104 lines
2.0 KiB
Markdown
104 lines
2.0 KiB
Markdown
# @babel/plugin-transform-block-scoping
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> Compile ES2015 block scoping (const and let) to ES5
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## Examples
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**In**
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```javascript
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{
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let a = 3
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}
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let a = 3
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```
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**Out**
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```javascript
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{
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var _a = 3;
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}
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var a = 3;
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```
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## Constant checks
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This plugin also validates all `const` variables.
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Reassignment of constants is a runtime error and it will insert the necessary error code for those.
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## Installation
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```sh
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npm install --save-dev @babel/plugin-transform-block-scoping
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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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Without options:
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```json
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{
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"plugins": ["@babel/plugin-transform-block-scoping"]
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}
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```
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With options:
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```json
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{
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"plugins": [
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["@babel/plugin-transform-block-scoping", {
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"throwIfClosureRequired": 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 @babel/plugin-transform-block-scoping 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-block-scoping"]
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});
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```
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## Options
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### `throwIfClosureRequired`
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`boolean`, defaults to `false`.
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In cases such as the following it's impossible to rewrite let/const without adding an additional function and closure while transforming:
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```javascript
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for (let i = 0; i < 5; i++) {
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setTimeout(() => console.log(i), 1);
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}
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```
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In extremely performance-sensitive code, this can be undesirable. If `"throwIfClosureRequired": true` is set, Babel throws when transforming these patterns instead of automatically adding an additional function.
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### `tdz`
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`boolean`, defaults to `false`.
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By default this plugin will ignore the *temporal dead zone (TDZ)* for block-scoped variables. The following code will **not throw an error when transpiled with Babel, which is not spec compliant**:
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```javascript
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i
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let i;
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```
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If you need these errors you can tell Babel to try and find them by setting `"tdz": true` for this plugin. However, the current implementation might not get all edge cases right and its best to just avoid code like this in the first place.
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