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tessl/npm-babel-plugin-transform-es2015-block-scoped-functions

Babel plugin to ensure function declarations at the block level are block scoped

Workspace: tessl
Visibility: Public
Created: 3 months ago
Last updated: 3 months ago
Describes: pkg:npm/babel-plugin-transform-es2015-block-scoped-functions@6.22.x

To install, run

npx @tessl/cli install tessl/npm-babel-plugin-transform-es2015-block-scoped-functions@6.22.0

babel-plugin-transform-es2015-block-scoped-functions

Babel plugin to ensure function declarations at the block level are block scoped. This plugin transforms ES2015 block-scoped function declarations to prevent hoisting issues and maintain proper block scope semantics by converting function declarations within block statements to let-scoped variable declarations with function expressions.

Package Information

Package Name: babel-plugin-transform-es2015-block-scoped-functions
Package Type: npm (Babel plugin)
Language: JavaScript
Installation: npm install --save-dev babel-plugin-transform-es2015-block-scoped-functions

Core Imports

The plugin exports a single default function that creates the Babel plugin configuration:

const plugin = require("babel-plugin-transform-es2015-block-scoped-functions");

For use in Babel configuration:

// Via require (CommonJS)
require("babel-core").transform("code", {
  plugins: ["transform-es2015-block-scoped-functions"]
});

Basic Usage

Via .babelrc (Recommended)

.babelrc

{
  "plugins": ["transform-es2015-block-scoped-functions"]
}

Via CLI

babel --plugins transform-es2015-block-scoped-functions script.js

Via Node API

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

Transformation Behavior

The plugin transforms function declarations within block statements to prevent ES5-style hoisting behavior:

Input:

{
  function foo() {
    return "block scoped";
  }
  
  if (condition) {
    function bar() {
      return "conditional";
    }
  }
}

Output:

{
  let foo = function () {
    return "block scoped";
  };
  
  if (condition) {
    let bar = function () {
      return "conditional";
    };
  }
}

Capabilities

Plugin Factory Function

The main export that creates the Babel plugin configuration.

/**
 * Creates a Babel plugin for transforming block-scoped function declarations
 * @param {Object} context - Babel plugin context
 * @param {Object} context.types - Babel types utility object (destructured as 't')
 * @returns {PluginConfig} Babel plugin configuration with visitor methods
 */
export default function ({ types: t }) {
  return {
    visitor: {
      BlockStatement(path) { /* transforms function declarations in block statements */ },
      SwitchCase(path) { /* transforms function declarations in switch cases */ }
    }
  };
}

Plugin Configuration Object

The object returned by the plugin factory function containing AST visitor methods.

interface PluginConfig {
  visitor: {
    /** Handles function declarations within block statements */
    BlockStatement(path: NodePath<BlockStatement>): void;
    /** Handles function declarations within switch case statements */
    SwitchCase(path: NodePath<SwitchCase>): void;
  };
}

Visitor Methods

BlockStatement Visitor

Transforms function declarations within block statements, excluding function bodies and export declarations.

/**
 * Processes BlockStatement nodes to transform function declarations
 * @param {NodePath} path - Babel AST path for the BlockStatement node
 */
function BlockStatement(path) {
  const { node, parent } = path;
  // Skip if parent is a function body or export declaration
  if (t.isFunction(parent, { body: node }) || t.isExportDeclaration(parent)) {
    return;
  }
  // Transform function declarations in the body
  statementList("body", path);
}

SwitchCase Visitor

Transforms function declarations within switch case consequent statements.

/**
 * Processes SwitchCase nodes to transform function declarations
 * @param {NodePath} path - Babel AST path for the SwitchCase node
 */
function SwitchCase(path) {
  // Transform function declarations in the consequent
  statementList("consequent", path);
}

Internal Implementation Details

Statement List Transformation

The internal helper function that performs the actual transformation:

/**
 * Internal helper that transforms function declarations in statement arrays
 * @param {string} key - Property name containing the statement array
 * @param {NodePath} path - Babel AST path object
 */
function statementList(key, path) {
  const paths = path.get(key);
  
  for (const path of paths) {
    const func = path.node;
    if (!path.isFunctionDeclaration()) continue;

    // Create let variable declaration with function expression
    const declar = t.variableDeclaration("let", [
      t.variableDeclarator(func.id, t.toExpression(func))
    ]);

    // Set hoisting priority
    declar._blockHoist = 2;
    
    // Remove original function name
    func.id = null;

    // Replace the function declaration
    path.replaceWith(declar);
  }
}

Types

Babel Context Types

interface BabelContext {
  /** Babel types utility object for AST manipulation */
  types: BabelTypes;
}

interface BabelTypes {
  /** Check if node is a function */
  isFunction(node: Node, opts?: Object): boolean;
  /** Check if node is an export declaration */
  isExportDeclaration(node: Node): boolean;
  /** Create a variable declaration node */
  variableDeclaration(kind: string, declarations: VariableDeclarator[]): VariableDeclaration;
  /** Create a variable declarator node */
  variableDeclarator(id: Identifier, init: Expression): VariableDeclarator;
  /** Convert a function declaration to function expression */
  toExpression(node: Node): Expression;
}

AST Node Path Types

interface NodePath<T = Node> {
  /** The AST node at this path */
  node: T;
  /** Parent node */
  parent: Node;
  /** Get child paths for a property */
  get(key: string): NodePath[];
  /** Check if node is a function declaration */
  isFunctionDeclaration(): boolean;
  /** Replace this node with another node */
  replaceWith(node: Node): void;
}

Dependencies

Runtime Dependencies

babel-runtime: ^6.22.0 - Babel runtime helpers for transformed code

Development Dependencies

babel-helper-plugin-test-runner: ^6.22.0 - Testing utilities for Babel plugins

Target AST Nodes

The plugin specifically processes these AST node types:

BlockStatement: Block statements containing function declarations
SwitchCase: Switch case statements with function declarations in consequent
FunctionDeclaration: The nodes being transformed to variable declarations

Block Scoping Rules

The plugin ensures proper ES2015 block scoping by:

Converting function declarations to let variable declarations
Preserving function behavior through function expressions
Setting appropriate hoisting priorities (_blockHoist = 2)
Maintaining block scope boundaries and preventing hoisting to function scope