in operator

The in operator returns true if the specified property is in the specified object or its prototype chain.

Syntax

prop in object

Parameters

prop
A string or symbol representing a property name or array index (non-symbols will be coerced to strings).
object
Object to check if it (or its prototype chain) contains the property with specified name (prop).

Examples

Basic usage

The following examples show some uses of the in operator.

// Arrays
let trees = ['redwood', 'bay', 'cedar', 'oak', 'maple']
0 in trees        // returns true
3 in trees        // returns true
6 in trees        // returns false
'bay' in trees    // returns false (you must specify the index number, not the value at that index)
'length' in trees // returns true (length is an Array property)
Symbol.iterator in trees // returns true (arrays are iterable, works only in ES2015+)

// Predefined objects
'PI' in Math          // returns true

// Custom objects
let mycar = {make: 'Honda', model: 'Accord', year: 1998}
'make' in mycar  // returns true
'model' in mycar // returns true

You must specify an object on the right side of the in operator. For example, you can specify a string created with the String constructor, but you cannot specify a string literal.

let color1 = new String('green')
'length' in color1 // returns true

let color2 = 'coral'
// generates an error (color2 is not a String object)
'length' in color2

Using in with deleted or undefined properties

If you delete a property with the delete operator, the in operator returns false for that property.

let mycar = {make: 'Honda', model: 'Accord', year: 1998}
delete mycar.make
'make' in mycar   // returns false

let trees = new Array('redwood', 'bay', 'cedar', 'oak', 'maple')
delete trees[3]
3 in trees  // returns false

If you set a property to undefined but do not delete it, the in operator returns true for that property.

let mycar = {make: 'Honda', model: 'Accord', year: 1998}
mycar.make = undefined
'make' in mycar   // returns true
let trees = new Array('redwood', 'bay', 'cedar', 'oak', 'maple')
trees[3] = undefined
3 in trees  // returns true

The in operator will return false for empty array slots. Even if accessing it directly returns undefined

let empties = new Array(3)
empties[2] // returns undefined
2 in empties  // returns false
To avoid this, make sure a new array is always filled with non-empty values or not write to indexes past the end of array
let empties = new Array(3).fill(undefined)
2 in empties  // returns true

Inherited properties

The in operator returns true for properties in the prototype chain. (If you want to check for only non-inherited properties, use Object.prototype.hasOwnProperty() instead.)

'toString' in {}  // returns true

Specifications

Specification
ECMAScript (ECMA-262)
The definition of 'Relational Operators' in that specification.

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
inChrome Full support 1Edge Full support 12Firefox Full support 1IE Full support 5.5Opera Full support 4Safari Full support 1WebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 10.1Safari iOS Full support 1Samsung Internet Android Full support 1.0nodejs Full support 0.1.100

Legend

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Full support

See also