Awesome

check-types.js

A little JavaScript library for asserting types and values.

Why would I want that?
How little is it?
How do I install it?
How do I use it?
- Loading the library
- Calling the exported functions
Are there TypeScript definitions?
Where can I use it?
What changed from 6.x to 7.x?
What changed from 5.x to 6.x?
What changed from 4.x to 5.x?
What changed from 3.x to 4.x?
What changed from 2.x to 3.x?
What changed from 1.x to 2.x?
What changed from 0.x to 1.x?
How do I set up the build environment?
What license is it released under?

Why would I want that?

Writing explicit conditions in your functions to check arguments and throw exceptions is a task that swiftly becomes tiresome and adds complexity to your codebase.

The purpose of check-types.js is to remove this burden from JavaScript application developers in an efficient and robust manner, abstracted by a simple API.

How little is it?

21 kb unminified with comments, 6.1 kb minified, 2.2 kb minified + gzipped.

How do I install it?

Via npm:

npm i check-types --save

Or if you just want the git repo:

git clone git@github.com:philbooth/check-types.js.git

If you're into other package managers, it is also available from Bower, Component and Jam.

How do I use it?

Loading the library

If you are running in Node.js, Browserify or another CommonJS-style environment, you can require check-types like so:

var check = require('check-types');

It also the supports the AMD-style format preferred by Require.js.

If you are including check-types.js with an HTML <script> tag, or neither of the above environments are detected, it will export the interface globally as check.

Calling the exported functions

Once you've loaded the library in your application, a whole bunch of functions are available to call.

Most of the functions are predicates, which can be executed in a number of different contexts:

check.xxx(thing): These functions are basic predicates, returning true or false depending on the type and value of thing.
check.not.xxx(thing): The not modifier negates predicates, returning true if the predicate returns false and false if the predicate returns true. It is also itself a function, which simply returns the negation of its argument.
check.maybe.xxx(thing): The maybe modifier tweaks predicates to return true if thing is null or undefined, otherwise their normal result is returned. It is also itself a function, which returns true when its argument is null or undefined, otherwise it returns its argument.
check.assert.xxx(thing, message): The assert modifier changes predicates to throw when their result is false, otherwise it returns thing. It can be applied to the not and maybe modifiers using the forms check.assert.not.xxx(thing, message) and check.assert.maybe.xxx(thing, message). It is also itself a function, which simply throws when its argument is false.
check.array.of.xxx(thing): The array.of modifier first checks that it is operating on an array and then applies the modified predicate to each item of the array. If the predicate fails for any item, it returns false, otherwise it returns true. It can also be prefixed by other modifiers, so check.maybe.array.of, check.not.array.of, check.assert.array.of, check.assert.maybe.array.of and check.assert.not.array.of all work as you would expect them to.
check.arrayLike.of.xxx(thing): The arrayLike.of modifier is synonymous with array.of, except it operates on array-like objects.
check.iterable.of.xxx(thing): The iterable.of modifier is synonymous with array.of, except it operates on iterables.
check.object.of.xxx(thing): The object.of modifier is synonymous with array.of, except it operates on an object's properties.

Additionally, there are some batch operations that allow you to apply different predicates to each value in an array or object. These are implemented by check.apply, check.map, check.any and check.all.

General predicates

check.equal(thing, thang): Returns true if thing === thang, false otherwise.
check.null(thing): Returns true if thing is null, false otherwise.
check.undefined(thing): Returns true if thing is undefined, false otherwise.
check.assigned(thing): Returns true if thing is not null or undefined, false otherwise.
check.primitive(thing): Returns true if thing is a primitive type, false otherwise. Primitive types are null, undefined, booleans, numbers, strings and symbols.
check.hasLength(thing, value): Returns true if thing has a length property that equals value, false otherwise.

String predicates

check.string(thing): Returns true if thing is a string, false otherwise.
check.emptyString(thing): Returns true if thing is the empty string, false otherwise.
check.nonEmptyString(thing): Returns true if thing is a non-empty string, false otherwise.
check.contains(thing, substring): Returns true if thing is a string that contains substring, false otherwise.
check.match(thing, regex): Returns true if thing is a string that matches regex, false otherwise.

Number predicates

check.number(thing): Returns true if thing is a number, false otherwise. Note that NaN, Number.POSITIVE_INFINITY and Number.NEGATIVE_INFINITY are not considered numbers here.
check.integer(thing): Returns true if thing is an integer, false otherwise.
check.zero(thing): Returns true if thing is zero, false otherwise.
check.infinity(thing): Returns true if thing is positive or negative infinity, false otherwise.
check.greater(thing, value): Returns true if thing is a number greater than value, false otherwise.
check.greaterOrEqual(thing, value): Returns true if thing is a number greater than or equal to value, false otherwise.
check.less(thing, value): Returns true if thing is a number less than value, false otherwise.
check.lessOrEqual(thing, value): Returns true if thing is a number less than or equal to value, false otherwise.
check.between(thing, a, b): Returns true if thing is a number between a and b (excluding a and b), false otherwise. The arguments a and b may be in any order, it doesn't matter which is greater.
check.inRange(thing, a, b): Returns true if thing is a number in the range a .. b (including a and b), false otherwise. The arguments a and b may be in any order, it doesn't matter which is greater.
check.positive(thing): Returns true if thing is a number greater than zero, false otherwise.
check.negative(thing): Returns true if thing is a number less than zero, false otherwise.
check.odd(thing): Returns true if thing is an odd number, false otherwise.
check.even(thing): Returns true if thing is an even number, false otherwise.

