Awesome

Nerdery JavaScript Standards() {

A sensible style guide for writing JavaScript. This is a living, breathing document that will continue to evolve as new language features are unveiled.

Following these conventions will:

Improve readability
Minimize common code smells
Reduce errors and improve maintainability

This document is not intended to:

Advocate specific frameworks or libraries
Give advice on design patterns and project architecture
Guide the reader in learning JavaScript

ESLint Config for the Nerdery JavaScript Standards

Types
Variables
Objects
Arrays
Destructuring
Strings
Functions
Arrow Functions
Classes
Modules
Iterators and Generators
Properties
Comparison
Comments
Whitespace
Commas
Semicolons
Naming Conventions
Accessors
Events
DOM Interaction
Asynchronous Operations
Deployment
License

Types

1.1 A variable should remain the same type it was originally assigned (a number, string, boolean, array, or object). Avoid reassigning variables to a different type.
```
// bad
let count = 1;
count = 'Ben Kenobi';

// good
let count = 1;
count = 2;
```

1.2 Values returned by functions should be of a consistent type. Avoid returning multiple different types.

// bad
pressYourLuck(bigMoney) {
    if (bigMoney) {
        return 'No whammies!';
    }

    return false;
};

1.3 Perform type coercion at the beginning of the statement.

1.4 Strings:

// this.reviewScore = 9;

// 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);

1.5 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix, no-implicit-coercion

const inputValue = '4';

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

// bad
const val = +inputValue;

// bad
const val = inputValue >> 0;

// good
const val = Number(inputValue);

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

1.6 Booleans:

const age = 0;

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

// bad
const hasAge = !!age;

// good
const hasAge = Boolean(age);

1.7 Avoid "trick" operators whose purpose is not immediately readable and obvious. If you must use a convoluted syntax 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;
```

1.8 When an exception is to be thrown, prefer use of one of the built-in Error types or a class that inherits from Error.

Why? This provide more semantic messaging and allow for the possibility of catching certain types of errors.

divide(numerator, denominator) {
    if (denominator === 0) {
        // bad
        throw 'string exceptions are harder to handle';

        // good
        throw new RangeError('cannot divide by 0');
    }
}

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Variables

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;
count += 1;

// good, use the let.
let count = 1;
count += 1;
```

2.3 Use one const declaration per variable. 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.
```
// bad
const items = getItems(),
    goSportsTeam = true,
    dragonball = 'z';

// good
const items = getItems();
const goSportsTeam = true;
const dragonball = 'z';
```

2.4 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 previous assigned variables.

// bad
let dragonball,
    items = getItems(),
    goSportsTeam = true;

// bad
const items = getItems();
let dragonball;
const goSportsTeam = true;

// good
const goSportsTeam = true;
const items = getItems();
let dragonball;

2.5 Assign variables near their first use.

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

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

    if (hasName === 'test') {
        return false;
    }

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

    return name;
}

// good
checkName(hasName) {
    if (hasName === 'test') {
        return false;
    }

    const name = getName();

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

    return name;
}

2.6 Always use const or let to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint: no-undef
```
// bad
superPower = new SuperPower();

// good
const superPower = new SuperPower();
```

2.7 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); // undefined
console.log(b); // 1
console.log(c); // 1

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

console.log(a); // undefined
console.log(b); // undefined
console.log(c); // undefined

// the same applies for `const`

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Objects

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

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

3.2 If your code will be executed in a browser context, don't use reserved words as keys. Use meaningful synonyms instead.

