95 lines
2.0 KiB
Markdown
95 lines
2.0 KiB
Markdown
# @babel/plugin-transform-parameters
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> Compile ES2015 default and rest parameters to ES5
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This plugin transforms ES2015 parameters to ES5, this includes:
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- Destructuring parameters
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- Default parameters
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- Rest parameters
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## Examples
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**In**
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```javascript
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function test(x = "hello", { a, b }, ...args) {
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console.log(x, a, b, args);
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}
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```
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**Out**
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```javascript
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function test() {
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var x = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : "hello";
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var _ref = arguments[1];
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var a = _ref.a,
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b = _ref.b;
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for (var _len = arguments.length, args = Array(_len > 2 ? _len - 2 : 0), _key = 2; _key < _len; _key++) {
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args[_key - 2] = arguments[_key];
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}
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console.log(x, a, b, args);
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}
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```
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## Installation
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```sh
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npm install --save-dev @babel/plugin-transform-parameters
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```
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## Caveats
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Default parameters desugar into `let` declarations to retain proper semantics. If this is
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not supported in your environment then you'll need the
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[@babel/plugin-transform-block-scoping](http://babeljs.io/docs/plugins/transform-block-scoping) plugin.
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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": ["@babel/plugin-transform-parameters"]
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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 @babel/plugin-transform-parameters 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: ["@babel/plugin-transform-parameters"]
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});
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```
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## Options
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### `loose`
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`boolean`, defaults to `false`.
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In loose mode, parameters with default values will be counted into the arity of the function. This is not spec behavior where these parameters do not add to function arity.
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The `loose` implementation is a more performant solution as JavaScript engines will fully optimize a function that doesn't reference `arguments`. Please do your own benchmarking and determine if this option is the right fit for your application.
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```javascript
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// Spec behavior
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function bar1 (arg1 = 1) {}
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bar1.length // 0
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// Loose mode
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function bar1 (arg1 = 1) {}
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bar1.length // 1
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```
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