Your Search Results

    in operator

    Summary

    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 Language Specification 5.1th Edition (ECMA-262) Standard  
    ECMAScript Language Specification 6th Edition (ECMA-262) Draft  

    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)

    See also

    Document Tags and Contributors

    Last updated by: fscholz,
    Hide Sidebar