Type check values:
is.string('๐ฆ') //=> true
$ npm install @sindresorhus/is
const is = require('@sindresorhus/is');
is('๐ฆ');
//=> 'string'
is(new Map());
//=> 'Map'
is.number(6);
//=> true
When using is
together with TypeScript, type guards are being used to infer the correct type inside if-else statements.
import is from '@sindresorhus/is';
const padLeft = (value: string, padding: string | number) => {
if (is.number(padding)) {
// `padding` is typed as `number`
return Array(padding + 1).join(' ') + value;
}
if (is.string(padding)) {
// `padding` is typed as `string`
return padding + value;
}
throw new TypeError(`Expected 'padding' to be of type 'string' or 'number', got '${is(padding)}'.`);
}
padLeft('๐ฆ', 3);
//=> ' ๐ฆ'
padLeft('๐ฆ', '๐');
//=> '๐๐ฆ'
Returns the type of value
.
Primitives are lowercase and object types are camelcase.
Example:
'undefined'
'null'
'string'
'symbol'
'Array'
'Function'
'Object'
Note: It will throw an error 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.
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
Returns true
for any bound
function.
is.boundFunction(() => {});
// => true
is.boundFunction(function () {}.bind(null));
// => true
is.boundFunction(function () {});
// => false
Returns true
if value
is a direct instance of class
.
is.directInstanceOf(new Error(), Error);
//=> true
class UnicornError extends Error {};
is.directInstanceOf(new UnicornError(), Error);
//=> false
Returns true
if value
is an instance of the URL
class.
const url = new URL('https://example.com');
is.urlInstance(url);
//=> true
Returns true
for all values that evaluate to true in a boolean context:
is.truthy('๐ฆ');
//=> true
is.truthy(undefined);
//=> 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.
is.arrayLike(document.forms);
//=> true
function () {
is.arrayLike(arguments);
//=> true
}
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]);
Check if value
(number) is in the range of 0
to upperBound
.
is.inRange(3, 10);
Returns true
if value
is a DOM Element.
Returns true
if value
is a Node.js stream.
const fs = require('fs');
is.nodeStream(fs.createReadStream('unicorn.png'));
//=> true
Returns true
if value
is an Observable
.
const {Observable} = require('rxjs');
is.observable(new Observable());
//=> true
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.
Returns true
if any of the input values
returns true in the predicate
:
is.any(is.string, {}, true, '๐ฆ');
//=> true
is.any(is.boolean, 'unicorns', [], new Map());
//=> false
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, '๐ฆ', [], 'unicorns');
//=> false
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.
- ow - Function argument validation for humans
- is-stream - Check if something is a Node.js stream
- is-observable - Check if a value is an Observable
- file-type - Detect the file type of a Buffer/Uint8Array
- is-ip - Check if a string is an IP address
- is-array-sorted - Check if an Array is sorted
- is-error-constructor - Check if a value is an error constructor
- is-empty-iterable - Check if an Iterable is empty
- is-blob - Check if a value is a Blob - File-like object of immutable, raw data
- has-emoji - Check whether a string has any emoji
MIT