shaka-player/build/generateExterns.js

DASH/EME video player library

#!/usr/bin/env node
/*! @license
 * Shaka Player
 * Copyright 2016 Google LLC
 * SPDX-License-Identifier: Apache-2.0
 */

/**
 * @fileoverview
 *
 * A node script that generates externs automatically from the uncompiled
 * source of a Closure project.  Designed for Shaka Player, but may be usable
 * in other projects as well.  Does not depend on the Closure compiler itself.
 *
 * We were not able to get our externs generated by the Closure compiler.  There
 * were many issues with the Closure-generated externs, including the order of
 * the externs and the replacement of record types and enums with their
 * underlying types.
 *
 * This uses a node module called esprima to parse JavaScript, then explores the
 * abstract syntax tree from esprima.  It finds exported symbols and generates
 * an appropriate extern definition for it.
 *
 * The generated externs are then topologically sorted according to the
 * goog.provide and goog.require calls in the sources.  No sorting is done
 * within source files, and no sorting is done based on parameter types.
 * Circular deps between source files will not be resolved, and deps not
 * represented in goog.provide/goog.require will not be discovered.
 *
 * Arguments: --output <EXTERNS> <INPUT> [<INPUT> ...]
 */

// Load required modules.
let assert = require('assert');
if (assert.strict) {
  // The "strict" mode was added in v9.9, use that if available.
  assert = assert.strict;
}
const esprima = require('esprima');
const fs = require('fs');

// The annotations we will consider "exporting" a symbol.
const EXPORT_REGEX = /@(?:export|exportInterface|expose)\b/;

// TODO: revisit this when Closure Compiler supports partially-exported classes.
let partiallyExportedClassesDetected = false;

/**
 * Topological sort of general objects using a DFS approach.
 * Will add a __mark field to each object as part of the sorting process.
 * @param {!Array.<T>} list
 * @param {function(T):!Array.<T>} getDeps
 * @return {!Array.<T>}
 * @template T
 * @see https://en.wikipedia.org/wiki/Topological_sorting#Depth-first_search
 */
function topologicalSort(list, getDeps) {
  const sorted = [];
  const NOT_VISITED = 0;
  const MID_VISIT = 1;
  const COMPLETELY_VISITED = 2;

  // Mark all objects as not visited.
  for (const object of list) {
    object.__mark = NOT_VISITED;
  }

  // Visit each object.
  for (const object of list) {
    visit(object);
  }

  // Return the sorted list.
  return sorted;

  /**
   * @param {T} object
   * @template T
   */
  function visit(object) {
    if (object.__mark == MID_VISIT) {
      assert.fail('Dependency cycle detected!');
    } else if (object.__mark == NOT_VISITED) {
      object.__mark = MID_VISIT;

      // Visit all dependencies.
      for (const dep of getDeps(object)) {
        visit(dep);
      }

      object.__mark = COMPLETELY_VISITED;

      // Push this object onto the list.  All transitive dependencies have
      // already been added to the list.
      sorted.push(object);
    }
  }
}

/**
 * @param {ASTNode} node A node from the abstract syntax tree.
 * @return {boolean} true if this is a call node.
 */
function isCallNode(node) {
  // Example node: {
  //   type: 'ExpressionStatement',
  //   expression: { type: 'CallExpression', callee: {...}, arguments: [...] },
  // }
  return node.type == 'ExpressionStatement' &&
         node.expression.type == 'CallExpression';
}


/**
 * Pretty-print a node via console.log.  Useful for debugging and development
 * to see what the AST looks like.
 * @param {ASTNode} node A node from the abstract syntax tree.
 */
function dumpNode(node) {
  console.log(JSON.stringify(node, null, '  '));
}


/**
 * @param {ASTNode} node A node from the abstract syntax tree.
 * @return {boolean} true if this is a call to goog.provide.
 */
function isProvideNode(node) {
  return isCallNode(node) &&
         getIdentifierString(node.expression.callee) == 'goog.provide';
}


/**
 * @param {ASTNode} node A node from the abstract syntax tree.
 * @return {boolean} true if this is a call to goog.require.
 */
function isRequireNode(node) {
  return isCallNode(node) &&
         getIdentifierString(node.expression.callee) == 'goog.require';
}


/**
 * @param {ASTNode} node A node from the abstract syntax tree.
 * @return {boolean} true if this is an exported symbol or property.
 */
function isExportNode(node) {
  const doc = getLeadingBlockComment(node);
  return doc && EXPORT_REGEX.test(doc);
}


