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minor fixes

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Ilya Kantor 2020-06-15 14:49:46 +03:00
parent b52aa942a8
commit 42b0220680
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1-js/09-classes/02-class-inheritance/3-class-extend-object/rabbit-extends-object.svg
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<svg xmlns="http://www.w3.org/2000/svg" width="458" height="344" viewBox="0 0 458 344"><defs><style>@import url(https://fonts.googleapis.com/css?family=Open+Sans:bold,italic,bolditalic%7CPT+Mono);@font-face{font-family:&apos;PT Mono&apos;;font-weight:700;font-style:normal;src:local(&apos;PT MonoBold&apos;),url(/font/PTMonoBold.woff2) format(&apos;woff2&apos;),url(/font/PTMonoBold.woff) format(&apos;woff&apos;),url(/font/PTMonoBold.ttf) format(&apos;truetype&apos;)}</style></defs><g id="inheritance" fill="none" fill-rule="evenodd" stroke="none" stroke-width="1"><g id="rabbit-extends-object.svg"><path id="Rectangle-1" fill="#FFF9EB" stroke="#E8C48E" stroke-width="2" d="M228 61v68h178V61H228z"/><text id="call:-function-bind:" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="238" y="80">call: function</tspan> <tspan x="238" y="95">bind: function</tspan> <tspan x="238" y="110">...</tspan></text><path id="Rectangle-1-Copy" fill="#FFF9EB" stroke="#E8C48E" stroke-width="2" d="M228 303h178v28H228z"/><text id="Function.prototype" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="227" y="53">Function.prototype</tspan></text><path id="Rectangle-1-Copy-4" fill="#FFF9EB" stroke="#E8C48E" stroke-width="2" d="M228 202v28h178v-28H228z"/><text id="constructor" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="238" y="220">constructor</tspan></text><text id="Object" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="227" y="194">Object</tspan></text><text id="Rabbit" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="233" y="292">Rabbit</tspan></text><path id="Line" fill="#EE6B47" fill-rule="nonzero" d="M316.5 143.5l7 14h-6v28h-2v-28h-6l7-14z"/><text id="[[Prototype]]" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="326" y="167">[[Prototype]]</tspan></text><path id="Line-Copy-3" fill="#EE6B47" fill-rule="nonzero" d="M316.5 247.5l7 14h-6v28h-2v-28h-6l7-14z"/><text id="[[Prototype]]-Copy" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="326" y="271">[[Prototype]]</tspan></text><text id="constructor-copy" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="236" y="321">constructor</tspan></text><path id="Rectangle-1-Copy-3" fill="#FFF9EB" stroke="#E8C48E" stroke-width="2" d="M8 61h178v68H8z"/><text id="call:-function-bind:-copy" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="18" y="80">call: function</tspan> <tspan x="18" y="95">bind: function</tspan> <tspan x="18" y="110">...</tspan></text><path id="Rectangle-1-Copy-2" fill="#FFF9EB" stroke="#E8C48E" stroke-width="2" d="M8 201h178v28H8z"/><text id="Function.prototype-Copy" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="7" y="53">Function.prototype</tspan></text><text id="Rabbit-Copy" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="13" y="190">Rabbit</tspan></text><path id="Line-Copy-4" fill="#EE6B47" fill-rule="nonzero" d="M96.5 145.5l7 14h-6v28h-2v-28h-6l7-14z"/><text id="[[Prototype]]-Copy-2" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="106" y="169">[[Prototype]]</tspan></text><text id="constructor-copy-2" fill="#8A704D" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="16" y="219">constructor</tspan></text><text id="class-Rabbit" fill="#EE6B47" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="14" y="18">class Rabbit</tspan></text><text id="class-Rabbit-extends" fill="#EE6B47" font-family="PTMono-Regular, PT Mono" font-size="14" font-weight="normal"><tspan x="224" y="18">class Rabbit extends Object</tspan></text></g></g></svg>
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1-js/09-classes/02-class-inheritance/3-class-extend-object/solution.md
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First, let's see why the latter code doesn't work.
The reason becomes obvious if we try to run it. An inheriting class constructor must call `super()`. Otherwise `"this"` won't be "defined".
So here's the fix:
```js run
class Rabbit extends Object {
constructor(name) {
*!*
super(); // need to call the parent constructor when inheriting
*/!*
this.name = name;
}
}
let rabbit = new Rabbit("Rab");
alert( rabbit.hasOwnProperty('name') ); // true
```
But that's not all yet.
Even after the fix, there's still important difference in `"class Rabbit extends Object"` versus `class Rabbit`.
As we know, the "extends" syntax sets up two prototypes:
1. Between `"prototype"` of the constructor functions (for methods).
2. Between the constructor functions themselves (for static methods).
In our case, for `class Rabbit extends Object` it means:
```js run
class Rabbit extends Object {}
alert( Rabbit.prototype.__proto__ === Object.prototype ); // (1) true
alert( Rabbit.__proto__ === Object ); // (2) true
```
So `Rabbit` now provides access to static methods of `Object` via `Rabbit`, like this:
```js run
class Rabbit extends Object {}
*!*
// normally we call Object.getOwnPropertyNames
alert ( Rabbit.getOwnPropertyNames({a: 1, b: 2})); // a,b
*/!*
```
But if we don't have `extends Object`, then `Rabbit.__proto__` is not set to `Object`.
Here's the demo:
```js run
class Rabbit {}
alert( Rabbit.prototype.__proto__ === Object.prototype ); // (1) true
alert( Rabbit.__proto__ === Object ); // (2) false (!)
alert( Rabbit.__proto__ === Function.prototype ); // as any function by default
*!*
// error, no such function in Rabbit
alert ( Rabbit.getOwnPropertyNames({a: 1, b: 2})); // Error
*/!*
```
So `Rabbit` doesn't provide access to static methods of `Object` in that case.
By the way, `Function.prototype` has "generic" function methods, like `call`, `bind` etc. They are ultimately available in both cases, because for the built-in `Object` constructor, `Object.__proto__ === Function.prototype`.
Here's the picture:
![](rabbit-extends-object.svg)
So, to put it short, there are two differences:
| class Rabbit | class Rabbit extends Object |
|--------------|------------------------------|
| -- | needs to call `super()` in constructor |
| `Rabbit.__proto__ === Function.prototype` | `Rabbit.__proto__ === Object` |

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1-js/09-classes/02-class-inheritance/3-class-extend-object/task.md
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importance: 3
---
# Class extends Object?
As we know, all objects normally inherit from `Object.prototype` and get access to "generic" object methods like `hasOwnProperty` etc.
For instance:
```js run
class Rabbit {
constructor(name) {
this.name = name;
}
}
let rabbit = new Rabbit("Rab");
*!*
// hasOwnProperty method is from Object.prototype
alert( rabbit.hasOwnProperty('name') ); // true
*/!*
```
But if we spell it out explicitly like `"class Rabbit extends Object"`, then the result would be different from a simple `"class Rabbit"`?
What's the difference?
Here's an example of such code (it doesn't work -- why? fix it?):
```js
class Rabbit extends Object {
constructor(name) {
this.name = name;
}
}
let rabbit = new Rabbit("Rab");
alert( rabbit.hasOwnProperty('name') ); // Error
```