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@babel/plugin-transform-block-scoping

NOTE: This plugin is included in @babel/preset-env

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​

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

Usage​

Without options:

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

With options:

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

Via CLI​

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

Via Node API​

JavaScript
require("@babel/core").transformSync("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.

You can read more about configuring plugin options here