The in operator

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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.

// 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.

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 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.