// bad
const superman = {
    class: 'alien',
};

// bad
const superman = {
    klass: 'alien',
};

// good
const superman = {
    type: 'alien',
};

3.3 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: 'San Francisco',
};
obj[getKey('enabled')] = true;

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

3.4 Use object method shorthand. eslint: object-shorthand

// bad
const atom = {
    value: 1,

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

// good
const atom = {
    value: 1,

    addValue(value) {
        return atom.value + value;
    },
};

3.5 Use property value shorthand when object keys and values are redundantly named. eslint: object-shorthand
```
// bad
makePoint(x, y) {
    return { x: x, y: y };
}

// good
makePoint(x, y) {
    return { x, y };
}
```

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

Why? It's easier to tell which properties are using the shorthand.
```
makePoint(x, y) {
    return {
        x,
        y,
        color: 'blue',
        opacity: 0.5
    };
}
```

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

Why? In general we consider it 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-blah': 5,
};

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

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` ಠ_ಠ
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 }

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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] = 'abracadabra';

// good
someStack.push('abracadabra');

4.3 Use array spreads ... or the slice() method to make a shallow copy of arrays.

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

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

// good
const itemsCopy = items.slice();

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

4.4 To convert an array-like object to an array, use Array#from.

const foo = document.querySelectorAll('.foo');
const nodes = Array.from(foo);

4.5 Use return statements in array method callbacks. It's ok to omit the return if the function body consists of a single statement. eslint: array-callback-return

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

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

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

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

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

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

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Destructuring

5.1 Use object destructuring when accessing and using multiple properties of an object.

Why? Destructuring saves you from creating temporary references for those properties.

// bad
getFullName(user) {
    const firstName = user.firstName;
    const lastName = user.lastName;

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

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

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

5.2 Use array 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
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
processInput(input) {
    // then a miracle occurs
    return { left, right, top, bottom };
}

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

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Strings

6.1 Use single quotes '' for strings. eslint: quotes

// bad
const name = "Capt. Janeway";

// good
const name = 'Capt. Janeway';

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 that was thrown because \
of Batman. When you stop to think about how Batman had anything to do \
with this, you would get nowhere \
fast.';

// bad
const errorMessage = 'This is a super long error that was thrown because ' +
  'of Batman. When you stop to think about how Batman had anything to do ' +
  'with this, you would get nowhere fast.';

// good
const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';

6.4 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
sayHi(name) {
    return 'How are you, ' + name + '?';
}

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

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

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

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

6.6 Never inject untrusted strings into the DOM unless the value has been sanitized. Untrusted strings include anything a user or external source can manipulate, such as query parameters, cookie values, or results from an AJAX call.

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Functions

7.1 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
foo(obj) {
    obj.key = 1;
}

// good
foo(obj) {
    const key = obj.key != null ? obj.key : 1;
}
```

7.2 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.
```
// bad
foo(name) {
    name = name || 'Tony Stark';
}

// good
foo(name) {
    const localName = name || 'Tony Stark';
}

// good
foo(name = 'Tony Stark') {
    const localName = name;
}
```

7.3 When arguments may be omitted completely, use default parameter syntax.

// bad
// This won't work as expected, the default parameter will
// only be assigned if the value provided is `undefined`
signup(name = 'Tony Stark') {
    // ...
}
signup(null);

// good
signup(name = 'Tony Stark') {
    // ...
}
signup();

7.4 Avoid side effects with default parameters.

Why? They are confusing to reason about.
```
// bad
count(a = b++) {
    // ...
}
```

7.5 Always put default parameters last.

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

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

7.6 Do not create functions with more than 5 parameters. When you must have that many parameters, pass in an object instead. max-params

// bad
signup(birthdate, address, city, state, zip, name) {
    // ...
}

// good
signup({ birthdate, address, city, state, zip, name }) {
    // ...
}

// good
// Pass an instance of a `UserInfo` object
signup(userInfo) {
    // ..
}

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

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

7.8 Never use arguments, opt to use rest syntax ... instead. 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
concatenateAll() {
    const args = Array.prototype.slice.call(arguments);
    return args.join('');
}

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

7.9 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-inner-declarations
```
// bad
if (currentUser) {
    function test() {
        console.log('Nope.');
    }
}

// good
let test;
if (currentUser) {
    test = () => {
        console.log('Yup.');
    };
}
```

7.10 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
const add = new Function('a', 'b', 'return a + b');
```

7.11 Prefer a single return at the end of the function. Avoid adding multiple returns in the middle of a function, since they make the flow harder to debug and encourage inconsistent return types.

