82 lines
2.6 KiB
Markdown
82 lines
2.6 KiB
Markdown
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# Extending built-in classes
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Built-in classes like Array, Map and others are extendable also.
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For instance, here `PowerArray` inherits from the native `Array`:
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```js run
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// add one more method to it (can do more)
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class PowerArray extends Array {
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isEmpty() {
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return this.length === 0;
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}
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}
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let arr = new PowerArray(1, 2, 5, 10, 50);
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alert(arr.isEmpty()); // false
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let filteredArr = arr.filter(item => item >= 10);
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alert(filteredArr); // 10, 50
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alert(filteredArr.isEmpty()); // false
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```
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Please note a very interesting thing. Built-in methods like `filter`, `map` and others -- return new objects of exactly the inherited type. They rely on the `constructor` property to do so.
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In the example above,
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```js
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arr.constructor === PowerArray
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```
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So when `arr.filter()` is called, it internally creates the new array of results exactly as `new PowerArray`.
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That's actually very cool, because we can keep using `PowerArray` methods further on the result.
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Even more, we can customize that behavior.
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There's a special static getter `Symbol.species`, if exists, it returns the constructor to use in such cases.
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If we'd like built-in methods like `map`, `filter` will return regular arrays, we can return `Array` in `Symbol.species`, like here:
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```js run
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class PowerArray extends Array {
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isEmpty() {
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return this.length === 0;
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}
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*!*
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// built-in methods will use this as the constructor
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static get [Symbol.species]() {
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return Array;
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}
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*/!*
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}
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let arr = new PowerArray(1, 2, 5, 10, 50);
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alert(arr.isEmpty()); // false
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// filter creates new array using arr.constructor[Symbol.species] as constructor
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let filteredArr = arr.filter(item => item >= 10);
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*!*
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// filteredArr is not PowerArray, but Array
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*/!*
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alert(filteredArr.isEmpty()); // Error: filteredArr.isEmpty is not a function
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```
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As you can see, now `.filter` returns `Array`. So the extended functionality is not passed any further.
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## No static inheritance in built-ins
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Built-in objects have their own static methods, for instance `Object.keys`, `Array.isArray` etc.
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And we've already been talking about native classes extending each other: `Array.[[Prototype]] = Object`.
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But statics are an exception. Built-in classes don't inherit static properties from each other.
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In other words, the prototype of built-in constructor `Array` does not point to `Object`. This way `Array` and `Date` do not have `Array.keys` or `Date.keys`. And that feels natural.
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Here's the picture structure for `Date` and `Object`:
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Note, there's no link between `Date` and `Object`. Both `Object` and `Date` exist independently. `Date.prototype` inherits from `Object.prototype`, but that's all.
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