/**
 * @param {ASTNode} node A node from the abstract syntax tree.
 * @return {boolean} true if this is a class assignment.
 */
function isClassAssignmentNode(node) {
  return node.type == 'ExpressionStatement' &&
      node.expression.type == 'AssignmentExpression' &&
      node.expression.right.type == 'ClassExpression';
}


/**
 * @param {ASTNode} node A node from the abstract syntax tree.
 * @return {boolean} true if this is a class assignment with exported members.
 */
function isPartiallyExportedClassAssignmentNode(node) {
  if (!isClassAssignmentNode(node)) {
    return false;
  }

  const rightSide = node.expression.right;
  // Example code: foo.bar = class bar2 extends foo.baz { /* ... */ };
  // Example right side: {
  //   id: { name: 'bar' },   // or null
  //   superClass: { type: 'MemberExpression', ... },  // or null
  //   body: { body: [ ... ] },
  // }

  for (const member of rightSide.body.body) {
    // Only look at exported members.  Constructors are exported implicitly
    // when the class is exported.
    const comment = getLeadingBlockComment(member);
    if (EXPORT_REGEX.test(comment)) {
      return true;
    }
  }

  return false;
}


/**
 * @param {ASTNode} node A node from the abstract syntax tree.
 * @return {string} A reconstructed leading comment block for the node.
 *   If there are multiple comments before this node, we will take the most
 *   recent block comment, as that is the one that would contain any applicable
 *   jsdoc/closure annotations for this symbol.
 */
function getLeadingBlockComment(node) {
  // Example code: /** @summary blah */ /** @export */ foo.bar = ...;
  // Example node: {
  //   type: 'ExpressionStatement',
  //   expression: { ... },
  //   leadingComments: [
  //     { type: 'Block', value: '* @summary blah ' },
  //     { type: 'Block', value: '* @export ' },
  //   ],
  // }
  if (!node.leadingComments || !node.leadingComments.length) {
    return null;
  }

  // Ignore non-block comments, since those are not jsdoc/Closure comments.
  const blockComments = node.leadingComments.filter((comment) => {
    return comment.type == 'Block';
  });
  if (!blockComments.length) {
    return null;
  }

  // In case there are multiple (for example, a file-level comment that also
  // preceeds the node), take the most recent one, which is closest to the node.
  const mostRecentComment = blockComments[blockComments.length - 1];

  // Reconstruct the original block comment by adding back /* and */.
  return '/*' + mostRecentComment.value + '*/';
}


/**
 * @param {number} idx An argument index from the call node.
 * @param {ASTNode} node A node from the abstract syntax tree.
 * @return {string} The argument value as a string.
 */
function getArgumentFromCallNode(idx, node) {
  // Example node: {
  //   type: 'ExpressionStatement',
  //   expression: { type: 'CallExpression', callee: {...}, arguments: [...] },
  // }
  assert(isCallNode(node));
  return node.expression.arguments[idx].value;
}


/**
 * @param {ASTNode} node An identifier or member node from the abstract syntax
 *   tree.
 * @return {string} The identifier as a string.
 */
function getIdentifierString(node) {
  if (node.type == 'Identifier') {
    // Example code: foo
    // Example node: { type: 'Identifier', name: 'foo' }
    return node.name;
  }

  assert.equal(node.type, 'MemberExpression');
  // Example code: foo.bar.baz
  // Example node: {
  //   type: 'MemberExpression',
  //   object: {
  //     type: 'MemberExpression',
  //     object: { type: 'Identifier', name: 'foo' },
  //     property: { type: 'Identifier', name: 'bar' },
  //   },
  //   property: { type: 'Identifier', name: 'baz' },
  // }
  return getIdentifierString(node.object) + '.' +
      getIdentifierString(node.property);
}