// bad
login(userId) {
    if (userId != null) {
        return getUser(userId);
    } else {
        return new User();
    }
};

// good
login(userId) {
    let user = null;

    if (userId != null) {
        user = getUser(userId);
    } else {
        user = new User();
    }

    return user;
};

7.12 An exception to the above for guard clauses: if asserting whether parameters are valid, exit early using return statements at the beginning of the function.
```
add(num1, num2) {
    if (isNaN(num1) || isNaN(num2)) {
        return false;
    }

    // ...
};
```

7.13 Add parantheses around immediately invoked function expressions. 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)
(function () {
    console.log('Welcome to the Internet. Please follow me.');
}());
```

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Arrow Functions

8.1 When you must use function expressions (as when passing an anonymous function), 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 function declaration.
```
// 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 your function takes a single argument, omit the parentheses. Otherwise, always include parentheses around arguments. eslint: arrow-parens

Why? Less visual clutter.
```
// bad
[1, 2, 3].map((x) => x * x);

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

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Classes

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.queue = [];

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

// good
class Queue {
    queue = [];

    constructor(contents = []) {
        this.queue = [...contents];
    }

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

9.2 Use static for declaring class-wide properties and methods.

// bad
class Queue {
    constructor() {
        Queue.instanceCount++;
    }
}

Queue.instanceCount = 0;

Queue.getInstanceCount = function() {
    return this.instanceCount;
}

// good
class Queue {
    static instanceCount = 0;

    constructor() {
        Queue.instanceCount++;
    }

    static getInstanceCount() {
        return this.instanceCount;
    }
}

9.3 Use extends for inheritance.

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

// bad
function PeekableQueue(contents) {
    Queue.apply(this, contents);
}
PeekableQueue.prototype = new Queue();
PeekableQueue.prototype.constructor = PeekableQueue;

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

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

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

class Jedi {
    constructor(options = {}) {
        this.name = options.name || 'no name';
    }

    getName() {
        return this.name;
    }

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

9.5 Avoid duplicate class members. eslint: [no-dupe-class-members] (http://eslint.org/docs/rules/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; }
}
```

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Modules

10.1 Create a separate module for each logical set of functionality in your application. Avoid grouping multiple areas of concern or the whole application into a single file.

10.2 Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system.

Why? Modules are the future, let's start using the future now.
```
// bad
const NerderyStyleGuide = require('./NerderyStyleGuide');

// good
import NerderyStyleGuide from './NerderyStyleGuide';
```

10.3 In modules with a single export, prefer default export over named export. eslint: import/prefer-default-export
```
// bad
export function foo() {}

// good
export default function foo() {}
```

10.4 Do not use wildcard imports.

Why? This makes sure you have a single default export.

// bad
import * as NerderyStyleGuide from './NerderyStyleGuide';

// good
import NerderyStyleGuide from './NerderyStyleGuide';

10.5 Self-host third-party libraries whenever possible. Avoid loading third-party scripts from external domains and CDN's. Exceptions are libraries that do not provide self-hosted versions, such as Google Maps or Analytics.

Why? Doing so exposes users to additional attack vectors, privacy violations (IP address tracking) and additional downtime risks.
```
// bad
import $ from 'https://ajax.googleapis.com/ajax/libs/jquery/2.2.0/jquery.min.js';

// good
import $ from './vendor/jquery.min.js';
```

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Iterators and Generators

11.1 Avoid using iterators and for-of loops. Prefer JavaScript's higher-order functions instead of loops like for-in or for-of.

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.

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

// bad
let sum = 0;
for (let num of numbers) {
  sum += num;
}

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

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

// 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 Don't use generators for now.

Why? They don't transpile well to ES5.

