, "gi");
const profileRegex = new RegExp(baseSite + "user\/(\w+)", "gi");

// good
const uploadsRegex = /http(?:s?):\/\/website\.com\/(?:top|new|user\/\w+\/(?:uploads|likes))(?:(?:\/\w+)?)\/(\d+)/gi;
const commentRegex = /http(?:s?):\/\/website\.com\/(?:top|new|user\/\w+\/(?:uploads|likes))(?:(?:\/\w+)?)\/(\d+)(?:(?::)comment(\d+))/gi;
const profileRegex = /http(?:s?):\/\/website\.com\/user\/(\w+)/gi;

⬆ back to top

Functions

7.1 Write the "use strict"; directive in each and every script you code and put it on the first line to scope it globally. Also, make an empty line below it. eslint: strict
Why? It is a good way to make your code safer. This is because strict mode doesn't allow the usage of dangerous features which could work in a way you wouldn't expect. Some things it does: 
- It disallows non-explicit global variables.
- Silent failing assignments will throw errors instead.
- It requires all property names in an object literal to be unique.
- Function parameter names must be unique as well. 
Also you do not need to worry about browser compartibility. It is not a statement, but a literal expression, ignored by earlier versions of JavaScript. Read more about strict mode and why you should use it. Note: Modules are exempt from this rule because they always run in strict-mode.
```
0 // bad
1 console.log("Foo");
2 let x = function(){
3 console.log("Bar");
4 };
5

---

0 // bad
1 console.log("Foo");
2 let x = function(){
3 "use strict";
4 // strict mode is enabled in the scope of this function only!
5 console.log("Bar");
6 };
7

---

0 "use strict";
1
2 // ^ Good
3 console.log("Foo");
4 let x = function(){
5 console.log("Bar");
6 };
7
```

7.2 Use named function expressions instead of function declarations. eslint: func-style

Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module! Don’t forget to explicitly name the expression, regardless of whether or not the name is inferred from the containing variable (which is often the case in modern browsers or when using compilers such as Babel). This eliminates any assumptions made about the Error's call stack. (Discussion)
```
// bad
function foo(){
    // ...
}

// okay
let foo = function(){
    // ...
};

// good
// lexical name distinguished from the variable-referenced invocation(s)
let short = function longUniqueMoreDescriptiveLexicalFoo(){
    // ...
};
```

7.3 Wrap immediately invoked function expressions in parentheses. eslint: wrap-iife

Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.
```
// immediately-invoked function expression (IIFE)

// bad
!function(){ /* ... */ }();
~function(){ /* ... */ }();
-function(){ /* ... */ }();
+function(){ /* ... */ }();
void function(){ /* ... */ }();

// good
(function(){
    console.log("I'm an IIFE");
}());
```

7.4 Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: no-loop-func

7.5 Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement.

// bad
if (currentUser){
    function test(){
        console.log("Foo");
    }
}

// good
let test;
if (currentUser){
    test = () => {
        console.log("Foo");
    };
}

7.6 Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.
```
// bad
function foo(name, options, arguments){
    // ...
}

// good
function foo(name, options, args){
    // ...
}
```

7.7 Never use arguments, opt to use rest syntax ... instead. eslint: prefer-rest-params

Why? ... is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like arguments.
```
// bad
function concatenateAll(){
    const args = Array.prototype.slice.call(arguments);
    return args.join("");
}

// good
function concatenateAll(...args){
    return args.join("");
}
```

7.8 Use default parameter syntax rather than mutating function arguments.

// really bad
function handleThings(opts){
    // No! We shouldn't mutate function arguments.
    // Double bad: if opts is falsy it'll be set to an object which may
    // be what you want but it can introduce subtle bugs.
    opts = opts || {};
    // ...
}

// still bad
function handleThings(opts){
    if (opts === void 0){
        opts = {};
    }
    // ...
}

// good
function handleThings(opts = {}){
    // ...
}

7.9 Avoid side effects with default parameters.

Why? They are confusing to reason about.

let b = 1;
// bad
function count(a = b++){
    console.log(a);
}
count();  // 1
count();  // 2
count(3); // 3
count();  // 3

7.10 Always put default parameters last.

// bad
function handleThings(opts = {}, name){
    // ...
}

// good
function handleThings(name, opts = {}){
    // ...
}

7.11 Never use the Function constructor to create a new function. eslint: no-new-func

Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.
```
// bad
let add = new Function("a", "b", "return a + b");

// still bad
let subtract = Function("a", "b", "return a - b");
```

7.12 Spacing in a function signature. eslint: space-before-function-paren space-before-blocks

Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.
```
// bad
const f = function () {};
const g = function (){};
const h = function() {};

// good
const x = function(){};
const y = function a(){};
```

7.13 Never mutate parameters. eslint: no-param-reassign

Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.
```
// bad
function f1(obj){
    obj.key = 1;
}

// good
function f2(obj){
    const key = Object.prototype.hasOwnProperty.call(obj, "key") ? obj.key : 1;
}
```

7.14 Never reassign parameters. eslint: no-param-reassign

Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimization issues, especially in V8 (and therefore NodeJS as well).
```
// bad
function f1(a){
    a = 1;
    // ...
}

function f2(a){
    if (!a) a = 1;
    // ...
}

// good
function f3(a){
    const b = a || 1;
    // ...
}

function f4(a = 1){
    // ...
}
```

7.15 Prefer the use of the spread operator ... to call variadic functions. eslint: prefer-spread

Why? It’s cleaner, you don’t need to supply a context, and you can not easily compose new with apply.

// bad
const x = [1, 2, 3, 4, 5];
console.log.apply(console, x);

// good
const x = [1, 2, 3, 4, 5];
console.log(...x);

// bad
new (Function.prototype.bind.apply(Date, [null, 2016, 8, 5]));

// good
new Date(...[2016, 8, 5]);

7.16 Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself and with a trailing comma on the last item. eslint: function-paren-newline

// bad
function foo(bar,
             baz,
             quux){
  // ...
}

// good
function foo(
    bar,
    baz,
    quux,
    ...
){
  // ...
}

// bad
console.log(foo,
    bar,
    baz);

// good
console.log(
    foo,
    bar,
    baz,
    ...
);

⬆ back to top

Arrow Functions

8.1 When you must use an anonymous function (as when passing an inline callback), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing

Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax.

Why not? If you have a fairly complicated function, you might move that logic out into its own named function expression.
```
// bad
[1, 2, 3].map(function (x){
    const y = x + 1;
    return x * y;
});

// good
[1, 2, 3].map((x) => {
    const y = x + 1;
    return x * y;
});
```

8.2 If the function body consists of a single statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style

Why? Syntactic sugar. It reads well when multiple functions are chained together.

