Object.prototype.constructor

Summary

Returns a reference to the Object function that created the instance's prototype. Note that the value of this property is a reference to the function itself, not a string containing the function's name. The value is only read-only for primitive values such as 1, true and "test".

Description

All objects inherit a constructor property from their prototype:

var o = {};
o.constructor === Object; // true

var a = [];
a.constructor === Array; // true

var n = new Number(3);
n.constructor === Number; // true

Examples

Example: Displaying the constructor of an object

The following example creates a prototype, Tree, and an object of that type, theTree. The example then displays the constructor property for the object theTree.

function Tree (name) {
   this.name = name;
}

var theTree = new Tree( "Redwood" );
console.log( "theTree.constructor is " + theTree.constructor );

This example displays the following output:

theTree.constructor is function Tree (name) {
    this.name = name;
}

Example: Changing the constructor of an object

The following example shows how to modify constructor value of generic objects. Only true, 1 and "test" will not be affected as they have read-only native constructors. This example shows that it is not always safe to rely on the constructor property of an object.

function Type () {}

var types = [
    new Array(),
    [],
    new Boolean(),
    true,             // remains unchanged
    new Date(),
    new Error(),
    new Function(),
    function () {},
    Math,	
    new Number(),
    1,                // remains unchanged
    new Object(),
    {},
    new RegExp(),
    /(?:)/,
    new String(),
    "test"            // remains unchanged
];

for( var i = 0; i < types.length; i++ ) {
    types[i].constructor = Type;
    types[i] = [ types[i].constructor, types[i] instanceof Type, types[i].toString() ];
}

console.log( types.join( "\n" ) );

Specifications

Browser compatibility