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


prop in objectName


A string or numeric expression representing a property name or array index.
Name of an object.


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

  Created by Mozilla Contributors, license: CC-BY-SA 2.5