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Properties

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

const luke = {
    jedi: true,
    age: 28,
};

// bad
const isJedi = luke['jedi'];

// good
const isJedi = luke.jedi;

12.2 Use bracket notation [] when accessing properties with a variable.

const luke = {
    jedi: true,
    age: 28,
};

const prop = 'jedi';
const isJedi = luke[prop];

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Comparison

13.1 Use === and !== over == and !=. eslint: eqeqeq

// bad
if (dragonball == 'z') {
    //...
}

// good
if (dragonball === 'z') {
    //...
}

13.2 The one allowable exception is null checks. Use == and != to compare against null.

Why? The == checks for both null and undefined in a single expression.
```
// bad
if (dragonball === null || dragonball === undefined) {
    //...
}

// good
if (dragonball == null) {
    //...
}
```

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

Why? JavaScript will try to coerce strings and numbers into a boolean value, which could lead to unintended results. Be more descriptive about what you want to compare. For more information see Truth Equality and JavaScript by Angus Croll.
```
// bad
if (isValid === true) {
    // ...
}

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

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

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

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

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

13.4 Use braces to create blocks in case and default clauses that 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 (number) {
    case 1:
        const x = 1;
        break;
    case 2:
        const y = 2;
        break;
    default:
        const z = 3;
}

// good
switch (number) {
    case 1: {
        const x = 1;
        break;
    }
    case 2: {
        const y = 2;
        break;
    }
    default: {
        const z = 3;
    }
}
```

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

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

// good
const maybeNull = value1 > value2 ? 'baz' : null;

const foo = maybe1 > maybe2 ? 'bar' : maybeNull;

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Blocks

14.1 Use braces with all multi-line blocks. eslint: curly

// bad
if (test)
    return false;

// good
if (test) {
    return false;
}

14.2 If you're using multi-line blocks with if and else, put else on the same line as your if block's closing brace. eslint: brace-style
```
// bad
if (test) {
    thing1();
}
else {
    thing2();
}

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

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Comments

15.1 Use /** ... */ for multi-line comments. Include types for all parameters and return values. eslint: valid-jsdoc

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

// good
/**
 * make() returns a new element
 * based on the passed in tag name
 *
 * @method make
 * @public
 * @param {String} tag
 * @return {Element} element
 */
make(tag) {
    // ...
    return element;
}

15.2 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
getType() {
    console.log('fetching type...');
    // set the default type to 'no type'
    const type = this._type || 'no type';

    return type;
}

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

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

    return type;
}

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

    return type;
}

15.3 Prefix any comments that are meant to be revisited later with FIXME or TODO.

Why? 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.

15.4 Use // FIXME: to annotate problems.

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

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

15.5 Use // TODO: to annotate refactoring recommendations.

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

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

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Whitespace

16.1 Use soft tabs set to 4 spaces. eslint: indent

// bad
test() {
∙∙console.log('test');
}

// good
test() {
∙∙∙∙console.log('test');
}

16.2 Place 1 space before the leading brace. eslint: space-before-blocks

// bad
test(){
    console.log('test');
}

// good
test() {
    console.log('test');
}

// bad
dog.set('attr',{
    age: '1 year',
    breed: 'Bernese Mountain Dog',
});

// good
dog.set('attr', {
    age: '1 year',
    breed: 'Bernese Mountain Dog',
});

16.3 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, space-before-function-paren
```
// bad
if(isJedi) {
    fight ();
}