// bad
[1, 2, 3].map(number => {
    const nextNumber = number + 1;
    `A string containing the ${nextNumber}.`;
});

// good
[1, 2, 3].map(number => `A string containing the ${number + 1}.`);

// good
[1, 2, 3].map((number) => {
    const nextNumber = number + 1;
    return `A string containing the ${nextNumber}.`;
});

// good
[1, 2, 3].map((number, index) => ({
    [index]: number,
}));

// No implicit return with side effects
function foo(callback){
    const val = callback();
    if (val === true){
        // Do something if callback returns true
    }
}

let bool = false;

// bad
foo(() => bool = true);

// good
foo(() => {
    bool = true;
});

8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.

Why? It shows clearly where the function starts and ends.

// bad
["get", "post", "put"].map(httpMethod => Object.prototype.hasOwnProperty.call(
        httpMagicObjectWithAVeryLongName,
        httpMethod,
    )
);

// good
["get", "post", "put"].map(httpMethod => (
    Object.prototype.hasOwnProperty.call(
        httpMagicObjectWithAVeryLongName,
        httpMethod,
    )
));

8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments for clarity and consistency. Note: it is also acceptable to always use parentheses, in which case use the “always” option for eslint. eslint: arrow-parens

Why? Less visual clutter.

// bad
[1, 2, 3].map((x) => x * x);

// good
[1, 2, 3].map(x => x * x);

// good
[1, 2, 3].map(number => (
    `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
));

// bad
[1, 2, 3].map(x => {
    const y = x + 1;
    return x * y;
});

// good
[1, 2, 3].map((x) => {
    const y = x + 1;
    return x * y;
});

8.5 Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint: no-confusing-arrow

// bad
const itemHeight = item => item.height > 256 ? item.largeSize : item.smallSize;

// bad
const itemHeight = (item) => item.height > 256 ? item.largeSize : item.smallSize;

// good
const itemHeight = item => (item.height > 256 ? item.largeSize : item.smallSize);

// good
const itemHeight = (item) => {
    const { height, largeSize, smallSize } = item;
    return height > 256 ? largeSize : smallSize;
};

8.6 Enforce the location of arrow function bodies with implicit returns. eslint: implicit-arrow-linebreak

// bad
(foo) =>
    bar;

(foo) =>
    (bar);

// good
(foo) => bar;
(foo) => (bar);
(foo) => (
     bar
)

⬆ back to top

Classes & Constructors

9.1 Always use class. Avoid manipulating prototype directly.

Why? class syntax is more concise and easier to reason about.

// bad
function Queue(contents = []){
    this.queue = [...contents];
}

Queue.prototype.pop = function(){
    const value = this.queue[0];
    this.queue.splice(0, 1);
    return value;
};

// good
class Queue {
    constructor(contents = []){
        this.queue = [...contents];
    }
    pop(){
        const value = this.queue[0];
        this.queue.splice(0, 1);
        return value;
    }
}

9.2 Use extends for inheritance.

Why? It is a built-in way to inherit prototype functionality without breaking instanceof.

// bad
const inherits = require("inherits");
function PeekableQueue(contents){
    Queue.apply(this, contents);
}

inherits(PeekableQueue, Queue);
PeekableQueue.prototype.peek = function(){
    return this.queue[0];
};

// good
class PeekableQueue extends Queue {
    peek(){
        return this.queue[0];
    }
}

9.3 Methods can return this to help with method chaining.

// bad
Player.prototype.jump = function(){
    this.jumping = true;
    return true;
};

Player.prototype.setHeight = function(height){
    this.height = height;
};

const foo = new Player();
foo.jump(); // => true
foo.setHeight(20); // => undefined

// good
class Player {
    jump(){
        this.jumping = true;
        return this;
    }

    setHeight(height){
        this.height = height;
        return this;
    }
}

const foo = new Player();

foo.jump().setHeight(20);

9.4 It’s okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

class Player {
    constructor(options = {}) {
        this.name = options.name || "no name";
    }

    getName(){
        return this.name;
    }

    toString(){
        return `Player - ${this.getName()}`;
    }
}

9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. eslint: no-useless-constructor

// bad
class Player {
    constructor() {}

    getName(){
        return this.name;
    }
}

// bad
class Foo extends Player {
    constructor(...args){
        super(...args);
    }
}

// good
class Foo extends Player {
    constructor(...args){
        super(...args);
        this.name = "Foo";
    }
}

9.6 Avoid duplicate class members. eslint: no-dupe-class-members

Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.
```
// bad
class Foo {
    bar(){ return 1; }
    bar(){ return 2; }
}

// good
class Foo {
    bar(){ return 1; }
}

// good
class Foo {
    bar(){ return 2; }
}
```

9.7 Class methods should use this or be made into a static method unless an external library or framework requires to use specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver. eslint: class-methods-use-this

// bad
class Foo {
    bar(){
        console.log("bar");
    }
}

// good - this is used
class Foo {
    bar(){
        console.log(this.bar);
    }
}

// good - constructor is exempt
class Foo {
    constructor(){
        // ...
    }
}

// good - static methods aren't expected to use this
class Foo {
    static bar(){
        console.log("bar");
    }
}

⬆ back to top

Modules

10.1 Stick to CommonJS Imports (RequireJS / module.exports).

Why? Not many of the current browser engines implements import/export from the ES6 standard. In NodeJS, require() is still the standard way of importing modules. Plus, in NodeJS you can make use of dynamic module loading and Require's API's in general, where you can control caching as well.
```
// bad
import foo from "bar";
export default bar;

// bad
import { foo } from "bar";
export default bar;

// good
let foo = require("bar");
module.exports = bar;
```

10.2 Only import from a path in one place. eslint: no-duplicate-imports

Why? Having multiple lines that import from the same path can make code harder to maintain.

// bad
let foo = require("bar");
// … some other imports … 
let { foo2, foo3 } = require("bar");

// good
let { foo, foo1, foo2 } = require("bar");

// good
let {
    foo,
    foo1,
    foo2,
} = require("bar");

10.3 Do not export mutable bindings. eslint: import/no-mutable-exports

Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.
```
// bad
let foo = 3;
module.exports = foo;

// good
const foo = 3;
module.exports = foo;
```

10.4 Put all imports above non-import statements. eslint: import/first

Why? Since imports are hoisted, keeping them all at the top prevents surprising behavior.
```
// bad
let foo = require("foo");
foo.init();

let bar = require("bar");

// good
let foo = require("foo");
let bar = require("bar");

foo.init();
```

10.5 Do not include JavaScript filename extensions eslint: import/extensions

Why? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.
```
// bad
import foo from "./foo.js";
import bar from "./bar.jsx";
import baz from "./baz/index.jsx";

// good
import foo from "./foo";
import bar from "./bar";
import baz from "./baz";
```

10.6 Multiline imports should be indented just like multiline array and object literals. eslint: object-curly-newline

Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide.
```
// bad
let { longNameA, longNameB, longNameC, longNameD, longNameE } = require("path");

// good
let {
    longNameA,
    longNameB,
    longNameC,
    longNameD,
    longNameE,
} = require("path");
```

Iterators and Generators

11.1 Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of. eslint: no-iterator no-restricted-syntax

Why? Dealing with pure functions that return values is easier to reason about than side effects.

