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JavaScript: The Definitive Guide

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JavaScript Reference

Object.constructor Property


Object.constructor Property---an object's constructor function


Navigator 3.0




The constructor property of any object is a read-only 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, then a.constructor will be 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 a Document object:

function isDocument(x) { 
    return ((typeof x == "object") && (x.constructor == "Document"));
Note, however, that this technique is not possible with all object types. In Navigator 3.0 there is no Window() constructor, for example, and Window objects have their constructor property set to Object.

See Also

"Object", Chapter 7, Objects

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