// good
if (isJedi) {
    fight();
}

// bad
fight () {
    console.log ('Swooosh!');
}

// good
fight() {
    console.log('Swooosh!');
}
```

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

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

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

// bad
export default class TacoTuesday {
  // ..
}

// bad
export default class TacoTuesday {
    // ..
}↵
↵

// good
export default class TacoTuesday {
    // ..
}↵

16.6 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

// 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();

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

16.7 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;

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

// bad
bar() {

    console.log(foo);

}

// also bad
if (baz) {

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

}

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

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

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

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

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

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

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

16.10 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]);

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

// bad
const foo = {clark: 'kent'};

// good
const foo = { clark: 'kent' };

16.12 Functions with multiline signatures, or invocations, should be indented with each item on a line by itself, with a trailing comma on the last item.

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

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

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

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

16.13 Avoid having lines of code that are longer than 100 characters (including whitespace). eslint: max-len

Why? This ensures readability and maintainability.

// bad
$.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));

// good
const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. ' +
  'Whatever wizard constrains a helpful ally. The counterpart ascends!';

// good
$.ajax({
    method: 'POST',
    url: 'https://airbnb.com/',
    data: { name: 'John' },
})
    .done(() => console.log('Congratulations!'))
    .fail(() => console.log('You have failed this city.'));

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Commas

17.1 Use trailing commas for multi-line arrays and objects. eslint: comma-style comma-dangle

Why? This leads to cleaner git diffs.

// bad
const heroes = [
    'Batman'
  , 'Superman'
];

// bad
const heroes = [
    'Batman',
    'Superman'
];

// good
const heroes = [
    'Batman',
    'Superman',
];

// bad
const hero = {
    firstName: 'Ada'
  , lastName: 'Lovelace'
};

// bad
const hero = {
    firstName: 'Ada',
    lastName: 'Lovelace'
};

// good
const hero = {
    firstName: 'Ada',
    lastName: 'Lovelace',
};

17.2 No trailing commas for single-line arrays and objects. eslint: comma-dangle

// bad
const heroes = ['Batman', 'Superman',];

// good
const heroes = ['Batman', 'Superman'];

// bad
const hero = { firstName: 'Ada', lastName: 'Lovelace', };

// good
const hero = { firstName: 'Ada', lastName: 'Lovelace' };

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Semicolons

18.1 Yup. eslint: semi

// bad
const name = 'Skywalker'
return name

// good
const name = 'Skywalker';
return name;

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Naming Conventions

19.1 Method names should be verbs, properties should be nouns.

// bad
class Plane {
    flew = 0;
    powering = 4;

    airborne() {
        // ...
    }

    grounded() {
        // ...
    }
}  

// good
class Plane {
    altitude = 0;
    engineCount = 4;

    takeoff() {
        // ...
    }

    land() {
        // ...
    }
}

19.2 If the property/method is a boolean, prefix with is, has, are, should.

// bad
if (visible)
    // ...
}

// good
if (isVisible)
    // ...
}

// bad
if (hero.superPower())
    // ...
}

// good
if (hero.hasSuperPower())
    // ...
}

19.3 Be descriptive with your naming. Avoid single letter names unless it corresponds with the problem domain, such as i for indexes or x/y/z for coordinates.
```
// bad
q() {
    // ...
}

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

// bad
return { a: 0, b: 0, c: 0 };