Use map() / every() / filter() / find() / findIndex() / reduce() / some() / ... to iterate over arrays, and Object.keys() / Object.values() / Object.entries() to produce arrays so you can iterate over objects.

Note: for-in and for-of are mostly OK as well. Especially for small operations. Higher-order functions however are best practice.

const numbers = [1, 2, 3, 4, 5];

// ok...
let sum = 0;
for (let num of numbers){
    sum += num;
}
sum === 15;

// good
let sum = 0;
numbers.forEach((num) => {
    sum += num;
});
sum === 15;

// best (use the functional force)
const sum = numbers.reduce((total, num) => total + num, 0);
sum === 15;

// bad
const increasedByOne = [];
for (let i = 0; i < numbers.length; i++){
    increasedByOne.push(numbers[i] + 1);
}

// good
const increasedByOne = [];
numbers.forEach((num) => {
    increasedByOne.push(num + 1);
});

// best (keeping it functional)
const increasedByOne = numbers.map(num => num + 1);

11.2 Use generators only in NodeJS for now.

Why? They don’t transpile well to ES5.

11.3 If you must use generators, or if you disagree with 11.2, make sure their function signature is spaced properly. eslint: generator-star-spacing

Why? function and * are part of the same conceptual keyword - * is not a modifier for function, function* is a unique construct, different from function.

// bad
function * foo(){
    // ...
}

// bad
let bar = function * (){
    // ...
};

// bad
let baz = function *(){
    // ...
};

// bad
let quux = function* (){
    // ...
};

// bad
function*foo(){
    // ...
}

// bad
function *foo(){
    // ...
}

// very bad
function
*
foo(){
    // ...
}

// very bad
let foo = function
*
(){
    // ...
};

// good
function* foo(){
    // ...
}

// good
let foo = function*(){
    // ...
};

⬆ back to top

Properties

12.1 Use dot notation when accessing properties. eslint: dot-notation

const foo = {
    bar: true,
    baz: 5,
};

// bad
const isTrue = foo["bar"];

// good
const isTrue = foo.bar;

12.2 Use bracket notation [] when accessing properties with a variable or if the key includes illegal characters.

const foo = {
    bar: true,
    baz: 5,
    "test-1": "foo"
};

function getProp(prop) {
    return foo[prop];
}

const isTrue = getProp("bar");
const bar = foo["test-1"];

12.3 Use exponentiation operator ** when calculating exponentiations. eslint: no-restricted-properties.
```
// bad
const binary = Math.pow(2, 10);

// good
const binary = 2 ** 10;
```

⬆ back to top

Variables

13.1 Always use const or let to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Variables declared without const, let or var are disallowed in strict mode as well. eslint: no-undef prefer-const
```
// bad
foo = new Foo();

// good
const foo = new Foo();
```

13.2 Use one const or let declaration per variable or assignment. eslint: one-var

Why? It’s easier to add new variable declarations this way, and you never have to worry about swapping out a ; for a , or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.
```
// bad
const items = getItems(),
    foo = true,
    bar = "baz";

// bad
// (compare to above, and try to spot the mistake)
const items = getItems(),
    foo = true;
    bar = "baz";

// good
const items = getItems();
const foo = true;
const bar = "baz";
```

13.3 Group all your consts and then group all your lets.

Why? This is helpful when later on you might need to assign a variable depending on one of the previously assigned variables.

// bad
let i, len, foo,
    items = getItems(),
    bar = true;

// bad
let i;
const items = getItems();
let foo;
const bar = true;
let len;

// good
const bar = true;
const items = getItems();
let foo;
let i;
let length;

13.4 Assign variables where you need them, but place them in a reasonable place.

Why? let and const are block scoped and not function scoped.

// bad - unnecessary function call
function checkName(hasName){
    const name = getName();

    if (hasName === "test") return false;

    if (name === "test"){
        this.setName("");
        return false;
    }

    return name;
}

// good
function checkName(hasName){
    if (hasName === "test") return false;

    const name = getName();

    if (name === "test"){
        this.setName("");
        return false;
    }

    return name;
}

13.5 Don’t chain variable assignments. eslint: no-multi-assign

Why? Chaining variable assignments creates implicit global variables.

// bad
(function example(){
    /**
     * JavaScript interprets this as
     * let a = ( b = ( c = 1 ) );
     * The let keyword only applies to variable a; variables b and c become
     * global variables.
     */
    let a = b = c = 1;
}());

console.log(a); // throws ReferenceError
console.log(b); // 1
console.log(c); // 1

// good
(function example(){
    let a = 1;
    let b = a;
    let c = a;
}());

console.log(a); // throws ReferenceError
console.log(b); // throws ReferenceError
console.log(c); // throws ReferenceError

// the same applies for `const`

13.6 Avoid using unary increments and decrements (++, --). eslint no-plusplus

Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.
```
// bad

const array = [1, 2, 3];
let num = 1;
num++;
--num;

let sum = 0;
let truthyCount = 0;
for (let i = 0; i < array.length; i++){
 let value = array[i];
 sum += value;
 if (value) truthyCount++;
}

// good

const array = [1, 2, 3];
let num = 1;
num += 1;
num -= 1;

const sum = array.reduce((a, b) => a + b, 0);
const truthyCount = array.filter(Boolean).length;
```

13.7 Avoid linebreaks before or after = in an assignment. If your assignment violates max-len, surround the value in parens. eslint operator-linebreak.

Why? Linebreaks surrounding = can obfuscate the value of an assignment.

// bad
const foo =
  superLongLongLongLongLongLongLongLongFunctionName();

// bad
const foo
  = "superLongLongLongLongLongLongLongLongString";

// good
const foo = (
    superLongLongLongLongLongLongLongLongFunctionName()
);

// good
const foo = "superLongLongLongLongLongLongLongLongString";

13.8 Disallow unused variables. eslint: no-unused-vars

Why? Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

// bad

let some_unused_var = 42;

// Write-only variables are not considered as used.
let y = 10;
y = 5;

// A read for a modification of itself is not considered as used.
let z = 0;
z = z + 1;

// Unused function arguments.
function getX(x, y){
    return x;
}

// good

function getXPlusY(x, y){
    return x + y;
}

let x = 1;
let y = a + 2;

alert(getXPlusY(x, y));

// 'type' is ignored even if unused because it has a rest property sibling.
// This is a form of extracting an object that omits the specified keys.
let { type, ...coords } = data;
// 'coords' is now the 'data' object without its 'type' property.