/**
 * @param {ASTNode} node A function definition node from the abstract syntax
 *   tree.
 * @return {!Array.<string>} a list of the parameter names.
 */
function getFunctionParameters(node) {
  assert(node.type == 'FunctionExpression' ||
         node.type == 'ArrowFunctionExpression');
  // Example code: function(x, y, z = null, ...varArgs) {...}
  // Example node: {
  //   params: [
  //     { type: 'Identifier', name: 'x' },
  //     { type: 'Identifier', name: 'y' },
  //     {
  //       type: 'AssignmentPattern',
  //       left: { type: 'Identifier', name: 'z' },
  //       right: { type: 'Literal', raw: 'null' },
  //     },
  //     {
  //       type: 'RestElement',
  //       argument: { type: 'Identifier', name: 'varArgs' },
  //     },
  //   ],
  //   body: {...},
  // }
  return node.params.map((param) => {
    if (param.type == 'Identifier') {
      return param.name;
    } else if (param.type == 'AssignmentPattern') {
      return param.left.name;
    } else {
      assert.equal(param.type, 'RestElement');
      return '...' + param.argument.name;
    }
  });
}


/**
 * Take the original block comment and prep it for the externs by removing
 * export annotations and blank lines.
 *
 * @param {string}
 * @return {string}
 */
function removeExportAnnotationsFromComment(comment) {
  // Remove @export annotations.
  comment = comment.replace(EXPORT_REGEX, '');

  // Split into lines, remove empty comment lines, then recombine.
  comment = comment.split('\n')
      .filter((line) => !/^ *\*? *$/.test(line))
      .join('\n');

  return comment;
}


/**
 * Recursively find all expression statements in all block nodes.
 * @param {ASTNode} node
 * @return {!Array.<ASTNode>}
 */
function getAllExpressionStatements(node) {
  assert(node.body && node.body.body);
  const expressionStatements = [];
  for (const childNode of node.body.body) {
    if (childNode.type == 'ExpressionStatement') {
      expressionStatements.push(childNode);
    } else if (childNode.body) {
      const childExpressions = getAllExpressionStatements(childNode);
      expressionStatements.push(...childExpressions);
    }
  }
  return expressionStatements;
}


/**
 * @param {!Set.<string>} names A set of the names of exported nodes.
 * @param {ASTNode} node An exported node from the abstract syntax tree.
 * @return {string} An extern string for this node.
 */
function createExternFromExportNode(names, node) {
  assert.equal(node.type, 'ExpressionStatement', 'Unknown node type');

  let comment = getLeadingBlockComment(node);
  comment = removeExportAnnotationsFromComment(comment);

  let name;
  let assignment;

  switch (node.expression.type) {
    case 'AssignmentExpression':
      // Example code: /** @export */ foo.bar = function(...) { ... };
      // Example node.expression: {
      //   operator: '=',
      //   left: {
      //     type: 'MemberExpression',
      //     object: { type: 'Identifier', name: 'foo' },
      //     property: { type: 'Identifier', name: 'bar' },
      //   }, right: {
      //     type: 'FunctionExpression', params: [ ... ], body: {...}
      //   }
      // }
      name = getIdentifierString(node.expression.left);
      assignment = createExternAssignment(name, node.expression.right,
          /* alwaysIncludeConstructor= */ true);
      break;

    case 'MemberExpression':
      // Example code: /** @export */ foo.bar;
      // Example node.expression: {
      //   object: { type: 'Identifier', name: 'foo' },
      //   property: { type: 'Identifier', name: 'bar' },
      // }
      name = getIdentifierString(node.expression);
      assignment = '';
      break;

    default:
      assert.fail('Unexpected expression type: ' + node.expression.type);
  }

  // Keep track of the names we've externed.
  names.add(name);
  // Generate the actual extern string.
  let externString = comment + '\n' + name + assignment + ';\n';

  // Find this.foo = bar in the constructor, and potentially generate externs
  // for that, too.
  if (node.expression.type == 'AssignmentExpression') {
    const rightSide = node.expression.right;

    if (rightSide.type == 'FunctionExpression' &&
        comment.includes('@constructor')) {
      externString += createExternsFromConstructor(name, rightSide);
    } else if (rightSide.type == 'ClassExpression') {
      const ctor = getClassConstructor(node.expression.right);
      if (ctor) {
        externString += createExternsFromConstructor(name, ctor);
      }
    }
  }
  return externString;
}


/**
 * Some classes are not exported, but contain exported members.  These need to
 * have externs generated, too.
 *
 * @param {!Set.<string>} names A set of the names of exported nodes.
 * @param {ASTNode} node An exported node from the abstract syntax tree.
 * @return {string} An extern string for this node.
 */
function createExternFromPartiallyExportedClassAssignmentNode(names, node) {
  assert.equal(node.type, 'ExpressionStatement', 'Unknown node type');
  assert.equal(node.expression.type, 'AssignmentExpression',
      'Should be assignment node');
  assert.equal(node.expression.right.type, 'ClassExpression',
      'Should be class assignment');

  const name = getIdentifierString(node.expression.left);
  const assignment = createExternAssignment(name, node.expression.right,
      /* alwaysIncludeConstructor= */ false);

  let externString = name + assignment + ';\n';