// good
return { x: 0, y: 0, z: 0 };
```

19.4 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() {}

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

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

const bad = new user({
    name: 'nope',
});

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

const good = new User({
    name: 'yup',
});

19.6 Use a leading underscore _ when naming private or protected properties.

// bad
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';

// good
this._firstName = 'Panda';

19.7 Don't save references to this. Use arrow functions or Function#bind.

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

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

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

19.8 If your file exports a single class, your filename should be exactly the name of the class.

// file contents
class CheckBox {
    // ...
}
export default CheckBox;

// in some other file
// bad
import CheckBox from './checkBox';

// bad
import CheckBox from './check_box';

// good
import CheckBox from './CheckBox';

19.9 Constant values should be in all capitals and underscore-separated.
```
const MAXIMUM_POWER = 9000;
```

19.10 Group related constants in an object. All properties should be named using the same convention for constants.

// bad
const SELECTOR_ACTIVE = '.isActive';
const SELECTOR_DISABLED = '.isDisabled';
const SELECTOR_MODAL_CLOSE = '.js-modal-close';
 
// good
const SELECTORS = {
    ACTIVE: '.isActive',
    DISABLED: '.isDisabled',
    MODAL_CLOSE: '.js-modal-close',
 };

19.11 If the values you set for constants are arbitrary and don't add meaning, use Symbol() instead.

// bad
const NAVIGATION = {
    HOME: 'home',
    ABOUT: 'about',
    CONTACT: 'contact',
}

// good
const NAVIGATION = {
    HOME: Symbol(),
    ABOUT: Symbol(),
    CONTACT: Symbol(),
}

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Accessors

20.1 When getting and setting properties, use get and set accessor methods.

//bad
class Dragon {
    _age = 0;

    getAge() {
        return this._age;
    }

    setAge(value) {
        this._age = value;
    }
}

const dragon = new Dragon();
dragon.setAge(25);
console.log(dragon.getAge()); // 25

prefer:

//good
class Dragon {
    _age = 0;

    get age() {
        return this._age;
    }

    set age(value) {
        this._age = value;
    }
}

const dragon = new Dragon();
dragon.age = 25;
console.log(dragon.age); // 25

20.2 Getter methods should not exhibit side effects.

//bad
get age() {
    if (this.isBirthday()) {
        this._age++;
    }

    return this._age;
}

//good
get age() {
    return this._age;
}

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Events

21.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass 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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DOM Interaction

22.1 Prefix jQuery object variables with a $.

// bad
const body = $(document.body);

// good
const $body = $(document.body);

22.2 DOM elements to be selected by JavaScript should be prefixed with js-. These selectors should not be related to any CSS styles and must exist solely for the accessing of those DOM elements.

// bad
<a href="#" class="previousButton">Previous</a>

const $previousButton = $('.previousButton');

// good
<a href="#" class="previousButton js-previousButton">Previous</a>

const $previousButton = $('.js-previousButton');

22.3 If the behavior of a DOM element is tied to a JavaScript class, the selector should be named the same as that JavaScript class.

// bad
<div class="js-carousel"></div>

new CarouselView($('.js-carousel'));

// good
<div class="js-CarouselView"></div>

new CarouselView($('.js-CarouselView'));

22.4 Cache jQuery lookups.

// bad
$('.sidebar').hide();
$('.sidebar').css({
    'background-color': 'pink'
});

// good
const $sidebar = $('.sidebar');
$sidebar.hide();
$sidebar.css({
    'background-color': 'pink'
});

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Asynchronous Operations

23.1 When performing an asynchronous operation, wrap that operation with a Promise. Use a Promise implementation that conforms with the Promises/A+ spec.

waitFor(milliseconds) {
    return new Promise((resolve, reject) => {
        window.setTimeout(
            () => { resolve(); },
            milliseconds
        );
    });
}

waitFor(1000)
    .then(() => { console.log('Done waiting!'); });

23.2 Avoid nesting promises several layers deep. Instead, compose a sequence of promises using a flat chain. Better yet, use the await syntax.

// bad
waitFor(1000)
    .then(() => {
        waitFor(2000)
            .then(() => {
                waitFor(3000)
                    .then(() => {
                        console.log('Done waiting!');
                    })
            })
    });

// good
waitFor(1000)
    .then(() => {
        return waitFor(2000);
    })
    .then(() => {
        return waitFor(3000);
    })
    .then(() => {
        console.log('Done waiting!');
    });

// best
async function mySequence() {
    await waitFor(1000);
    await waitFor(2000);
    await waitFor(3000);
    console.log('Done waiting!');
}

23.3 Always add a catch() handler to promise chains.

Why? In some browsers, if code inside of a promise executes and generates a runtime error, it will silently fail and never be reported to the console.
```
waitFor(1000)
    .then(() => { console.log('Done waiting!'); })
    .catch(exception => { console.error('Error in waitFor():', exception); });
```

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Deployment

24.1 All JavaScript deployed to a production environment must be minified and combined. Test your minified/combined code early, as it may behave differently or exhibit unforeseen JavaScript errors.

License

(The MIT License)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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Awesome

Nerdery JavaScript Standards() {

Table of Contents

Types

Variables

Objects

Arrays

Destructuring

Strings

Functions

Arrow Functions

Classes

Modules

Iterators and Generators

Properties

Comparison

Blocks

Comments

Whitespace

Commas

Semicolons

Naming Conventions

Accessors

Events

DOM Interaction

Asynchronous Operations

Deployment

License

};