13.9 Do not floor variables with the bitwise or (x | 0). Use Math.floor() or if you must Math.trunc().

Why? First off, it does not floor the number. It truncates it (rounding towards 0). It causes odd Comparative behavior as well: Math.floor(NaN) === NaN, while (NaN | 0) === 0. Also, it works with 32-bit signed integers only. As mentioned above, use Math.trunc() if you have to. It is the ES5 equivalent of | 0 and it is able to work with numbers higher or equal to 2^31.
```
let x = 42.835

// Bad
let y = x | 0;

// Okay
let y = Math.trunc(x);

// Good 
let y = Math.floor(x);
```

⬆ back to top

Hoisting

14.1 var declarations get hoisted to the top of their closest enclosing function scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It’s important to know why typeof is no longer safe.

// this wouldn’t work (assuming there
// is no notDefined global variable)
function example(){
    console.log(notDefined); // => throws a ReferenceError
}

// creating a variable declaration after you
// reference the variable will work due to
// variable hoisting. Note: the assignment
// value of `true` is not hoisted.
function example(){
    console.log(declaredButNotAssigned); // => undefined
    var declaredButNotAssigned = true;
}

// the interpreter is hoisting the variable
// declaration to the top of the scope,
// which means our example could be rewritten as:
function example(){
    let declaredButNotAssigned;
    console.log(declaredButNotAssigned); // => undefined
    declaredButNotAssigned = true;
}

// using const and let
function example(){
    console.log(declaredButNotAssigned); // => throws a ReferenceError
    console.log(typeof declaredButNotAssigned); // => throws a ReferenceError
    const declaredButNotAssigned = true;
}

14.2 Anonymous function expressions hoist their variable name, but not the function assignment.

function example(){
    console.log(anonymous); // => undefined

    anonymous(); // => TypeError anonymous is not a function

    var anonymous = function(){
        console.log("anonymous function expression");
    };
}

14.3 Named function expressions hoist the variable name, not the function name or the function body.

function example(){
    console.log(named); // => undefined

    named(); // => TypeError named is not a function

    foo(); // => ReferenceError foo is not defined

    var named = function foo(){
        console.log("bar");
    };
}

// the same is true when the function name
// is the same as the variable name.
function example(){
    console.log(named); // => undefined

    named(); // => TypeError named is not a function

    var named = function named(){
        console.log("named");
    };
}

14.4 Function declarations hoist their name and the function body.

function example(){
    foo(); // => bar

    function foo(){
        console.log("bar");
    }
}

For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

⬆ back to top

Comparison Operators & Equality

15.1 Use === and !== over == and != (Strict comparison). eslint: eqeqeq

15.2 Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:
- Objects evaluate to true
- Undefined evaluates to false
- Null evaluates to false
- Booleans evaluate to the value of the boolean
- Numbers evaluate to false if +0, -0, or NaN, otherwise true
- Strings evaluate to false if an empty string '', otherwise true
```
if ([0] && []){
    // true
    // an array (even an empty one) is an object, objects will evaluate to true
}
```

15.3 Use shortcuts for booleans, but explicit comparisons for strings and numbers.

// bad
if (isValid === true){
    // ...
}

// good
if (isValid){
    // ...
}

// bad
if (name){
    // ...
}

// good
if (name !== ""){
    // ...
}

// bad
if (collection.length){
    // ...
}

// good
if (collection.length > 0){
    // ...
}

15.4 For more information see Truth Equality and JavaScript by Angus Croll.

15.5 Use braces to create blocks in case and default clauses. This is especially useful for those, which contain lexical declarations (e.g. let, const, function, and class). eslint: no-case-declarations

Why? Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.

// bad
switch (foo){
    case 1:
        let x = 1;
        break;
    case 2:
        const y = 2;
        break;
    case 3:
        function f(){
            // ...
        }
        break;
    default:
        class C {}
}

// good
switch (foo) {
    case 1: {
        let x = 1;
        break;
    }
    case 2: {
        const y = 2;
        break;
    }
    case 3: {
        function f(){
            // ...
        }
        break;
    }
    case 4: {
        bar();
        break;
    }
    default: {
        class C {}
    }
}

15.6 Ternaries should not be nested and generally be single line expressions. eslint: no-nested-ternary

// bad
const foo = maybe1 > maybe2
  ? "bar"
  : value1 > value2 ? "baz" : null;

// split into 2 separated ternary expressions
const maybeNull = value1 > value2 ? "baz" : null;

// better
const foo = maybe1 > maybe2
  ? "bar"
  : maybeNull;

// best
const foo = maybe1 > maybe2 ? "bar" : maybeNull;

15.7 Avoid unneeded ternary statements. eslint: no-unneeded-ternary

// bad
const foo = a ? a : b;
const bar = c ? true : false;
const baz = c ? false : true;

// good
const foo = a || b;
const bar = !!c;
const baz = !c;

15.8 When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators (+, -, *, & /) since their precedence is broadly understood. eslint: no-mixed-operators

Why? This improves readability and clarifies the developer’s intention.

// bad
const foo = a && b < 0 || c > 0 || d + 1 === 0;

// bad
const bar = a ** b - 5 % d;

// bad
// one may be confused into thinking (a || b) && c
if (a || b && c){
    return d;
}

// good
const foo = (a && b < 0) || c > 0 || (d + 1 === 0);

// good
const bar = (a ** b) - (5 % d);

// good
if (a || (b && c)){
    return d;
}

// good
const bar = a + b / c * d;

⬆ back to top

Blocks

16.1 Use braces with all multi-line blocks. eslint: nonblock-statement-body-position

Note: Use one-line statements only with short commands. Like if (x) y(); They look nicer but can be hard to debug

// bad
if (test)
  return false;

// good
if (test) return false;

// bad - it is too bulky and doesn't fit nicely in one row
if (test1) if (test2) stuff();

// good
if (test1){
    if (test2) stuff();
}

// good (multiline)
if (test){
    doStuff();
    // ...
    return false;
}

// bad
function foo(){ return false; }

// good
function bar(){
    return false;
}

16.2 If you're using multi-line blocks with if and else, do not put else on the same line as your if block’s closing brace. eslint: brace-style

// bad
if (test){
    thing1();
    thing2();
} else {
    thing3();
    thing4();
}

// good
if (test){
    thing1();
    thing2();
}
else {
    thing3();
    thing4();
}

// bad 
if (test){
    thing1();
    thing2();
} else thing3();

// good
if (test){
    thing1();
    thing2();
} 
else thing3();

16.3 If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint: no-else-return

// bad
function foo(){
    if (x) return x;
    else return y;
}

// bad
function bar(){
    if (x) return x;
    else if (y) return y;
}

// bad
function baz(){
    if (x) return x;
    else {
        if (y) return y;
    }
}

// good
function foo(){
    if (x) return x;
    return y;
}

// good
function bar(){
    if (x) return x;
    if (y) return y;
}

// good
function baz(x){
    if (x){
        if (z) return y;
    } 
    else return z;
}

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Control Statements

17.1 In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.

Why? Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. This also improves readability by making it easier to visually follow complex logic.

