Metro Babel Transformer

const { transform } = require("metro-babel-transformer");

import { transform, BabelTransformerArgs, BabelTransformerOptions } from "metro-babel-transformer";
import type { File as BabelNodeFile } from "@babel/types";

const { transform } = require("metro-babel-transformer");

// Transform JavaScript source code
const result = transform({
  filename: "example.js",
  src: `
    const greeting = (name) => {
      return \`Hello, \${name}!\`;
    };
    export default greeting;
  `,
  options: {
    dev: true,
    enableBabelRuntime: false,
    enableBabelRCLookup: false,
    globalPrefix: "__metro__",
    hot: false,
    inlineRequires: undefined, // Reserved for future use
    minify: false,
    platform: null,
    publicPath: "/assets",
    projectRoot: "/path/to/project",
  },
  plugins: [
    // Optional Babel plugins array
    ["@babel/plugin-transform-modules-commonjs", {}],
  ],
});

console.log("Transformed AST:", result.ast);
console.log("Metadata:", result.metadata);

/**
 * Transform source code using Babel with configurable options
 * @param args - Transformation arguments containing source, options, and plugins
 * @returns Object with transformed AST and metadata
 */
function transform(args: BabelTransformerArgs): {
  ast: BabelNodeFile;
  functionMap?: BabelFileFunctionMapMetadata; // Deprecated - will be removed in future release. Function maps are now generated by Babel plugins and included in metadata.
  metadata?: MetroBabelFileMetadata;
};

interface BabelTransformerArgs {
  /** Path to the source file being transformed */
  readonly filename: string;
  /** Transformer configuration options */
  readonly options: BabelTransformerOptions;
  /** Optional array of Babel plugins to apply */
  readonly plugins?: Array<string | [string, any] | BabelPluginInstance>;
  /** Source code string to transform */
  readonly src: string;
}

interface BabelTransformerOptions {
  /** Custom transform options passed to plugins */
  readonly customTransformOptions?: CustomTransformOptions;
  /** Development mode flag - affects BABEL_ENV and optimizations */
  readonly dev: boolean;
  /** Enable lookup of .babelrc files */
  readonly enableBabelRCLookup?: boolean;
  /** Enable Babel runtime helpers (boolean or specific runtime package) */
  readonly enableBabelRuntime: boolean | string;
  /** Path to babel config file to extend */
  readonly extendsBabelConfigPath?: string;
  /** Enable experimental import statement support */
  readonly experimentalImportSupport?: boolean;
  /** Use Hermes parser instead of Babel parser */
  readonly hermesParser?: boolean;
  /** Hot reload mode flag */
  readonly hot: boolean;
  /** Inline requires setting (reserved for future use) */
  readonly inlineRequires?: void;
  /** Minification flag */
  readonly minify: boolean;
  /** Disable ES6 transforms (experimental) */
  readonly unstable_disableES6Transforms?: boolean;
  /** Target platform (e.g., 'ios', 'android', 'web') */
  readonly platform: string | null;
  /** Project root directory path */
  readonly projectRoot: string;
  /** Public path for assets */
  readonly publicPath: string;
  /** Transform profile for different optimization levels */
  readonly unstable_transformProfile?: TransformProfile;
  /** Global prefix for generated identifiers */
  readonly globalPrefix: string;
}

type TransformProfile = 'default' | 'hermes-stable' | 'hermes-canary';

interface CustomTransformOptions {
  [key: string]: unknown;
}

interface BabelTransformer {
  /** Main transform function */
  transform: (args: BabelTransformerArgs) => {
    ast: BabelNodeFile;
    functionMap?: BabelFileFunctionMapMetadata; // Deprecated - will be removed in future release. Function maps are now generated by Babel plugins and included in metadata.
    metadata?: MetroBabelFileMetadata;
  };
  /** Optional cache key generator for caching transformed results */
  getCacheKey?: () => string;
}

/** Babel AST node representing a complete file */
type BabelNodeFile = import('@babel/types').File;

/** Base Babel file metadata */
interface BabelFileMetadata {
  [key: string]: any;
}

/** Babel core configuration options */
interface BabelCoreOptions {
  plugins?: Array<string | [string, any] | BabelPluginInstance>;
  [key: string]: any;
}

/** Babel plugin instance */
interface BabelPluginInstance {
  (babel: any, options?: any): {
    visitor: any;
    [key: string]: any;
  };
}

interface MetroBabelFileMetadata extends BabelFileMetadata {
  /** Metro-specific metadata extensions */
  metro?: {
    /** Function mapping data for debugging */
    functionMap?: BabelFileFunctionMapMetadata;
    /** Import declaration locations for optimization */
    unstable_importDeclarationLocs?: BabelFileImportLocsMetadata;
  };
}

interface BabelFileFunctionMapMetadata {
  /** Array of function names in the source */
  readonly names: ReadonlyArray<string>;
  /** Source map style mappings for function locations */
  readonly mappings: string;
}

type BabelFileImportLocsMetadata = ReadonlySet<string>;

const { transform } = require("metro-babel-transformer");

const result = transform({
  filename: "app.js",
  src: "const app = () => 'Hello World';",
  options: {
    dev: false,
    enableBabelRuntime: false,
    enableBabelRCLookup: true,
    globalPrefix: "__app__",
    hot: false,
    minify: true,
    platform: "web",
    publicPath: "/static",
    projectRoot: process.cwd(),
  },
});

const result = transform({
  filename: "component.js",
  src: "export const Component = () => <div>React Component</div>;",
  options: {
    dev: true,
    enableBabelRuntime: "@babel/runtime",
    hermesParser: true, // Use Hermes parser instead of Babel
    hot: true,
    minify: false,
    platform: "ios",
    projectRoot: "/Users/dev/my-app",
    publicPath: "/",
    globalPrefix: "__metro__",
  },
  plugins: [
    ["@babel/plugin-transform-react-jsx", { runtime: "automatic" }],
  ],
});

const result = transform({
  filename: "feature.ts",
  src: "const feature: Feature = { enabled: true };",
  options: {
    dev: true,
    enableBabelRuntime: false,
    hot: false,
    minify: false,
    platform: "android",
    projectRoot: "/project",
    publicPath: "/assets",
    globalPrefix: "__app__",
    customTransformOptions: {
      experimentalDecorators: true,
      strictMode: false,
    },
    unstable_transformProfile: "hermes-stable",
  },
});

try {
  const result = transform(transformArgs);
  // Process result
} catch (error) {
  console.error("Transform failed:", error.message);
  // Handle transformation error
}