The in operator

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

Syntax

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.

// Arraysvar trees = ["redwood", "bay", "cedar", "oak", "maple"];0 in trees        // returns true3 in trees        // returns true6 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 objectsvar 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 truevar 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 falsevar 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

SpecificationStatusCommentECMAScript 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.StandardInitial definition. Implemented in JavaScript 1.4.

Browser compatibility

• Desktop 
• Mobile

FeatureChromeFirefox (Gecko)EdgeInternet ExplorerOperaSafariBasic support(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)

See also

• delete
• Object.prototype.hasOwnProperty()
• Reflect.has()
• Enumerability and ownership of properties