Object.prototype.constructor
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"
.
Description
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
Examples
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
One can assign the constructor
property for any value except null
and undefined
since those don't have a corresponding constructor function (like String
, Number
, Boolean
etc.), but values which are primitives won't keep the change (with no exception thrown). This is due to the same mechanism, which allows one to set any property on primitive values (except null
and undefined
) with no effect. Namely whenever one uses such a primitive as an object an instance of the corresponding constructor is created and discarded right after the statement was executed.
let val = null;
val.constructor = 1; //TypeError: val is null
val = 'abc';
val.constructor = Number; //val.constructor === String
val.foo = 'bar'; //An implicit instance of String('abc') was created and assigned the prop foo
val.foo === undefined; //true, since a new instance of String('abc') was created for this comparison, which doesn't have the foo property
So basically one can change the value of the constructor
property for anything, except the primitives mentioned above, note that changing the constructor
property does not affect the instanceof operator:
let a = [];
a.constructor = String
a.constructor === String // true
a instanceof String //false
a instanceof Array //true
a = new Foo();
a.constructor = 'bar'
a.constructor === 'bar' // true
//etc.
If the object is sealed/frozen then the change has no effect and no exception is thrown:
let a = Object.seal({});
a.constructor = Number;
a.constructor === Object; //true
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() {
Parent.call(this)
}
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.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 to work properly we need either to keep Parent
as the constructor or reassign static properties to Child
's constructor:
...
Child = Object.assign(Child, ParentWithStatic); // Notice that we assign it before we create(...) a prototype below
Child.prototype = Object.create(ParentWithStatic.prototype);
...
or assign Parent
's 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()
...
Note: Manually updating or setting the constructor can lead to different 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.
Specifications
Specification |
---|
ECMAScript Language Specification # sec-object.prototype.constructor |
Browser compatibility
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See also
- Class declaration
- Class constructor
- Glossary: constructor