The in
operator returns true
if the specified property is in the specified object.
Syntax
prop in objectName
Parameters
prop
- A string or numeric expression representing a property name or array index.
objectName
- Name of an object.
Description
The following examples show some uses of the in
operator.
// Arrays var trees = new Array("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) // 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 1st Edition. | Standard | Initial definition. Implemented in JavaScript 1.4 |
ECMAScript 5.1 (ECMA-262) The definition of 'The in Operator' in that specification. |
Standard | |
ECMAScript 6 (ECMA-262) The definition of 'Relational Operators' in that specification. |
Release Candidate |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (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) |