JavaScript: The Definitive Guide, Fourth Edition

Name

Object.constructor — an object’s constructor function

Availability

JavaScript 1.1; JScript 2.0; ECMAScript v1

Synopsis

                  object.constructor

Description

The constructor property of any object is a reference to the function that was used as the constructor for that object. For example, if you create an array a with the Array( ) constructor, a.constructor is an Array:

a = new Array(1,2,3);   // Create an object
a.constructor == Array  // Evaluates to true

One common use of the constructor property is to determine the type of unknown objects. Given an unknown value, you can use the typeof operator to determine whether it is a primitive value or an object. If it is an object, you can use the constructor property to determine what type of object it is. For example, the following function determines whether a given value is an array:

function isArray(x) {
    return ((typeof x == "object") && (x.constructor == Array));
}

Note, however, that while this technique works for the objects built-in to core JavaScript, it is not guaranteed to work with “host objects” such as the Window object of client-side JavaScript. The default implementation of the Object.toString( ) method provides another way to determine the type of an unknown object.

See Also

Object.toString( )