  // Find this.foo = bar in the constructor, and potentially generate externs
  // for that, too.
  const rightSide = node.expression.right;
  const ctor = getClassConstructor(node.expression.right);
  if (ctor) {
    externString += createExternsFromConstructor(name, ctor);
  }

  // Keep track of the names we've externed.
  names.add(name);

  return externString;
}


/**
 * @param {ASTNode} node A method node from the abstract syntax tree.
 * @return {string} The extern string for this method.
 */
function createExternMethod(node) {
  // Example code: foo.bar = class {
  //   baz() { ... }
  // };
  // Example node: {
  //   leadingComments: [ ... ],
  //   static: false,
  //   key: Identifier,
  //   value: FunctionExpression,
  // }
  const id = getIdentifierString(node.key);
  let comment = getLeadingBlockComment(node);
  if (!comment) {
    if (id == 'constructor') {
      // ES6 constructors don't necessarily need comments; a comment along the
      // lines of "Creates a Foo object." doesn't really add anything.
      comment = '';
    } else {
      throw new Error('No leading block comment for: ' + id);
    }
  }
  comment = removeExportAnnotationsFromComment(comment);

  const params = getFunctionParameters(node.value);

  let methodString = (comment ? '  ' + comment + '\n' : '') + '  ';
  if (node.static) {
    methodString += 'static ';
  }
  methodString += id + '(' + params.join(', ') + ') {}';
  return methodString;
}


/**
 * Find the constructor of an ES6 class, if it exists.
 *
 * @param {ASTNode} className
 * @return {ASTNode}
 */
function getClassConstructor(classNode) {
  // Example class node: {
  //   type: 'ClassExpression',
  //   body: {
  //     type: 'ClassBody',
  //     body: [ MethodDefinition, ... ],
  //   }
  // }
  //
  // Example method node: {
  //   type: 'MethodDefinition',
  //   key: { type: 'Identifier', name: 'constructor' },
  //   value: {
  //     type: 'FunctionExpression',
  //     params: [ [Identifier], [Identifier], [Identifier] ],
  //     body: { type: 'BlockStatement', body: [Array] },
  // }

  assert.equal(classNode.type, 'ClassExpression');

  for (const member of classNode.body.body) {
    if (member.type == 'MethodDefinition' && member.key.name == 'constructor') {
      return member.value;
    }
  }

  return null;
}


/**
 * @param {string} name The name of the thing we are assigning.
 * @param {ASTNode} node An assignment node from the abstract syntax tree.
 * @param {boolean} alwaysIncludeConstructor Include the constructor of a class
 *   expression, even if there is no export annotation.
 * @return {string} The assignment part of the extern string for this node.
 */
function createExternAssignment(name, node, alwaysIncludeConstructor) {
  switch (node.type) {
    case 'ClassExpression': {
      // Example code: foo.bar = class bar2 extends foo.baz { /* ... */ };
      // Example node: {
      //   id: { name: 'bar' },   // or null
      //   superClass: { type: 'MemberExpression', ... },  // or null
      //   body: { body: [ ... ] },
      // }
      let classString = ' = class ';
      if (node.id) {
        classString += getIdentifierString(node.id) + ' ';
      }
      if (node.superClass) {
        classString += 'extends ' + getIdentifierString(node.superClass) + ' ';
      }
      classString += '{\n';
      for (const member of node.body.body) {
        const comment = getLeadingBlockComment(member);

        if (EXPORT_REGEX.test(comment)) {
          // This has an export annotation, so fall through and generate
          // externs.
        } else {
          // If there's no export annotation, we may make an exception for the
          // constructor in some situations.
          if (member.key.name == 'constructor' && alwaysIncludeConstructor) {
            // Fall through and generate externs.
          } else {
            // Skip extern generation.
            continue;
          }
        }

        assert.equal(
            member.type, 'MethodDefinition',
            'Unexpected exported member type in exported class!');

        classString += createExternMethod(member) + '\n';
      }
      classString += '}';
      return classString;
    }

