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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.

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/*
 * Copyright 2016 The Closure Compiler Authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

'require util/defineproperty';
'require util/global';

/** @const {string} */
$jscomp.SYMBOL_PREFIX = 'jscomp_symbol_';


/**
 * Initializes the Symbol function.
 * @suppress {reportUnknownTypes}
 */
$jscomp.initSymbol = function() {
  // Only need to do this once. All future calls are no-ops.
  $jscomp.initSymbol = function() {};

  if (!$jscomp.global['Symbol']) {
    $jscomp.global['Symbol'] = $jscomp.Symbol;
  }
};


/**
 * Produces "symbols" (actually just unique strings).
 * @param {string=} opt_description
 * @return {symbol}
 */
$jscomp.Symbol = /** @type {function(): !Function} */ (function() {
  var counter = 0;
  /**
   * @param {string=} opt_description
   * @return {symbol}
   * @suppress {reportUnknownTypes}
   */
  function Symbol(opt_description) {
    return /** @type {?} */ (
        $jscomp.SYMBOL_PREFIX + (opt_description || '') + (counter++));
  }
  return Symbol;
})();


/**
 * Initializes Symbol.iterator (if it's not already defined) and adds a
 * Symbol.iterator property to the Array prototype.
 * @suppress {reportUnknownTypes}
 */
$jscomp.initSymbolIterator = function() {
  $jscomp.initSymbol();
  var symbolIterator = $jscomp.global['Symbol'].iterator;
  if (!symbolIterator) {
    symbolIterator = $jscomp.global['Symbol'].iterator =
        $jscomp.global['Symbol']('iterator');
  }

  if (typeof Array.prototype[symbolIterator] != 'function') {
    $jscomp.defineProperty(
        Array.prototype, symbolIterator, {
          configurable: true,
          writable: true,
          /**
           * @this {Array}
           * @return {!IteratorIterable}
           */
          value: function() {
            return $jscomp.arrayIterator(this);
          }
        });
  }

  // Only need to do this once. All future calls are no-ops.
  $jscomp.initSymbolIterator = function() {};
};


/**
 * Returns an iterator from the given array.
 * @param {!Array} array
 * @return {!IteratorIterable}
 * @template T
 */
$jscomp.arrayIterator = function(array) {
  var index = 0;
  return $jscomp.iteratorPrototype(function() {
    if (index < array.length) {
      return {
        done: false,
        value: array[index++],
      };
    } else {
      return {done: true};
    }
  });
};


/**
 * Returns an iterator with the given `next` method.  Passing
 * all iterators through this function allows easily extending
 * the definition of `%IteratorPrototype%` if methods are ever
 * added to it in the future.
 *
 * @param {function(this: Iterator): T} next
 * @return {!IteratorIterable}
 * @template T
 * @suppress {reportUnknownTypes}
 */
$jscomp.iteratorPrototype = function(next) {
  $jscomp.initSymbolIterator();

  var iterator = {next: next};
  /**
   * @this {IteratorIterable}
   * @return {!IteratorIterable}
   */
  iterator[$jscomp.global['Symbol'].iterator] = function() { return this; };
  return /** @type {!IteratorIterable} */ (iterator);
};