89 lines
2.3 KiB
Markdown
89 lines
2.3 KiB
Markdown
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# The "constructor" property
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[todo make me more interesting]
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Every function by default already has the `"prototype"` property.
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It's an object of this form:
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```js
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function Rabbit() {}
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Rabbit.prototype = {
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constructor: Rabbit
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};
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```
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Here, `Rabbit.prototype` is assigned manually, but the same object is its value by default.
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We can check it:
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```js
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function Rabbit() {}
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// Rabbit.prototype = { constructor: Rabbit }
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alert( Rabbit.prototype.constructor == Rabbit ); // true
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```
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Here's the picture:
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Naturally, the `"constructor"` property becomes available to all rabbits through the `[[Prototype]]`:
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```js run
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function Rabbit() {}
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let rabbit = new Rabbit();
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alert(rabbit.constructor == Rabbit); // true
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```
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We can use it to create a new object using the same constructor as the existing one:
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```js run
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function Rabbit(name) {
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this.name = name;
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alert(name);
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}
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let rabbit = new Rabbit("White Rabbit");
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let rabbit2 = new rabbit.constructor("Black Rabbit");
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```
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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.
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Probably the most important thing about `"constructor"` is that...
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**JavaScript itself does not use the `"constructor"` property at all.**
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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.
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It is created automatically, but what happens with it later -- is totally on us.
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In particular, if we assign our own `Rabbit.prototype = { jumps: true }`, then there will be no `"constructor"` in it any more.
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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:
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```js
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function Rabbit() {}
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// Not overwrite Rabbit.prototype totally
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// just add to it
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Rabbit.prototype.jumps = true
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// the default Rabbit.prototype.constructor is preserved
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```
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Or, alternatively, recreate it manually:
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```js
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Rabbit.prototype = {
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jumps: true,
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*!*
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constructor: Rabbit
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*/!*
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};
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```
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