Boolean predicates

check.boolean(thing): Returns true if thing is a boolean, false otherwise.

Object predicates

check.object(thing): Returns true if thing is a plain-old JavaScript object, false otherwise.
check.emptyObject(thing): Returns true if thing is an empty object, false otherwise.
check.nonEmptyObject(thing): Returns true if thing is a non-empty object, false otherwise.
check.instanceStrict(thing, prototype): Returns true if thing is an instance of prototype, false otherwise.
check.instance(thing, prototype): Returns true if thing is an instance of prototype, false otherwise. Falls back to testing constructor.name and Object.prototype.toString if the instanceof test fails.
check.like(thing, duck): Duck-typing checker. Returns true if thing has all of the properties of duck, false otherwise.

Array predicates

check.array(thing): Returns true if thing is an array, false otherwise.
check.emptyArray(thing): Returns true if thing is an empty array, false otherwise.
check.nonEmptyArray(thing): Returns true if thing is a non-empty array, false otherwise.
check.arrayLike(thing): Returns true if thing has a numeric length property, false otherwise.
check.iterable(thing): Returns true if thing implements the iterable protocol, false otherwise. In pre-ES6 environments, this predicate falls back to arrayLike behaviour.
check.includes(thing, value): Returns true if thing includes value, false otherwise.

Date predicates

check.date(thing): Returns true if thing is a valid date, false otherwise.

Function predicates

check.function(thing): Returns true if thing is a function, false otherwise.

Modifiers

check.not(value): Returns the negation of value.
check.not.xxx(...): Returns the negation of the predicate.
check.maybe(value): Returns true if value is null or undefined, otherwise it returns value.
check.maybe.xxx(...): Returns true if thing is null or undefined, otherwise it propagates the return value from its predicate.
check.array.of.xxx(value): Returns true if value is an array and the predicate is true for every item. Also works with the not and maybe modifiers.
check.arrayLike.of.xxx(thing): The arrayLike.of modifier is synonymous with array.of, except it operates on array-like objects.
check.iterable.of.xxx(thing): The iterable.of modifier is synonymous with array.of, except it operates on iterables.
check.object.of.xxx(thing): The object.of modifier is synonymous with array.of, except it operates on an object's properties.
check.assert(value, message, ErrorType): Throws a TypeError if value is falsy, otherwise it returns value. message and ErrorType are optional arguments that control the message and type of the thrown error object.
check.assert.xxx(...): Throws a TypeError if the predicate returns false, otherwise it returns the subject value. The last two arguments are an optional message and error type that control the message and type of the thrown error object. Also works with the not, maybe and ...of modifiers.

Batch operations

check.apply(things, predicates): Applies each value from the things array to the corresponding predicate and returns the array of results. Passing a single predicate instead of an array applies all of the values to the same predicate.
check.map(things, predicates): Maps each value from the things object to the corresponding predicate and returns an object containing the results. Supports nested objects. Passing a single predicate instead of an object applies all of the values to the same predicate, ignoring nested objects.
check.all(results): Returns true if all the result values are true in an array (returned from apply) or object (returned from map).
check.any(predicateResults): Returns true if any result value is true in an array (returned from apply) or object (returned from map).

Some examples

check.even(3);
// Returns false

check.not.even(3);
// Returns true

check.maybe.even(null);
// Returns true

check.assert.like({ foo: 'bar' }, { baz: 'qux' });
// Throws `new TypeError('Invalid type')`

check.assert.not.like({ foo: 'bar' }, { baz: 'qux' });
// Doesn't throw, returns `{ foo: 'bar' }`

check.assert.maybe.like(undefined, { foo: 'bar' });
// Doesn't throw, returns `undefined`

check.assert(myFunction(), 'Something went wrong', CustomError);
// Throws `new CustomError('Something went wrong')` if myFunction returns `false`

check.apply([ 'foo', 'bar', '' ], check.nonEmptyString);
// Returns [ true, true, false ]

check.map({
    foo: 2,
    bar: { baz: 'qux' }
}, {
    foo: check.odd,
    bar: { baz: check.nonEmptyString }
});
// Returns { foo: false, bar: { baz: true } }

check.all(
    check.map(
        { foo: 0, bar: '' },
        { foo: check.number, bar: check.string }
    )
);
// Returns true

check.any(
    check.apply(
        [ 1, 2, 3, '' ],
        check.string
    )
);
// Returns true

Are there TypeScript definitions?

Yes!

Thanks to @idchlife, type definitions were added to DefinitelyTyped. You can add them to your project via npm:

npm i @types/check-types --save-dev

Where can I use it?

As of version 2.0, this library no longer supports ES3. That means you can't use it in IE 7 or 8.

Everywhere else should be fine.

If those versions of IE are important to you, worry not! The 1.x versions all support old IE and any future 1.x versions will adhere to that too.

See the releases for more information.

What changed from 6.x to 7.x?

Breaking changes were made to the API in version 7.0.0.

Specifically, the instance predicate was renamed to instanceStrict and the builtIn and userDefined predicates were combined to form a new instance predicate.