// bad
if ((foo === 123 || bar === "abc") && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()){
    thing1();
}

// bad
if (foo === 123 &&
    bar === "abc"){
    thing1();
}

// bad
if (foo === 123
    && bar === "abc"){
    thing1();
}

// bad
if (
    foo === 123 &&
    bar === "abc"
){
    thing1();
}

// good
if (
    foo === 123
    && bar === "abc"
){
    thing1();
}

// good
if (
    (foo === 123 || bar === "abc")
    && doesItLookGoodWhenItBecomesThatLong()
    && isThisReallyHappening()
){
    thing1();
}

// good
if (foo === 123 && bar === "abc"){
    thing1();
}

17.2 Don't use selection operators in place of control statements.

// bad
!isRunning && startRunning();

// good
if (!isRunning) startRunning();

⬆ back to top

Comments

18.1 Stick to the english language. Always write variable names, function names, comments and co in english.
Why? Some reasons:
- Consistency.
- English is a global language. What if you're part of a german developer team, write code in german and then want to hire someone from another country?
- JavaScript's keywords are english.
- Some languages use symbols from different charsets (ö, ä, ü, ß, Ѱ, Ω, etc. pp.). Some of them are illegal as variable/function names and others could break your encoding.

18.2 Use /** ... */ for multi-line comments.

// bad
// make() returns a new element
// based on the passed in tag name
//
// @param {String} tag
// @return {Element} element
function make(tag){

    // ...

    return element;
}

// good
/**
 * make() returns a new element
 * based on the passed-in tag name
 */
function make(tag){

    // ...

    return element;
}

18.3 Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it’s on the first line of a block.

// bad
const active = true;  // is current tab

// good
// is current tab
const active = true;

// bad
function getType(){
    console.log("fetching type...");
    // set the default type to 'no type'
    const type = this.type || "no type";

    return type;
}

// good
function getType(){
    console.log("fetching type...");

    // set the default type to 'no type'
    const type = this.type || "no type";

    return type;
}

// also good
function getType(){
    // set the default type to 'no type'
    const type = this.type || "no type";

    return type;
}

18.4 Start all comments with a space to make it easier to read. eslint: spaced-comment

// bad
//is current tab
const active = true;

// good
// is current tab
const active = true;

// bad
/**
 *make() returns a new element
 *based on the passed-in tag name
 */
function make(tag){

    // ...

    return element;
}

// good
/**
 * make() returns a new element
 * based on the passed-in tag name
 */
function make(tag){

    // ...

    return element;
}

18.5 Prefixing your comments with FIXME or TODO (action-items) helps other developers quickly understand if you're pointing out a problem that needs to be revisited, or if you're suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME: -- need to figure this out or TODO: -- need to implement.

18.6 Use // FIXME: to annotate problems.

class Calculator extends Abacus {
    constructor(){
        super();

        // FIXME: shouldn’t use a global here
        total = 0;
    }
}

18.7 Use // TODO: to annotate solutions to problems.

class Calculator extends Abacus {
    constructor(){
        super();

        // TODO: total should be configurable by an options param
        this.total = 0;
    }
}

⬆ back to top

Whitespace

19.1 Use soft tabs (space character instead of tabulator) set to 4 spaces. eslint: indent

// bad
function foo(){
∙∙let name;
  let foo;
}

// bad
function bar(){
∙let name;
 let foo;
}

// good
function baz(){
∙∙∙∙let name;
    let foo;
}

19.2 Place 1 space before the leading brace if the character before it is not a parenthesis. eslint: space-before-blocks

Why? Technically speaking, the block is part of the expression. A space doesn't really make sense unless it is simpler to read.

// bad
function test() {
    console.log("test");
}

// good
function test(){
    console.log("test");
}

// bad
foo.set("attr",{
    bar: "baz"
});

// good
foo.set("attr", {
    bar: "baz"
});

// bad
class Foo{
    // ...
}

// good 
class Foo {
    // ...
}

// bad
constructor() {
    // ...
}

// good
constructor(){
    // ...
}

// bad
if (foo) {
    bar();
}
else{
    baz();
}

// good
if (foo){
    bar();
}
else {
    baz();
}

// bad
let x ={
    foo: "bar",
};

// good
let x = {
    foo: "bar",
};

19.3 Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space between the argument list and the function name in function calls and declarations. eslint: keyword-spacing
```
// bad
if(foo){
    bar ();
}

// good
if (foo){
    bar();
}

// bad
function foo (){
    console.log ("bar");
}

// good
function foo(){
    console.log("bar");
}
```

19.4 Set off operators with spaces. eslint: space-infix-ops
```
// bad
const x=y+5;

// good
const x = y + 5;
```

19.5 Use Unix/Linux-Style Linebreaks - LF (\n) instead of CR + LF (\r\n). eslint: linebreak-style

19.6 End files with a single newline character. eslint: eol-last

Why? Because that's how the POSIX standard defines a line. Read more...

