in operator

The in operator returns true if the specified property is in the specified object.

Syntax

JavaScript
prop in objectName

Parameters

prop
A string or symbol representing a property name or array index (non-symbols will be coerced to strings).
objectName
Name of an object.

Description

The following examples show some uses of the in operator.

JavaScript
// 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 ES6+)

// 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.

JavaScript
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.

JavaScript
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.

JavaScript
var mycar = {make: "Honda", model: "Accord", year: 1998};
mycar.make = undefined;
"make" in mycar;  // returns true
JavaScript
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.

JavaScript
"toString" in {}; // returns true

Specifications

Specification Status Comment
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Relational Operators' in that specification.
Draft  
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

Feature Chrome Firefox (Gecko) Edge Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

See also

License

© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-us/docs/web/javascript/reference/operators/in

JavaScript Operator operator Relational Operators