    case 'ArrowFunctionExpression':
    case 'FunctionExpression': {
      // Example code: foo.square = function(x) { return x * x; };
      // Example node: { params: [ { type: 'Identifier', name: 'x' } ] }
      const params = getFunctionParameters(node);
      return ' = function(' + params.join(', ') + ') {}';
    }

    case 'ObjectExpression': {
      // Example code: foo.Bar = { 'ABC': 1, DEF: 2 };
      // Example node: {
      //   properties: [ {
      //     kind: 'init',
      //     key: { type: 'Literal', value: 'ABC' }
      //     value: { type: 'Literal', value: 1 }
      //   }, {
      //     kind: 'init',
      //     key: { type: 'Identifier', name: 'DEF' }
      //     value: { type: 'Literal', value: 2 }
      //   } ]
      // }
      const propertyStrings = node.properties.map((prop) => {
        assert.equal(prop.kind, 'init');
        assert(prop.key.type == 'Literal' || prop.key.type == 'Identifier');
        // Literal indicates a quoted name in the source, while Identifier is
        // an unquoted name.  In the case of Literal, key.raw gets us the
        // unquoted name, we end up with an unquoted name in both cases.
        const name = prop.key.type == 'Literal' ? prop.key.raw : prop.key.name;
        assert.equal(prop.value.type, 'Literal');
        return '  ' + name + ': ' + prop.value.raw;
      });
      return ' = {\n' + propertyStrings.join(',\n') + '\n}';
    }

    case 'Identifier':
      // Example code: /** @const {string} @export */ foo.version = VERSION;
      // Example extern: /** @const {string} */ foo.version;
      return '';

    case 'Literal':
      // Example code: /** @const {string} @export */ foo.version = 'v1.0.0';
      // Example extern: /** @const {string} */ foo.version;
      return '';

    default:
      assert.fail('Unexpected export type: ' + node.type);
      return '';  // Shouldn't be hit, but linter wants a return statement.
  }
}


/**
 * Look for exports in a constructor body.  If we don't do this, we may end up
 * with errors about classes not fully implementing their interfaces.  In
 * reality, the interface is implemented by assigning members on "this".
 *
 * @param {string} className
 * @param {ASTNode} constructorNode
 * @return {string}
 */
function createExternsFromConstructor(className, constructorNode) {
  // Example code:
  //
  // /** @interface @exportInterface */
  // FooLike = function() {};
  //
  // /** @exportInterface @type {number} */
  // FooLike.prototype.bar;
  //
  // /** @export @implements {FooLike} */
  // class Foo {
  //   constructor() {
  //     /** @override @exportInterface */
  //     this.bar = 10;
  //   }
  // };
  //
  // Example externs:
  //
  // /**
  //  * Generated by createExternFromExportNode:
  //  * @implements {FooLike}
  //  */
  // class Foo {
  //   constructor() {}
  // }
  //
  // /**
  //  * Generated by createExternsFromConstructor:
  //  * @override
  //  */
  // Foo.prototype.bar;

  const expressionStatements = getAllExpressionStatements(constructorNode);
  let externString = '';

  for (const statement of expressionStatements) {
    const left = statement.expression.left;
    const right = statement.expression.right;

    // Skip anything that isn't an assignment to a member of "this".
    if (statement.expression.type != 'AssignmentExpression' ||
        left.type != 'MemberExpression' ||
        left.object.type != 'ThisExpression') {
      continue;
    }

    assert(left);
    assert(right);

    // Skip anything that isn't exported.
    let comment = getLeadingBlockComment(statement);
    if (!EXPORT_REGEX.test(comment)) {
      continue;
    }

    comment = removeExportAnnotationsFromComment(comment);

    assert.equal(left.property.type, 'Identifier');
    const name = className + '.prototype.' + left.property.name;
    externString += comment + '\n' + name + ';\n';
  }

  return externString;
}


/**
 * @param {!Set.<string>} names A set of the names of exported nodes.
 * @param {string} inputPath
 * @return {{
 *   path: string,
 *   provides: !Array.<string>,
 *   requires: !Array.<string>,
 *   externs: string,
 * }}
 */
function generateExterns(names, inputPath) {
  // Load and parse the code, with comments attached to the nodes.
  const code = fs.readFileSync(inputPath, 'utf-8');
  const program = esprima.parse(code, {attachComment: true});
  assert.equal(program.type, 'Program');

  const body = program.body;
  const provides = program.body.filter(isProvideNode)
      .map((node) => getArgumentFromCallNode(0, node));
  const requires = program.body.filter(isRequireNode)
      .map((node) => getArgumentFromCallNode(0, node));

  // Get all exported nodes and all classes, in order.
  const rawExterns = program.body.map((node) => {
    if (isExportNode(node)) {
      // Explicitly-exported nodes are handled here.
      return createExternFromExportNode(names, node);
    } else if (isPartiallyExportedClassAssignmentNode(node)) {
      // Some classes are not exported, but contain exported members.  These
      // need to have externs generated, too.

