CODEWITHSUNDEEP
javascriptconstructorprototype
Jinghui Niu
Jinghui Niu

Reputation: 1140

Why does `.constructor` point to `object` in this case?

In the example below:

var A = function() {};
var a = new A();
var b = new A();
A.prototype.member1 = 10;

A.prototype = {}
var c = new A();

Now the a.constructor and b.constructor are both pointing to function A, which is expected. However, c.constructor is pointing to a void object.

What is the rationale behind this? Thanks.

Upvotes: 0

Views: 47

Answers (1)

RobG
RobG

Reputation: 147403

Now the a.constructor and b.constructor are both pointing to function A, which is expected. However, c.constructor is pointing to a void object. a and b both have the original A.prototype object as their internal [[Prototype]], which has a constructor property referencing A. So when you do:

What is the rationale behind this? Thanks.

a.constructor

it is the inherited constructor property that is used.

You then replace the original prototype object with a new, "empty" object that doesn't have a constructor property. Replacing the constructor's prototype does not change the internal [[Prototype]] of existing instances, they still reference the object that was the prototype at the time they were constructed.

The new object will only be used as the [[Prototype]] for new instances, so when you construct c, it's internal [[Prototype]] is this new object that has no constructor property, so it continues along the [[Prototype]] chain until a constructor property is found on Object.prototoype, which references the Object constructor.

var A = function() {};
var a = new A();
var b = new A();

A.prototype.member1 = 10;

// Replace A.prototype
A.prototype = {}
var c = new A();

// a still uses original prototype, so inherits member1
console.log(a.member1); // 10

// c uses the plain object, so doesn't have member1 property
console.log(c.member1); // undefined

// c inherits constructor property from Object.prototype, so
// references Object constructor
console.log(c.constructor === Object); // true

Upvotes: 1

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