// bad
let foo = require("foo");
// ...
module.exports = foo;

// bad
let foo = require("foo");
// ...
module.exports = foo;↵
↵

// good
let foo = require("foo");
// ...
module.exports = foo;↵

19.7 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which emphasizes that the line is a method call, not a new statement. eslint: newline-per-chained-call no-whitespace-before-property

// bad
$("#items").find(".selected").highlight().end().find(".open").updateCount();

// bad
$("#items").
    find(".selected").
        highlight().
        end().
    find(".open").
        updateCount();

// good
$("#items")
    .find(".selected")
        .highlight()
        .end()
    .find(".open")
        .updateCount();

// bad
const leds = stage.selectAll(".led").data(data).enter().append("svg:svg").classed("led", true)
    .attr("width", (radius + margin) * 2).append("svg:g")
    .attr("transform", `translate(${radius + margin}, ${radius + margin})`)
    .call(tron.led);

// good
const leds = stage.selectAll(".led")
        .data(data)
    .enter().append("svg:svg")
        .classed("led", true)
        .attr("width", (radius + margin) * 2)
    .append("svg:g")
        .attr("transform", `translate(${radius + margin}, ${radius + margin})`)
        .call(tron.led);

// good
const leds = stage.selectAll(".led").data(data);

19.8 Leave a blank line after blocks and before the next statement.

// bad
if (foo){
    return bar;
}
return baz;

// good
if (foo){
    return bar;
}

return baz;

// bad
const obj = {
    foo(){
    },
    bar(){
    }
};
return obj;

// good
const obj = {
    foo(){
    },

    bar(){
    }
};

return obj;

// bad
const arr = [
    function foo(){
    },
    function bar(){
    }
];
return arr;

// good
const arr = [
    function foo(){
    },

    function bar(){
    }
];

return arr;

19.9 Do not pad your blocks with blank lines. eslint: padded-blocks

// bad
function bar(){

    console.log(foo);

}

// bad
if (baz){

    console.log(qux);
} 
else {
    console.log(foo);

}

// bad
class Foo {

    constructor(bar){
        this.bar = bar;
    }
}

// good
function bar(){
    console.log(foo);
}

// good
if (baz){
    console.log(qux);
} 
else {
    console.log(foo);
}

19.10 Do not add spaces inside parentheses. eslint: space-in-parens

// bad
function bar( foo ){
    return foo;
}

// good
function bar(foo){
    return foo;
}

// bad
if ( foo ){
    console.log(foo);
}

// good
if (foo){
    console.log(foo);
}

19.11 Do not add spaces inside brackets. eslint: array-bracket-spacing

// bad
const foo = [ 1, 2, 3 ];
console.log(foo[ 0 ]);

// good
const foo = [1, 2, 3];
console.log(foo[0]);

19.12 Add spaces inside curly braces. eslint: object-curly-spacing

// bad
const foo = {foo: "bar"};

// good
const foo = { foo: "bar" };

19.13 Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per above, long strings are exempt from this rule, and should not be broken up. eslint: max-len

Why? This ensures readability and maintainability.

// bad
const foo = jsonData && jsonData.foo && jsonData.foo.bar && jsonData.foo.bar.baz && jsonData.foo.bar.baz.quux && jsonData.foo.bar.baz.quux.xyzzy;

// bad
$.ajax({ method: "POST", url: "https://nulldev.org", data: { foo: "bar" } }).done(() => console.log("Done")).fail(() => console.log("Error"));

// good
const foo = jsonData
    && jsonData.foo
    && jsonData.foo.bar
    && jsonData.foo.bar.baz
    && jsonData.foo.bar.baz.quux
    && jsonData.foo.bar.baz.quux.xyzzy;

// good
$.ajax({
    method: "POST", 
    url: "https://nulldev.org", 
    data: { 
        foo: "bar" 
    },
}).done(() => {
    console.log("Done");
}).fail(() => {
    console.log("Error");
});

19.14 Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint: block-spacing
```
// bad
function foo(){return true;}
if (foo){ bar = 0;}

// good
function foo(){ return true; }
if (foo){ bar = 0; }
```

19.15 Avoid spaces before commas and require a space after commas. eslint: comma-spacing

// bad
let foo = 1,bar = 2;
let arr = [1 , 2];

// good
let foo = 1, bar = 2;
let arr = [1, 2];

19.16 Enforce spacing inside of computed property brackets. eslint: computed-property-spacing

// bad
obj[foo ]
obj[ "foo"]
let x = {[ b ]: a}
obj[foo[ bar ]]

// good
obj[foo]
obj["foo"]
let x = { [b]: a }
obj[foo[bar]]

19.17 Avoid spacing between functions and their invocations. eslint: func-call-spacing
```
// bad
func ();

func
();

// good
func();
```

19.18 Enforce spacing between keys and values in object literal properties. eslint: key-spacing

// bad
let obj = { "foo" : 42 };
let obj2 = { "foo":42 };

// good
let obj = { "foo": 42 };

19.19 Avoid trailing spaces at the end of lines. eslint: no-trailing-spaces

19.20 Avoid multiple empty lines and only allow one newline at the end of files. eslint: no-multiple-empty-lines

```
// bad
let x = 1;



let y = 2;

// good
let x = 1;

let y = 2;
```

⬆ back to top

Commas

20.1 Do not write leading commas. eslint: comma-style

// bad
const x = [
      foo
    , bar
    , baz
];

// good
const x = [
    foo,
    bar,
    baz,
];

// bad
const x = {
      foo: "foo1"
    , bar: "bar1"
    , baz: "baz1"
    , abc: "abc1"
};

// good
const x = {
    foo: "foo1",
    bar: "bar1",
    baz: "baz1",
    abc: "abc1",
};

20.2 Write additional trailing commas. eslint: comma-dangle

Why? It leads to cleaner git diffs and allows easier copy-pasting. Careful: A comma must not appear after a rest element. Read more...

// git diff without trailing comma (bad)
const hero = {
     bar: "bar",
-    baz: "baz"
+    baz: "baz",
+    abc: [1, 2, 3]
};

// git diff with trailing comma (good)
const foo = {
     bar: "bar",
     baz: "baz",
+    abc: [1, 2, 3],
};

// bad
const foo = {
    bar: true,
    baz: false
};

const foo = [
    "bar",
    "baz"
];

// good
const foo = {
    bar: true,
    baz: false,
};

const foo = [
    "bar",
    "baz",
];

// bad
function foo(
    arg1,
    arg2,
    agr3
){
    // ..
}

// good
function foo(
    arg1,
    arg2,
    agr3,
){
    // ..
}

// bad
createUser(
    firstName,
    lastName,
    birthday
);

// good
createUser(
    firstName,
    lastName,
    birthday,
);

⬆ back to top

Semicolons

21.1 Use semicolons. eslint: semi

Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called Automatic Semicolon Insertion to determine whether or not it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues. In other words: You could say ASI is a feature to help out, when you forget a semicolon. This doesn't mean you shouldn't use them. Not using semicolons may also slow down the execution because of the additional parsing.
```
// bad - raises exception
const foo = {}
const bar = {}
[foo, bar].forEach(baz => baz.x = "test")

// bad - raises exception
const abc = "Another test"
(async function tempFoo(){
    // ...
}())

// bad - returns `undefined` instead of the value on the next line
// this always happens when `return` is on a line by itself because of ASI!
function tempBar(){
  return
      "Some string..."
}

// good
const foo = {};
const bar = {};
[foo, bar].forEach((baz) => {
    baz.x = "test";
});

// good
const abc = "Another test";
(async function tempFoo(){
    // ...
}());

// good
function tempBar(){
    return "Some string...";
}
```
Read more.

