102 lines
3.2 KiB
Markdown
102 lines
3.2 KiB
Markdown
# babel-plugin-transform-class-constructor-call (deprecated)
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> Proposal Withdrawn: can be solved with decorators.
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This plugin allows Babel to transform class constructors.
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It basically allows to use the [new.target](http://mdn.io/new.target) feature on ES2015 classes:
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```js
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class Point {
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constructor(x, y) {
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this.x = x;
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this.y = y;
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}
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call constructor(x, y) {
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return new Point(x, y);
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}
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}
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let p1 = new Point(1, 2); // OK
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let p2 = Point(3, 4); // OK
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```
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[Try in REPL](http://babeljs.io/repl/#?evaluate=true&presets=es2015%2Cstage-0&code=class%20Point%20%7B%0A%0A%20%20constructor(x%2C%20y)%20%7B%0A%20%20%20%20this.x%20%3D%20x%3B%0A%20%20%20%20this.y%20%3D%20y%3B%0A%20%20%7D%0A%0A%20%20call%20constructor(x%2C%20y)%20%7B%0A%20%20%20%20return%20new%20Point(x%2C%20y)%3B%0A%20%20%7D%0A%0A%7D%0A%0Alet%20p1%20%3D%20new%20Point(1%2C%202)%3B%20%2F%2F%20OK%0Alet%20p2%20%3D%20Point(3%2C%204)%3B%20%2F%2F%20OK)
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## Example
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### Date example
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The javascript [Date](http://mdn.io/date) works this way:
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```js
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// You can get a Date instance using the new keyword
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let now = new Date();
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console.log(now.getMonth()); // Prints '3'
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console.log(now.toString()); // Prints 'Mon Apr 11 2016 13:26:07 GMT+0100 (BST)'
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// You can get a string of the current date using Date as a function:
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let nowStr = Date();
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console.log(nowStr); // Prints 'Mon Apr 11 2016 13:26:07 GMT+0100 (BST)'
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```
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It is currently possible to implement something like that using [new.target](http://mdn.io/new.target) (see [example in proposal](https://github.com/tc39/ecma262/blob/master/workingdocs/callconstructor.md#motivating-example)) and this new feature makes it available for ES2015 classes.
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A date implementation could be:
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```js
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class Date {
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constructor() {
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// ...
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}
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call constructor() {
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let date = new Date();
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return date.toString();
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}
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}
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let now = new Date(); // Get a Date instance
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let nowStr = Date(); // Use the 'call constructor()' part to get a string value of the current date
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```
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[Try in REPL](http://babeljs.io/repl/#?evaluate=true&presets=es2015%2Cstage-0&code=class%20Date%20%7B%0A%20%20constructor()%20%7B%0A%20%20%20%20%2F%2F%20...%0A%20%20%7D%0A%0A%20%20call%20constructor()%20%7B%0A%20%20%20%20let%20date%20%3D%20new%20Date()%3B%0A%20%20%20%20return%20date.toString()%3B%0A%20%20%7D%0A%7D%0A%0Alet%20now%20%3D%20new%20Date()%3B%20%2F%2F%20Get%20a%20Date%20instance%0Alet%20nowStr%20%3D%20Date()%3B%20%2F%2F%20Use%20the%20'call%20constructor()'%20part%20to%20get%20a%20string%20value%20of%20the%20current%20date)
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## Installation
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```sh
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npm install --save-dev babel-plugin-transform-class-constructor-call
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```
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## Usage
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### Via `.babelrc` (Recommended)
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**.babelrc**
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```json
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{
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"plugins": ["transform-class-constructor-call"]
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}
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```
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### Via CLI
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```sh
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babel --plugins transform-class-constructor-call script.js
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```
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### Via Node API
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```javascript
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require("babel-core").transform("code", {
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plugins: ["transform-class-constructor-call"]
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});
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```
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## References
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* [Inactive Proposals](https://github.com/tc39/proposals/blob/master/inactive-proposals.md)
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* [Proposal: Call Constructor](https://github.com/tc39/ecma262/blob/master/workingdocs/callconstructor.md)
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* [Blog post: ECMAScript proposal: function-callable classes](http://www.2ality.com/2015/10/call-constructor-esprop.html)
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