The check-types npm package provides a collection of functions for asserting types and values in JavaScript. It is designed to help developers enforce type safety and data integrity in their applications by offering a simple and intuitive API for type checking and validation.

What are check-types's main functionalities?

Primitive Type Checking

This feature allows you to check if a value is of a specific primitive type, such as a number, string, or boolean. The code sample demonstrates how to use the package to verify that a value is a number, a string, and a boolean.

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

console.log(check.number(42)); // true
console.log(check.string('hello')); // true
console.log(check.boolean(false)); // true

Object and Array Checking

With this feature, you can check if a value is an array or an object. The code sample shows how to validate that a value is an array and an object.

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

console.log(check.array([1, 2, 3])); // true
console.log(check.object({ key: 'value' })); // true

Function and Null Checking

This feature enables you to verify if a value is a function or null. The provided code sample illustrates how to check for a function and a null value.

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

console.log(check.function(function() {})); // true
console.log(check.null(null)); // true

Other packages similar to check-types

check-types.js

A little JavaScript library for asserting types and values, with zero dependencies.

Why would I want that?

Writing explicit conditions in your functions to check arguments and throw errors 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?

25 kb unminified with comments, 7.4 kb minified, 2.5 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@gitlab.com:philbooth/check-types.js.git

How do I use it?

Loading the library

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

const 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 to help you apply predicates to each value of an array or object. These are implemented by 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, options): Returns true if thing is the empty string, false otherwise.
check.nonEmptyString(thing, options): Returns true if thing is a non-empty string, false otherwise.
check.contains(string, substring): Returns true if string contains substring, false otherwise.
check.in(substring, string): Returns true if substring is in string, false otherwise.
check.match(string, regex): Returns true if string 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.float(thing): Returns true if thing is a non-integer number, false otherwise.
check.zero(thing): Returns true if thing is zero, false otherwise.
check.one(thing): Returns true if thing is one, 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.thenable(thing): Returns true if thing has a then method, 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.contains(object, value): Returns true if object contains value, false otherwise.
check.in(value, object): Returns true if value is in object, false otherwise.
check.containsKey(object, key): Returns true if object contains key key, false otherwise.
check.keyIn(key, object): Returns true if key key is in object, false otherwise.
check.like(thing, duck): Duck-typing checker. Returns true if thing has all of the properties of duck, false otherwise.
check.identical(thing, thang): Deep equality checker. Returns true if thing has all of the same values as thang, 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.contains(array, value): Returns true if array contains value, false otherwise.
check.in(value, array): Returns true if value is in array, 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.
check.throws(() => thing()): Returns true if thing is a function that throws, 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.map(things, predicates): Maps each value from the things array or object to the corresponding predicate and returns the array or object of results. Passing a single predicate instead of an array or object maps all of the values to the same predicate.
check.all(results): Returns true if all the result values are true in an array or object returned by map.
check.any(results): Returns true if any result value is true in an array or object returned by map.

Some examples

check.even(3);
// Returns false

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

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

check.assert.even(3);
// Throws `new TypeError('assert failed: expected 3 to be even number')`

check.assert.not.even(3);
// Doesn't throw

check.assert.maybe.even(null);
// Doesn't throw

check.contains('foo', 'oo')
// Returns true

check.contains('foe', 'oo')
// Returns false

check.contains(['foo', 'bar'], 'bar')
// Returns true

check.contains(['foo', 'bar'], 'ar')
// Returns false

check.like({ foo: 'bar' }, { foo: 'baz' });
// Returns true

check.like({ foo: 'bar' }, { baz: 'qux' });
// Returns false

check.array.of.nonEmptyString([ 'foo', 'bar' ]);
// Returns true

check.array.of.nonEmptyString([ 'foo', 'bar', '' ]);
// Returns false

check.array.of.inRange([ 0, 1, 2 ], 0, 2);
// Returns true

check.array.of.inRange([ 0, 1, 2 ], 0, 1);
// Returns false

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

check.map([ '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.map(
        [ 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 10.x to 11.x?

Breaking changes were made to the API in version 11.0.0.

Specifically, the options argument was removed from the emptyString and nonEmptyString predicates because it caused problematic behaviour in the assert modifier. Callers who were previously using options.trim with these predicates should call check.match instead:

check.match(stringWithSpaces, /^\s*$/);