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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.

Description

The following examples show some uses of the in operator.

// Arrays
var 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
var 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.

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

var 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.

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

var 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.

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

Inherited properties

The in operator returns true for properties in the prototype chain.

'toString' in {}; // returns true

Specifications

Specification Status Comment
ECMAScript Latest Draft (ECMA-262)
The definition of 'Relational Operators' in that specification.
Living Standard  
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Relational Operators' in that specification.
Standard  
ECMAScript 5.1 (ECMA-262)
The definition of 'The in Operator' in that specification.
Standard  
ECMAScript 3rd Edition (ECMA-262)
The definition of 'The in Operator' in that specification.
Standard Initial definition. Implemented in JavaScript 1.4.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic Support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
FeatureAndroidChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic Support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

See also

Document Tags and Contributors

 Last updated by: fscholz,