The constructor property returns a reference to the Object constructor function that created the instance object. 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".


All objects (with the exception of objects created with Object.create(null)) will have a constructor property. Objects created without the explicit use of a constructor function (such as object- and array-literals) will have a constructor property that points to the Fundamental Object constructor type for that object.

let o = {}
o.constructor === Object // true

let o = new Object
o.constructor === Object // true

let a = []
a.constructor === Array // true

let a = new Array
a.constructor === Array // true

let n = new Number(3)
n.constructor === Number // true


Displaying the constructor of an object

The following example creates a constructor (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

let 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

Changing the constructor of an object

The following example shows how to modify the 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 () {}

let types = [
  new Array(),
  new Boolean(),
  true,             // remains unchanged
  new Date(),
  new Error(),
  new Function(),
  function () {},
  new Number(),
  1,                // remains unchanged
  new Object(),
  new RegExp(),
  new String(),
  'test'            // remains unchanged

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


This example displays the following output (comments added for reference):

function Type() {},false,                                     // new Array()
function Type() {},false,                                     // []
function Type() {},false,false                                // new Boolean()
function Boolean() {
    [native code]
},false,true                                                  // true
function Type() {},false,Mon Sep 01 2014 16:03:49 GMT+0600    // new Date()
function Type() {},false,Error                                // new Error()
function Type() {},false,function anonymous() {

}                                                             // new Function()
function Type() {},false,function () {}                       // function () {}
function Type() {},false,[object Math]                        // Math
function Type() {},false,0                                    // new Number()
function Number() {
    [native code]
},false,1                                                     // 1
function Type() {},false,[object Object]                      // new Object()
function Type() {},false,[object Object]                      // {}
function Type() {},false,/(?:)/                               // new Regexp()
function Type() {},false,/(?:)/                               // /(?:)/
function Type() {},false,                                     // new String()
function String() {
    [native code]
},false,test                                                  // 'test'

Changing the constructor of a function

Mostly this property is used for defining a function as a function-constructor with further calling it with new and prototype-inherits chain.

function Parent() { /* ... */ }
Parent.prototype.parentMethod = function parentMethod() {}

function Child() {
   Parent.call(this) // Make sure everything is initialized properly
Child.prototype = Object.create(Parent.prototype) // re-define child prototype to Parent prototype

Child.prototype.constructor = Child // return original constructor to Child

But when do we need to perform the last line here? Unfortunately, the answer is: it depends.

Let's try to define the cases in which re-assignment of the original constructor will play a major role, and when it will be one superfluous line of code.

Take the following case: the object has the create() method to create itself.

function Parent() { /* ... */ }
function CreatedConstructor() {

CreatedConstructor.prototype = Object.create(Parent.prototype)

CreatedConstructor.prototype.create = function create() {
  return new this.constructor()

new CreatedConstructor().create().create() // TypeError undefined is not a function since constructor === Parent

In the example above the exception will be shown since the constructor links to Parent.

To avoid this, just assign the necessary constructor you are going to use.

function Parent() { /* ... */ }
function CreatedConstructor() { /* ... */ }

CreatedConstructor.prototype = Object.create(Parent.prototype)
CreatedConstructor.prototype.constructor = CreatedConstructor // sets the correct constructor for future use

CreatedConstructor.prototype.create = function create() {
  return new this.constructor()

new CreatedConstructor().create().create() // it's pretty fine

Ok, now it's pretty clear why changing the constructor can be useful.

Let's consider one more case.

function ParentWithStatic() {}

ParentWithStatic.startPosition = { x: 0, y:0 } // Static member property
ParentWithStatic.getStartPosition = function getStartPosition() {
  return this.startPosition

function Child(x, y) {
  this.position = {
    x: x,
    y: y

Child = Object.assign(ParentWithStatic)
Child.prototype = Object.create(ParentWithStatic.prototype)
Child.prototype.constructor = Child

Child.prototype.getOffsetByInitialPosition = function getOffsetByInitialPosition() {
  let position = this.position
  let startPosition = this.constructor.getStartPosition() // error undefined is not a function, since the constructor is Child

  return {
    offsetX: startPosition.x - position.x,
    offsetY: startPosition.y - position.y

For this example we need either to stay parent constructor to continue to work properly or reassign static properties to child's constructor:

Child = Object.assign(ParentWithStatic) // Notice that we assign it before we create(...) a prototype below
Child.prototype = Object.create(ParentWithStatic.prototype)

or assign parent constructor identifier to a separate property on the Child constructor function and access it via that property:

Child.parentConstructor = ParentWithStatic
Child.prototype = Object.create(ParentWithStatic.prototype)
   let startPosition = this.constructor.parentConstructor.getStartPosition()

Summary: Manually updating or setting the constructor can lead to differrent and sometimes confusing consequences. To prevent this, just define the role of constructor in each specific case. In most cases, constructor is not used and reassignment of it is not necessary.


ECMAScript (ECMA-262)
The definition of 'Object.prototype.constructor' in that specification.

Browser compatibility

ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
constructorChrome Full support 1Edge Full support 12Firefox Full support 1IE Full support 4Opera Full support 4Safari Full support 1WebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 10.1Safari iOS Full support 1Samsung Internet Android Full support 1.0nodejs Full support Yes


Full support
Full support

See also