TypeScript icon, indicating that this package has built-in type declarations

0.5.1-0 • Public • Published

is Build Status

Type check values: is.string('🦄') //=> true


$ npm install @sindresorhus/is


const is = require('@sindresorhus/is');

//=> 'string'

is(new Map());
//=> 'Map'

//=> true



Returns the type of value.

Primitives are lowercase and object types are camelcase.


  • 'undefined'
  • 'null'
  • 'string'
  • 'symbol'
  • 'Array'
  • 'Function'
  • 'Object'

Note: It will throw if you try to feed it object-wrapped primitives, as that's a bad practice. For example new String('foo').


All the below methods accept a value and returns a boolean for whether the value is of the desired type.



Built-in types


Keep in mind that functions are objects too.


Returns true for any object with a .then() and .catch() method. Prefer this one over .nativePromise() as you usually want to allow userland promise implementations too.


Returns true for any object that implements its own .next() and .throw() methods and has a function definition for Symbol.iterator.


Returns true for any async function that can be called with the await operator.

is.asyncFunction(async () => {});
// => true

is.asyncFunction(() => {});
// => false

Typed arrays


Structured data




Returns true for all values that evaluate to true in a boolean context:

//=> true

//=> false

Returns true if value is one of: false, 0, '', null, undefined, NaN.


JavaScript primitives are as follows: null, undefined, string, number, boolean, symbol.


Returns true if value is a safe integer.


An object is plain if it's created by either {}, new Object(), or Object.create(null).


Returns true for instances created by a ES2015 class.


A value is array-like if it is not a function and has a value.length that is a safe integer greater than or equal to 0.

//=> true

function () {
    //=> true
.inRange(value, range)

Check if value (number) is in the given range. The range is an array of two values, lower bound and upper bound, in no specific order.

is.inRange(3, [0, 5]);
is.inRange(3, [5, 0]);
is.inRange(0, [-2, 2]);
.inRange(value, upperBound)

Check if value (number) is in the range of 0 to upperBound.

is.inRange(3, 10);

Returns true if value is a DOM Element.


Check if value is Infinity or -Infinity.


Returns true if value is an even integer.


Returns true if value is an odd integer.


Returns true if value is falsy or an empty string, array, object, map, or set.


Returns true if is.empty(value) or a string that is all whitespace.

.any(predicate, ...values)

Returns true if any of the input values returns true in the predicate:

is.any(is.string, {}, true, '🦄');
//=> true

is.any(is.boolean, 'unicorns', [], new Map());
//=> false
.all(predicate, ...values)

Returns true if all of the input values returns true in the predicate:

is.all(is.object, {}, new Map(), new Set());
//=> true

is.all(is.string, '🦄', [], 'unicorns');
//=> false


Why yet another type checking module?

There are hundreds of type checking modules on npm, unfortunately, I couldn't find any that fit my needs:

  • Includes both type methods and ability to get the type
  • Types of primitives returned as lowercase and object types as camelcase
  • Covers all built-ins
  • Unsurprising behavior
  • Well-maintained
  • Comprehensive test suite

For the ones I found, pick 3 of these.

The most common mistakes I noticed in these modules was using instanceof for type checking, forgetting that functions are objects, and omitting symbol as a primitive.


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