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# @babel/plugin-transform-block-scoping
> Compile ES2015 block scoping (const and let) to ES5
## Examples
**In**
```javascript
{
let a = 3
}
let a = 3
```
**Out**
```javascript
{
var _a = 3;
}
var a = 3;
```
## Constant checks
This plugin also validates all `const` variables.
Reassignment of constants is a runtime error and it will insert the necessary error code for those.
## Installation
```sh
npm install --save-dev @babel/plugin-transform-block-scoping
```
## Usage
### Via `.babelrc` (Recommended)
**.babelrc**
Without options:
```json
{
"plugins": ["@babel/plugin-transform-block-scoping"]
}
```
With options:
```json
{
"plugins": [
["@babel/plugin-transform-block-scoping", {
"throwIfClosureRequired": true
}]
]
}
```
### Via CLI
```sh
babel --plugins @babel/plugin-transform-block-scoping script.js
```
### Via Node API
```javascript
require("@babel/core").transform("code", {
plugins: ["@babel/plugin-transform-block-scoping"]
});
```
## Options
### `throwIfClosureRequired`
`boolean`, defaults to `false`.
In cases such as the following it's impossible to rewrite let/const without adding an additional function and closure while transforming:
```javascript
for (let i = 0; i < 5; i++) {
setTimeout(() => console.log(i), 1);
}
```
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.
### `tdz`
`boolean`, defaults to `false`.
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**:
```javascript
i
let i;
```
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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