@babel/plugin-transform-block-scoping

Compile ES2015 block scoping (const and let) to ES5

Examples

In

{
  let a = 3
}

let a = 3

Out

{
  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

npm install --save-dev @babel/plugin-transform-block-scoping

Usage

Via .babelrc (Recommended)

.babelrc

Without options:

{
  "plugins": ["@babel/plugin-transform-block-scoping"]
}

With options:

{
  "plugins": [
    ["@babel/plugin-transform-block-scoping", {
      "throwIfClosureRequired": true
    }]
  ]
}

Via CLI

babel --plugins @babel/plugin-transform-block-scoping script.js

Via Node API

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:

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:

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.