The "constructor" property

[todo make me more interesting] Every function by default already has the "prototype" property.

It's an object of this form:

function Rabbit() {}

Rabbit.prototype = {
  constructor: Rabbit
};

Here, Rabbit.prototype is assigned manually, but the same object is its value by default.

We can check it:

function Rabbit() {}
// Rabbit.prototype = { constructor: Rabbit }

alert( Rabbit.prototype.constructor == Rabbit ); // true

Here's the picture:

Naturally, the "constructor" property becomes available to all rabbits through the [[Prototype]]:

function Rabbit() {}

let rabbit = new Rabbit();

alert(rabbit.constructor == Rabbit); // true

We can use it to create a new object using the same constructor as the existing one:

function Rabbit(name) {
  this.name = name;
  alert(name);
}

let rabbit = new Rabbit("White Rabbit");

let rabbit2 = new rabbit.constructor("Black Rabbit");

That may come in handy when we have an object, but don't know which constructor was used for it (e.g. it comes from a 3rd party library), and we need to create the same.

Probably the most important thing about "constructor" is that...

JavaScript itself does not use the "constructor" property at all.

Yes, it exists in the default "prototype" for functions, but that's literally all about it. No language function relies on it and nothing controls its validity.

It is created automatically, but what happens with it later -- is totally on us.

In particular, if we assign our own Rabbit.prototype = { jumps: true }, then there will be no "constructor" in it any more.

Such assignment won't break native methods or syntax, because nothing in the language uses the "constructor" property. But we may want to keep "constructor" for convenience or just in case, by adding properties to the default "prototype" instead of overwriting it as a whole:

function Rabbit() {}

// Not overwrite Rabbit.prototype totally
// just add to it
Rabbit.prototype.jumps = true
// the default Rabbit.prototype.constructor is preserved

Or, alternatively, recreate it manually:

Rabbit.prototype = {
  jumps: true,
*!*
  constructor: Rabbit
*/!*
};

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The "constructor" property

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