babel-plugin-transform-es2015-block-scoping

What is babel-plugin-transform-es2015-block-scoping?

The babel-plugin-transform-es2015-block-scoping package is a Babel plugin that transforms ES2015 block scoping (let and const) to ES5. This is useful for ensuring compatibility with older JavaScript environments that do not support ES2015 features.

What are babel-plugin-transform-es2015-block-scoping's main functionalities?

Transform let to var

This feature transforms ES2015 let declarations to ES5 var declarations, ensuring compatibility with older JavaScript environments.

const babel = require('@babel/core');
const code = 'let x = 1;';
const output = babel.transform(code, { plugins: ['babel-plugin-transform-es2015-block-scoping'] });
console.log(output.code); // var x = 1;

Transform const to var

This feature transforms ES2015 const declarations to ES5 var declarations, ensuring compatibility with older JavaScript environments.

const babel = require('@babel/core');
const code = 'const y = 2;';
const output = babel.transform(code, { plugins: ['babel-plugin-transform-es2015-block-scoping'] });
console.log(output.code); // var y = 2;

Block scoping with loops

This feature ensures that block-scoped variables within loops are transformed to function-scoped variables, maintaining the correct scoping behavior in older JavaScript environments.

const babel = require('@babel/core');
const code = 'for (let i = 0; i < 5; i++) { console.log(i); }';
const output = babel.transform(code, { plugins: ['babel-plugin-transform-es2015-block-scoping'] });
console.log(output.code); // for (var i = 0; i < 5; i++) { console.log(i); }

Other packages similar to babel-plugin-transform-es2015-block-scoping

babel-plugin-transform-es2015-parameters

The babel-plugin-transform-es2015-parameters package transforms ES2015 parameter features (such as default parameters and rest parameters) to ES5. While it does not directly handle block scoping, it is often used in conjunction with block scoping transformations to ensure full ES2015 compatibility.

babel-plugin-transform-es2015-destructuring

The babel-plugin-transform-es2015-destructuring package transforms ES2015 destructuring assignments to ES5. This plugin is complementary to block scoping transformations, as destructuring is often used in conjunction with let and const declarations.

README

babel-plugin-transform-es2015-block-scoping

Compile ES2015 block scoping (const and let) to ES5

Installation

npm install --save-dev babel-plugin-transform-es2015-block-scoping

Usage

Via .babelrc (Recommended)

.babelrc

Without options:

{
  "plugins": ["transform-es2015-block-scoping"]
}

With options:

{
  "plugins": [
    ["transform-es2015-block-scoping", {
      "throwIfClosureRequired": true
    }]
  ]
}

Via CLI

babel --plugins transform-es2015-block-scoping script.js

Via Node API

require("babel-core").transform("code", {
  plugins: ["transform-es2015-block-scoping"]
});

Options throwIfClosureRequired

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.