112 lines
2.0 KiB
Markdown
112 lines
2.0 KiB
Markdown
# babel-plugin-transform-es2015-for-of
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> Compile ES2015 for...of to ES5
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## Installation
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```sh
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npm install --save-dev babel-plugin-transform-es2015-for-of
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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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```js
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// without options
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{
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"plugins": ["transform-es2015-for-of"]
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}
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// with options
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{
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"plugins": [
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["transform-es2015-for-of", {
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"loose": true
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}]
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]
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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-es2015-for-of 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-es2015-for-of"]
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});
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```
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## Options `loose`
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#### Abrupt completions
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In loose mode an iterators `return` method will not be called on abrupt completions caused by thrown errors.
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Please see [google/traceur-compiler#1773](https://github.com/google/traceur-compiler/issues/1773) and
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[babel/babel#838](https://github.com/babel/babel/issues/838) for more information.
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#### Arrays
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Under loose mode the `forOf` transformer will output more verbose iteration code.
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For example the following:
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```javascript
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for (var i of foo) {}
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```
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is normally output as:
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```javascript
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for (var _iterator = foo[Symbol.iterator](), _step; !(_step = _iterator.next()).done;) {
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var i = _step.value;
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}
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```
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Under loose mode however it's output as:
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```javascript
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for (var _iterator = foo, _isArray = Array.isArray(_iterator), _i = 0, _iterator = _isArray ? _iterator : _iterator[Symbol.iterator]();;) {
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var i;
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if (_isArray) {
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if (_i >= _iterator.length) break;
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i = _iterator[_i++];
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} else {
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_i = _iterator.next();
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if (_i.done) break;
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i = _i.value;
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}
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}
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```
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The result is that arrays are put in a fast path, heavily increasing performance.
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All other iterables will continue to work fine but array iteration will be
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significantly faster.
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### Optimization
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If a basic array is used, Babel will compile the for-of loop down to a regular for loop.
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**In**
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```js
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for (let a of [1,2,3]) {}
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```
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**Out**
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```js
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var _arr = [1, 2, 3];
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for (var _i = 0; _i < _arr.length; _i++) {
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var a = _arr[_i];
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}
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```
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