Code Transformation

/**
 * Transforms the passed in code string
 * @param {string} code - Source code to transform
 * @param {Object} opts - Transformation options
 * @returns {Object} Result object with code, map, ast, metadata, options, ignored properties
 */
function transform(code, opts);

// Transform result object structure:
// {
//   code: string,        // Transformed code (if opts.code !== false)
//   map: Object,         // Source map (if opts.sourceMaps is truthy)
//   ast: Object,         // AST (if opts.ast !== false)
//   metadata: Object,    // Transformation metadata (if opts.metadata !== false)
//   options: Object,     // Resolved options used
//   ignored: boolean     // Whether file was ignored
// }

const babel = require("babel-core");

// Basic transformation
const result = babel.transform("const fn = () => 42;", {
  presets: ["es2015"]
});
console.log(result.code); // "var fn = function fn() { return 42; };"

// With source maps
const result = babel.transform("const greeting = 'Hello';", {
  presets: ["es2015"],
  sourceMaps: true,
  filename: "input.js"
});
console.log(result.code); // transformed code
console.log(result.map);  // source map object

// Return only AST
const result = babel.transform("class MyClass {}", {
  code: false,
  ast: true
});
console.log(result.ast); // AST object
console.log(result.code); // null

/**
 * Asynchronously transforms entire file contents
 * @param {string} filename - Path to file to transform
 * @param {Object} opts - Transformation options (optional)
 * @param {Function} callback - Callback function (err, result)
 */
function transformFile(filename, opts, callback);

const babel = require("babel-core");

// Transform file with callback
babel.transformFile("src/input.js", {
  presets: ["es2015"],
  sourceMaps: true
}, function(err, result) {
  if (err) {
    console.error("Transform error:", err);
    return;
  }
  console.log(result.code);
  console.log(result.map);
});

// With options parameter optional
babel.transformFile("src/input.js", function(err, result) {
  if (err) throw err;
  console.log(result.code);
});

/**
 * Synchronously transforms entire file contents
 * @param {string} filename - Path to file to transform
 * @param {Object} opts - Transformation options (optional, defaults to {})
 * @returns {Object} Transform result object
 */
function transformFileSync(filename, opts);

const babel = require("babel-core");

// Transform file synchronously
try {
  const result = babel.transformFileSync("src/input.js", {
    presets: ["es2015"],
    sourceMaps: "inline"
  });
  console.log(result.code);
} catch (err) {
  console.error("Transform error:", err);
}

// With default options
const result = babel.transformFileSync("src/input.js");
console.log(result.code);

// Transform options object structure:
const transformOptions = {
  // Output control
  ast: Boolean,           // Include AST in result (default: true)
  code: Boolean,          // Generate code (default: true)
  comments: Boolean,      // Output comments (default: true)
  compact: Boolean,       // Remove whitespace (default: "auto")
  
  // Source handling
  filename: String,       // Filename for errors/source maps (default: "unknown")
  filenameRelative: String, // Filename relative to sourceRoot
  sourceMaps: Boolean,    // Source map generation (true, false, "inline", "both")
  sourceMapTarget: String, // Source map file property
  sourceRoot: String,     // Source map sourceRoot property
  sourceType: String,     // Parse as "script" or "module"
  
  // Configuration files
  babelrc: Boolean,       // Use .babelrc files (default: true)
  extends: String,        // Path to .babelrc file to extend
  
  // File filtering  
  ignore: Array,          // Files to ignore (strings, RegExp, or Functions)
  only: Array,            // Only transform these files (strings, RegExp, or Functions)
  
  // Plugins and presets
  plugins: Array,         // List of plugins to load and use
  presets: Array,         // List of presets to load and use
  
  // Advanced options
  env: Object,            // Environment-specific options
  retainLines: Boolean,   // Retain line numbers (default: false)
  minified: Boolean,      // Generate minified output (default: false)
  
  // Parser and generator options
  parserOpts: Object,     // Options passed to babylon parser
  generatorOpts: Object   // Options passed to babel-generator
};

// Plugin as function
{
  plugins: [
    function(babel) {
      return {
        visitor: {
          Identifier(path) {
            path.node.name = path.node.name.split("").reverse().join("");
          }
        }
      };
    }
  ]
}

// Plugin with options
{
  plugins: [
    ["transform-es2015-arrow-functions", { spec: true }]
  ]
}

// Preset configuration
{
  presets: [
    ["es2015", { modules: false }],
    "react"
  ]
}

try {
  const result = babel.transform(invalidCode, options);
} catch (err) {
  if (err._babel) {
    // Babel-specific error
    console.error("Babel error:", err.message);
    console.error("Code frame:", err.codeFrame);
    console.error("Location:", err.loc);
  } else {
    // Other error
    console.error("General error:", err.message);
  }
}