⬆ back to top

Type Casting & Coercion

22.1 Perform type coercion at the beginning of the statement.

22.2 Strings: Prefer String() over .toString() eslint: no-new-wrappers

Why? .toString() is a prototype of Number. String() on the other hand, is globally available and thus allows casting of any type. Also, .toString() can be overridden as seen in section 9.4

// => this.reviewScore = 9;

// bad
const totalScore = new String(this.reviewScore); // typeof totalScore is "object" not "string"

// bad
const totalScore = this.reviewScore + ""; // invokes this.reviewScore.valueOf()

// bad
const totalScore = this.reviewScore.toString(); // isn't guaranteed to return a string

// good
const totalScore = String(this.reviewScore);

22.3 Numbers: Use Number() for type casting and parseInt() only with a radix for parsing strings. Do prefer Number() over parseInt() though. eslint: radix no-new-wrappers

Why? Mostly because of the same reasons listed in the section above. Also, since parseInt() always expects a string, it does show odd behaviour when parsing very small numbers (source)
```
const inputValue = "4";

// bad
const val = new Number(inputValue);

// bad
const val = +inputValue;

// bad
const val = inputValue >> 0;

// bad
const val = parseInt(inputValue);

// best
const val = Number(inputValue);

// good
const val = parseInt(inputValue, 10);
```

22.4 If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you're doing.
```
// good
/**
 * parseInt was the reason my code was slow.
 * Bitshifting the String to coerce it to a
 * Number made it a lot faster.
 */
const val = inputValue >> 0;
```

22.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. More info. Largest signed 32-bit Int is 2,147,483,647:
```
2147483647 >> 0; // => 2147483647
2147483648 >> 0; // => -2147483648
2147483649 >> 0; // => -2147483647
```

22.6 Booleans: eslint: no-new-wrappers

const age = 0;

// bad
const hasAge = new Boolean(age);

// good
const hasAge = Boolean(age);

// best
const hasAge = !!age;

22.6 Only compare returned strings by typeof to valid strings: eslint: valid-typeof


// bad - will be prevented by linter
typeof foo === "strnig";

// good
typeof foo === "string";
typeof bar === typeof foo;

⬆ back to top

Naming Conventions

23.0 Stick to the english language. Always write variable names, function names, comments and co in english.
Why? Some reasons:
- Consistency.
- English is a global language. What if you're part of a german developer team, write code in german and then want to hire someone from another country?
- JavaScript's keywords are english.
- Some languages use symbols from different charsets (ö, ä, ü, ß, Ѱ, Ω, etc. pp.). Some of them are illegal as variable/function names and others could break your encoding.

23.1 Avoid single letter names. Be descriptive with your naming. eslint: id-length
```
// bad
function q(){
    // ...
}

// good
function query(){
    // ...
}
```

23.2 Use camelCase when naming objects, functions, and instances. eslint: camelcase

// bad
const OBJEcttsssss = {};
const this_is_my_object = {};
function c(){}

// good
const thisIsMyObject = {};
function thisIsMyFunction(){}

23.3 Use PascalCase only when naming constructors or classes. eslint: new-cap

// bad
function user(options){
    this.name = options.name;
}

const bad = new user({
    name: "...",
});

// good
class User {
    constructor(options){
        this.name = options.name;
    }
}

const good = new User({
    name: "...",
});

23.4 Do not use trailing or leading underscores. eslint: no-underscore-dangle

Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won’t count as breaking, or that tests aren’t needed. tl;dr: if you want something to be “private”, it must not be observably present.
```
// bad
this.__firstName__ = "Foo";
this.firstName_ = "Foo";
this._firstName = "Foo";

// good
this.firstName = "Foo";

// good, in environments where WeakMaps are available
// see https://kangax.github.io/compat-table/es6/#test-WeakMap
const firstNames = new WeakMap();
firstNames.set(this, "Foo");
```

23.5 Don’t save references to this. Use arrow functions or Function#bind.

// bad
function foo(){
    const self = this;
    return function(){
        console.log(self);
    };
}

// bad
function foo(){
    const that = this;
    return function(){
        console.log(that);
    };
}

// good
function foo(){
    return () => {
        console.log(this);
    };
}

23.6 A base filename should exactly match the name of its default export.

// file 1 contents
class CheckBox {
    // ...
}
module.exports = CheckBox;

// file 2 contents
module.exports = function fortyTwo(){ return 42; }

// file 3 contents
module.exports = function insideDirectory(){}

// in some other file
// bad
let CheckBox = require("./checkBox"); // PascalCase import/export, camelCase filename
let FortyTwo = require("./FortyTwo"); // PascalCase import/filename, camelCase export
let InsideDirectory = require("./InsideDirectory"); // PascalCase import/filename, camelCase export

// bad
let CheckBox = require("./check_box"); // PascalCase import/export, snake_case filename
let forty_two = require("./forty_two"); // snake_case import/filename, camelCase export
let inside_directory = require("./inside_directory"); // snake_case import, camelCase export
let index = require("./inside_directory/index"); // requiring the index file explicitly
let insideDirectory = require("./insideDirectory/index"); // requiring the index file explicitly

// good
let CheckBox = require("./CheckBox"); // PascalCase export/import/filename
let fortyTwo = require("./fortyTwo"); // camelCase export/import/filename
let insideDirectory = require("./insideDirectory"); // camelCase export/import/directory name/implicit "index"
// ^ supports both insideDirectory.js and insideDirectory/index.js

23.7 Use camelCase when you export-default a function. Your filename should be identical to your function’s name.
```
function makeStyleGuide(){
    // ...
}

module.exports = makeStyleGuide;
```

23.8 Use PascalCase when you export a constructor / class / singleton / function library / bare object.
```
const Foo = {
    bar: {
    }
};

module.exports = Foo;
```

23.9 Acronyms and initialisms should always be all capitalized, or all lowercased.

Why? Names are for readability, not to appease a computer algorithm.

