@babel/plugin-transform-block-scoping

What is @babel/plugin-transform-block-scoping?

The @babel/plugin-transform-block-scoping npm package is a Babel plugin that transforms JavaScript code to convert block scoping (let and const) to function scoping (var) where necessary. This is particularly useful for ensuring compatibility with older environments that do not support ES6 block-scoped variables.

What are @babel/plugin-transform-block-scoping's main functionalities?

Transform let and const to var

This feature transforms block-scoped variables (let and const) into function-scoped variables (var) to ensure compatibility with older JavaScript engines.

if (true) {
  let x = 'hello';
}
// Transformed to:
if (true) {
  var x = 'hello';
}

Block scoping for loops

This feature correctly scopes the loop variable within closures, preventing common errors when using setTimeout or similar asynchronous functions within loops.

for (let i = 0; i < 3; i++) {
  setTimeout(function() { console.log(i); }, 0);
}
// Transformed to:
for (var i = 0; i < 3; i++) {
  (function(i) {
    setTimeout(function() { console.log(i); }, 0);
  })(i);
}

Other packages similar to @babel/plugin-transform-block-scoping

babel-plugin-transform-es2015-block-scoping

This package is a predecessor to @babel/plugin-transform-block-scoping and offers similar functionality, transforming ES2015 block-scoped variables to ES5. It is now deprecated in favor of the @babel suite of plugins.

fast-async

While not a direct alternative, fast-async is a Babel plugin that transforms async/await and generators to ES5, including handling block scoping within async functions. It provides a different set of features but deals with modern JavaScript syntax transformation like @babel/plugin-transform-block-scoping.

babel-preset-env

babel-preset-env is a Babel preset that includes @babel/plugin-transform-block-scoping among other plugins. It automatically determines the Babel plugins and polyfills you need based on your supported environments. It's a more comprehensive solution that includes similar functionality.

README

@babel/plugin-transform-block-scoping

Compile ES2015 block scoping (const and let) to ES5

See our website @babel/plugin-transform-block-scoping for more information.

Install

Using npm:

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

or using yarn:

yarn add @babel/plugin-transform-block-scoping --dev

Changelog

v7.25.9 (2024-10-22)

:bug: Bug Fix

• babel-parser, babel-template, babel-types
  • #16905 fix: Keep type annotations in syntacticPlaceholders mode (@liuxingbaoyu)
• babel-helper-compilation-targets, babel-preset-env
  • #16907 fix: support BROWSERSLIST{,_CONFIG} env (@JLHwung)
• Other
  • #16884 Analyze ClassAccessorProperty to prevent the no-undef rule (@victorenator)

:house: Internal

• babel-helper-transform-fixture-test-runner
  • #16914 remove test options flaky (@JLHwung)

:running_woman: Performance

• babel-parser, babel-types
  • #16918 perf: Make VISITOR_KEYS etc. faster to access (@liuxingbaoyu)