@babel/helper-hoist-variables

What is @babel/helper-hoist-variables?

The @babel/helper-hoist-variables package is a utility that is used within the Babel compiler to hoist variable declarations to the top of their respective scopes during the code transformation process. This is part of the Babel's internal mechanism to ensure that the transformed code adheres to the scoping rules of ECMAScript and to avoid issues with variable declarations in the transpiled output.

What are @babel/helper-hoist-variables's main functionalities?

Variable Hoisting

This feature allows the package to traverse a given AST (Abstract Syntax Tree) path and find all variable declarations. It then hoists these declarations to the top of the function or program scope. The 'emitOnScope' parameter is a callback that is called with an identifier for each variable that needs to be hoisted.

function hoistVariables(path, emitOnScope) {
  const names = Object.create(null);
  path.traverse({
    VariableDeclarator({ node }) {
      if (node.id.type === 'Identifier') {
        names[node.id.name] = true;
      }
    }
  });
  Object.keys(names).forEach(name => {
    emitOnScope.push({ id: t.identifier(name) });
  });
}

Other packages similar to @babel/helper-hoist-variables

babel-plugin-transform-es2015-block-scoping

This package is a Babel plugin that transforms ES2015 block scoping (let and const) into ES5-compatible code. It hoists declarations to the appropriate scope, similar to what @babel/helper-hoist-variables does, but as a standalone plugin for Babel.

babel-plugin-transform-strict-mode

This package introduces strict mode to the entire file. It doesn't directly hoist variables, but by enforcing strict mode, it can prevent variable hoisting issues by catching them early during the development process.

lebab

Lebab is a tool that transforms older JavaScript into newer JavaScript. It includes features for transforming var declarations into let/const, which involves understanding and potentially altering the scope of variables, somewhat similar to the hoisting behavior of @babel/helper-hoist-variables.

README

@babel/helper-hoist-variables

Installation

npm install @babel/helper-hoist-variables --save

API

declare export default hoistVariables(path: NodePath, emit: Function, kind: "var" | "let" = "var");

Usage

TODO