// bad
let SmsContainer = require("./containers/SmsContainer");

// bad
const HttpRequests = [
    // ...
];

// good
let SMSContainer = require("./containers/SMSContainer");

// good
const HTTPRequests = [
    // ...
];

// also good (sticks to camelCase)
const httpRequests = [
    // ...
];

// best
let TextMessageContainer = require("./containers/TextMessageContainer");

// best
const requests = [
    // ...
];

23.10 You may optionally uppercase a constant only if it (1) is exported, (2) is a const (it can not be reassigned), and (3) the programmer can trust it (and its nested properties) to never change. Note: This is about ES6 import/export, not CommonJS require()

Why? This is an additional tool to assist in situations where the programmer would be unsure if a variable might ever change. UPPERCASE_VARIABLES are letting the programmer know that they can trust the variable (and its properties) not to change. 
- What about all const variables? This is unnecessary, so uppercasing should not be used for constants within a file. It should be used for exported constants however. 
- What about exported objects? Uppercase at the top level of export (e.g. EXPORTED_OBJECT.key) and maintain that all nested properties do not change.
```
// bad
const PRIVATE_VARIABLE = "should not be unnecessarily uppercased within a file";

// bad
export const THING_TO_BE_CHANGED = "should obviously not be uppercased";

// bad
export let REASSIGNABLE_VARIABLE = "do not use let with uppercase variables";

// ---

// allowed but does not supply semantic value
export const apiKey = "SOMEKEY";

// better in most cases
export const API_KEY = "SOMEKEY";

// ---

// bad - unnecessarily uppercases key while adding no semantic value
export const MAPPING = {
 KEY: "value",
};

// good
export const MAPPING = {
 key: "value",
};
```

23.11 Write isCondition instead of conditionState for boolean state checks.

Why? It makes the intentions clear.

// bad - this is very vague
if (activeState) doSomething();

// bad - the "=== true" check makes the intentions clear, but is also unnecessary
if (activeState === true) doSomething();

// good - short and clear
if (isActive) doSomething();

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Accessors

24.1 Accessor functions for properties are not required.

24.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal("foo").
```
// bad
class Foo {
    get bar(){
        // ...
    }

    set bar(value){
        // ...
    }
}

// good
class Foo {
    getBar(){
        // ...
    }

    setBar(value){
        // ...
    }
}
```

24.3 If the property/method is a boolean, use isVal() or hasVal().

// bad
if (!foo.bar()){
    return false;
}

// good
if (!foo.hasBar()){
    return false;
}

24.4 It’s okay to create get() and set() functions, but be consistent.

class Foo {
    constructor(options = {}){
        const bar = options.bar || "Default bar";
        this.set("bar", bar);
    }

    set(key, val){
        this[key] = val;
    }

    get(key){
        return this[key];
    }
}

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Events

25.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass an object literal (also known as a "hash") instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:
```
// bad
$(this).trigger("listingUpdated", listing.id);

// ...

$(this).on("listingUpdated", (e, listingID) => {
    // do something with listingID
});
```
prefer:
```
// good
$(this).trigger("listingUpdated", { 
    listingID: listing.id 
});

// ...

$(this).on("listingUpdated", (e, data) => {
    // do something with data.listingID
});
```

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Standard Library

The Standard Library contains utilities that are functionally broken but remain for legacy reasons.

26.1 Use Number.isNaN instead of global isNaN. eslint: no-restricted-globals

Why? The global isNaN coerces non-numbers to numbers, returning true for anything that coerces to NaN. If this behavior is desired, make it explicit.
```
// bad
isNaN("1.2"); // false
isNaN("1.2.3"); // true

// good
Number.isNaN("1.2.3"); // false
Number.isNaN(Number("1.2.3")); // true
```

26.2 Use Number.isFinite instead of global isFinite. eslint: no-restricted-globals

Why? The global isFinite coerces non-numbers to numbers, returning true for anything that coerces to a finite number. If this behavior is desired, make it explicit.
```
// bad
isFinite("2e3"); // true

// good
Number.isFinite("2e3"); // false
Number.isFinite(parseInt("2e3", 10)); // true
```

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jQuery

27.1 Prefix jQuery object variables with a $.

// bad
const sidebar = $(".sidebar");

// good
const $sidebar = $(".sidebar");

// good
const $sidebarBtn = $(".sidebar-btn");

27.2 Cache jQuery lookups.

// bad
function setSidebar(){
    $(".sidebar").hide();

    // ...

    $(".sidebar").css({
        "background-color": "green",
    });
}

// good
function setSidebar(){
    const $sidebar = $(".sidebar");
    $sidebar.hide();

    // ...

    $sidebar.css({
        "background-color": "green",
    });
}

27.3 For DOM queries use Cascading $(".sidebar ul") or parent > child $(".sidebar > ul"). jsPerf

27.4 Use find with scoped jQuery object queries.

// bad
$("ul", ".sidebar").hide();

// bad
$(".sidebar").find("ul").hide();

// good
$(".sidebar ul").hide();

// good
$(".sidebar > ul").hide();

// good
$sidebar.find("ul").hide();

27.5 Use .on instead of the event name (shorthands) when doing bindings.

Why? .on() works on dynamically added elements and is better in performance. Read more...

// bad
$("a.test").click(function(){
    // ...
});

$("div.test").mouseover(function(){
    // ...
});

// good
$("a.test").on("click", function(){
    // ...
});

$("div.test").on("mouseover", function(){
    // ...
});

27.6 Don't bloat the $(document).ready() function

Why? It actively harms readability and generally the structure of the code.


// Bad
$(document).ready(function(){
    $(foo).on("click", function(){
        doStuff();
        doMoreStuff();
        doEvenMoreStuff(function(stuff){
            // ...
        });
    });
    
    $(bar).on("click", function(){
        doMoreStuff();
        doStuff();
    });
    
    $(baz).on("click", function(){
        doMoreStuff();
        doEvenMoreStuff(function(stuff){
            // ...
        });
    });
});

// Good
function stuffHandler(){
    // ...
}

$(document).ready(function(){
    $(foo).on("click", stuffHandler);
    $(bar).on("click", stuffHandler);
    $(baz).on("click", stuffHandler);
});

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ECMAScript 5 Compatibility

28.1 Refer to Kangax’s ES5 compatibility table.

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ECMAScript 6+ (ES 2015+) Styles

29.1 This is a collection of links to the various ES6+ features.

Arrow Functions
Classes
Object Shorthand
Object Concise
Object Computed Properties
Template Strings
Destructuring
Default Parameters
Rest
Array Spreads
Let and Const
Exponentiation Operator
Iterators and Generators
Modules

29.2 Do not use TC39 proposals that have not reached stage 3.

Why? They are not finalized, and they are subject to change or to be withdrawn entirely. We want to use JavaScript, and proposals are not JavaScript yet.

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Testing

30.1 Tests
- Whichever testing framework you use, you should be writing tests!
- Strive to write many small pure functions, and minimize where mutations occur.
- Be cautious about stubs and mocks - they can make your tests more brittle.
- Recommendations: mocha and jest or tape for small, separate modules.
- 100% test coverage is a good goal to strive for, even if it’s not always practical to reach it.
- Whenever you fix a bug, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.