      // But wait!  The latest compiler won't actually export those correctly!
      // TODO: File a bug against the Closure Compiler.
      // In the mean time, log these now and throw an error at the end to make
      // sure we are generating usable releases.  This tends to affect our
      // plugin registration APIs, and apps should definitely be able to use
      // those!
      if (!partiallyExportedClassesDetected) {
        partiallyExportedClassesDetected = true;
        console.log('The Closure Compiler does not handle partially-exported ' +
            'classes correctly!  The following classes need to be exported:');
      }

      const name = getIdentifierString(node.expression.left);
      console.log(' * ' + name);

      return createExternFromPartiallyExportedClassAssignmentNode(names, node);
    } else {
      // Ignore anything else, and don't generate any externs.
      return '';
    }
  });

  const externs = rawExterns.join('');

  return {
    path: inputPath,
    provides: provides,
    requires: requires,
    externs: externs,
  };
}


/**
 * Generate externs from exported code.
 * Arguments: --output <EXTERNS> <INPUT> [<INPUT> ...]
 *
 * @param {!Array.<string>} args The args to this script, not counting node and
 *   the script name itself.
 */
function main(args) {
  const inputPaths = [];
  let outputPath;

  for (let i = 0; i < args.length; ++i) {
    if (args[i] == '--output') {
      outputPath = args[i + 1];
      ++i;
    } else {
      inputPaths.push(args[i]);
    }
  }
  assert(outputPath, 'You must specify output file with --output <EXTERNS>');
  assert(inputPaths.length, 'You must specify at least one input file.');

  // Generate externs for all input paths.
  const names = new Set();
  const results = inputPaths.map((path) => generateExterns(names, path));

  // TODO: revisit this when the compiler supports partially-exported classes.
  if (partiallyExportedClassesDetected) {
    throw new Error(
        'Partially exported classes are not supported in the compiler!');
  }

  // Sort them in dependency order.
  const sorted = topologicalSort(results, /* getDeps= */ (object) => {
    return object.requires.map((id) => {
      const dep = results.find((x) => x.provides.includes(id));
      assert(dep, 'Cannot find dependency: ' + id);
      return dep;
    });
  });

  // Generate namespaces for all externs.  For example, if we extern
  // foo.bar.baz, foo and foo.bar will both need to be declared first.
  const namespaces = new Set();
  const namespaceDeclarations = [];
  for (const name of Array.from(names).sort()) {
    // Add the full name "foo.bar.baz" and its prototype ahead of time.  We
    // should never generate these as namespaces.
    namespaces.add(name);
    namespaces.add(name + '.prototype');

    // For name "foo.bar.baz", iterate over partialName "foo" and "foo.bar".
    const pieces = name.split('.');
    for (let i = 1; i < pieces.length; ++i) {
      const partialName = pieces.slice(0, i).join('.');
      if (!namespaces.has(partialName)) {
        let declaration;
        if (i == 1) {
          declaration = '/** @namespace */\n';
          declaration += 'window.';
        } else {
          declaration = '/** @const */\n';
        }
        declaration += partialName + ' = {};\n';
        namespaceDeclarations.push(declaration);
        namespaces.add(partialName);
      }
    }
  }

  // Get externs.
  const externs = sorted.map((x) => x.externs).join('');

  // Get license header.
  const licenseHeader = fs.readFileSync(__dirname + '/license-header', 'utf-8');

  // Output generated externs, with an appropriate header.
  fs.writeFileSync(outputPath,
      licenseHeader +
      '/**\n' +
      ' * @fileoverview Generated externs.  DO NOT EDIT!\n' +
      ' * @externs\n' +
      ' * @suppress {duplicate} To prevent compiler errors with the\n' +
      ' *   namespace being declared both here and by goog.provide in the\n' +
      ' *   library.\n' +
      ' */\n\n' +
      namespaceDeclarations.join('') + '\n' + externs);
}


// Skip argv[0], which is the node binary, and argv[1], which is the script.
main(process.argv.slice(2));