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Babel Helper Create Class Features Plugin

A Babel helper plugin that provides essential functionality for transforming modern JavaScript class features to ES6-compatible code. It handles compilation of class public and private fields, private methods, and decorators, serving as a foundational component for other Babel plugins that need to transform class-related syntax.

Package Information

  • Package Name: @babel/helper-create-class-features-plugin
  • Package Type: npm
  • Language: TypeScript
  • Installation: npm install @babel/helper-create-class-features-plugin

Core Imports

import { createClassFeaturePlugin, FEATURES } from "@babel/helper-create-class-features-plugin";

For specific utilities:

import { 
  enableFeature, 
  injectInitialization, 
  buildCheckInRHS,
  buildNamedEvaluationVisitor 
} from "@babel/helper-create-class-features-plugin";

Basic Usage

import { createClassFeaturePlugin, FEATURES } from "@babel/helper-create-class-features-plugin";

// Create a plugin for class properties
export default createClassFeaturePlugin({
  name: "transform-class-properties",
  api,
  feature: FEATURES.fields,
  loose: options.loose,
  manipulateOptions(opts, parserOpts) {
    parserOpts.plugins.push("classProperties");
  },
});

// Create a plugin for private methods
export default createClassFeaturePlugin({
  name: "transform-private-methods", 
  api,
  feature: FEATURES.privateMethods | FEATURES.fields,
  loose: options.loose,
});

Architecture

This helper is built around several key components:

  • Plugin Factory: createClassFeaturePlugin function that generates configured Babel plugins
  • Feature System: Bit flag-based feature enablement with consistency checking across plugins
  • Transform Engine: AST transformation logic for class fields, private members, and decorators
  • Private Name Management: Comprehensive system for handling private field and method names
  • Decorator Support: Multiple decorator specification version support (2018-09, 2021-12, 2022-03, 2023-01, 2023-05, 2023-11)

Capabilities

Plugin Creation

Primary factory function for creating Babel plugins that transform class features with configurable options and feature flags.

function createClassFeaturePlugin(options: Options): PluginObject;

interface Options {
  name: string;
  feature: number;
  loose?: boolean;
  inherits?: PluginObject["inherits"];
  manipulateOptions?: PluginObject["manipulateOptions"];
  api?: PluginAPI;
  decoratorVersion?: DecoratorVersionKind | "2018-09";
}

Plugin Creation

Feature Management

Core feature enablement and configuration system that manages the bit flag-based feature system and ensures consistency across multiple class feature plugins.

const FEATURES: {
  readonly fields: number;        // 1 << 1 (2)
  readonly privateMethods: number; // 1 << 2 (4)  
  readonly decorators: number;    // 1 << 3 (8)
  readonly privateIn: number;     // 1 << 4 (16)
  readonly staticBlocks: number;  // 1 << 5 (32)
};

function enableFeature(file: File, feature: number, loose: boolean): void;
function isLoose(file: File, feature: number): boolean;
function shouldTransform(path: NodePath<t.Class>, file: File): boolean;

Feature Management

Field Transformation

Comprehensive private and public field transformation including private name mapping, field initialization, and usage transformation.

function buildPrivateNamesMap(
  className: string,
  privateFieldsAsSymbolsOrProperties: boolean,
  props: PropPath[],
  file: File
): PrivateNamesMap;

function buildFieldsInitNodes(
  ref: t.Identifier | null,
  superClass: t.Expression | null,
  props: PropPath[],
  privateNamesMap: PrivateNamesMap,
  file: File,
  setPublicClassFields: boolean,
  privateFieldsAsSymbolsOrProperties: boolean,
  noUninitializedPrivateFieldAccess: boolean,
  constantSuper: boolean,
  innerBinding: t.Identifier | null
): FieldsInitResult;

Field Transformation

Decorator Support

Multi-version decorator transformation supporting various decorator specifications with proper handling of decorator metadata and class modification.

function hasDecorators(node: t.Class): boolean;
function buildNamedEvaluationVisitor(): Visitor;

type DecoratorVersionKind = 
  | "2023-11" | "2023-05" | "2023-01" 
  | "2022-03" | "2021-12";

Decorator Support

Utilities

Helper utilities for class initialization, computed key extraction, and AST manipulation.

function injectInitialization(
  path: NodePath<t.Class>,
  constructor: NodePath<t.ClassMethod> | undefined,
  instanceNodes: t.ExpressionStatement[],
  referenceVisitor: (visitor: Visitor, state: any) => void,
  lastInstanceNodeReturnsThis: boolean
): void;

function buildCheckInRHS(
  rhs: t.Expression,
  file: File,
  inRHSIsObject?: boolean
): t.Expression;

Utilities

Types

Core Types

type PropNode = 
  | t.ClassProperty 
  | t.ClassPrivateProperty 
  | t.ClassPrivateMethod 
  | t.StaticBlock;

type PropPath = NodePath<PropNode>;

interface PrivateNameMetadata {
  id: t.Identifier;
  static: boolean;
  method: boolean;
  getId?: t.Identifier;
  setId?: t.Identifier;
  methodId?: t.Identifier;
  initAdded?: boolean;
  getterDeclared?: boolean;
  setterDeclared?: boolean;
}

type PrivateNamesMap = Map<string, PrivateNameMetadata>;