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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.
 */

/**
 * @fileoverview
 * @suppress {lintVarDeclarations}
 */

'require es6/conformance';
'require es6/symbol';
'require es6/util/makeiterator';
'require es6/weakmap';
'require util/defines';
'require util/owns';
'require util/polyfill';


/**
 * Internal record type for entries.
 * @record
 * @template KEY, VALUE
 * @suppress {reportUnknownTypes}
 */
$jscomp.MapEntry = function() {
  /** @type {!$jscomp.MapEntry} */
  this.previous;
  /** @type {!$jscomp.MapEntry} */
  this.next;
  /** @type {?Object} */
  this.head;
  /** @type {KEY} */
  this.key;
  /** @type {VALUE} */
  this.value;
};


$jscomp.polyfill('Map',
    /**
     * @param {*} NativeMap
     * @return {*}
     * @suppress {reportUnknownTypes}
     */
    function(NativeMap) {

  /**
   * Checks conformance of the existing Map.
   * @return {boolean} True if the browser's implementation conforms.
   * @suppress {missingProperties} "entries" unknown prototype
   */
  function isConformant() {
    if ($jscomp.ASSUME_NO_NATIVE_MAP ||
        !NativeMap ||
        typeof NativeMap != "function" ||
        !NativeMap.prototype.entries ||
        typeof Object.seal != 'function') {
      return false;
    }
    // Some implementations don't support constructor arguments.
    try {
      NativeMap = /** @type {function(new: Map, !Iterator=)} */ (NativeMap);
      var key = Object.seal({x: 4});
      var map = new NativeMap($jscomp.makeIterator([[key, 's']]));
      if (map.get(key) != 's' || map.size != 1 || map.get({x: 4}) ||
          map.set({x: 4}, 't') != map || map.size != 2) {
        return false;
      }
      var /** !Iterator */ iter = map.entries();
      var item = iter.next();
      if (item.done || item.value[0] != key || item.value[1] != 's') {
        return false;
      }
      item = iter.next();
      if (item.done || item.value[0].x != 4 ||
          item.value[1] != 't' || !iter.next().done) {
        return false;
      }
      return true;
    } catch (err) { // This should hopefully never happen, but let's be safe.
      return false;
    }
  }

  if ($jscomp.USE_PROXY_FOR_ES6_CONFORMANCE_CHECKS) {
    if (NativeMap && $jscomp.ES6_CONFORMANCE) return NativeMap;
  } else {
    if (isConformant()) return NativeMap;
  }

  /** @const {!WeakMap} */
  var idMap = new WeakMap();

  // Numeric indices to avoid ambiguation failures breaking code that extends
  // the Map class.  They will actually be extending the polyfill if it is used.
  // We need to make sure that ambiguation will not try to reuse the property
  // names for any properties that the extending class adds.
  /**
   * @const Index for PolyfillMap's data property, of type
   *        !Object>>
   */
  var DATA = 0;
  /**
   * @const Index for PolyfillMap's head property, of type
   *        !$jscomp.MapEntry
   */
  var HEAD = 1;

  /**
   * Polyfill for the global Map data type.
   * @constructor
   * @unrestricted
   * @extends {Map}
   * @implements {Iterable>}
   * @template KEY, VALUE
   * @param {!Iterable>|!Array>|null=}
   *     opt_iterable Optional data to populate the map.
   */
  // TODO(sdh): fix param type if heterogeneous arrays ever supported.
  var PolyfillMap = function(opt_iterable) {
    this[DATA] = {};
    this[HEAD] = createHead();

    // Note: this property should not be changed.  If we're willing to give up
    // ES3 support, we could define it as a property directly.  It should be
    // marked readonly if such an annotation ever comes into existence.
    /** @type {number} */
    this.size = 0;

    if (opt_iterable) {
      var iter = $jscomp.makeIterator(opt_iterable);
      var entry;
      while (!(entry = iter.next()).done) {
        var item =
            /** @type {!IIterableResult>} */ (entry).value;
        this.set(/** @type {KEY} */ (item[0]), /** @type {VALUE} */ (item[1]));
      }
    }
  };


  /** @override */
  PolyfillMap.prototype.set = function(key, value) {
    // normalize -0/+0 to +0
    key = key === 0 ? 0 : key;
    var r = maybeGetEntry(this, key);
    if (!r.list) {
      r.list = (this[DATA][r.id] = []);
    }
    if (!r.entry) {
      r.entry = {
        next: this[HEAD],
        previous: this[HEAD].previous,
        head: this[HEAD],
        key: key,
        value: value,
      };
      r.list.push(r.entry);
      this[HEAD].previous.next = r.entry;
      this[HEAD].previous = r.entry;
      this.size++;
    } else {
      r.entry.value = value;
    }
    return this;
  };


  /** @override */
  PolyfillMap.prototype.delete = function(key) {
    var r = maybeGetEntry(this, key);
    if (r.entry && r.list) {
      r.list.splice(r.index, 1);
      if (!r.list.length) delete this[DATA][r.id];
      r.entry.previous.next = r.entry.next;
      r.entry.next.previous = r.entry.previous;
      r.entry.head = null;
      this.size--;
      return true;
    }
    return false;
  };


  /** @override */
  PolyfillMap.prototype.clear = function() {
    this[DATA] = {};
    this[HEAD] =
        this[HEAD].previous = createHead();
    this.size = 0;
  };


  /** @override */
  PolyfillMap.prototype.has = function(key) {
    return !!(maybeGetEntry(this, key).entry);
  };


  /** @override */
  PolyfillMap.prototype.get = function(key) {
    var entry = maybeGetEntry(this, key).entry;
    // NOTE: this cast is a lie, but so is the extern.
    return /** @type {VALUE} */ (entry &&
      /** @type {VALUE} */ (entry.value));
  };


  /** @override */
  PolyfillMap.prototype.entries = function() {
    return makeIterator(this, /** @return {!Array<(KEY|VALUE)>} */ function(
        /** !$jscomp.MapEntry */ entry) {
      return ([entry.key, entry.value]);
    });
  };


  /** @override */
  PolyfillMap.prototype.keys = function() {
    return makeIterator(this, /** @return {KEY} */ function(
        /** !$jscomp.MapEntry */ entry) {
      return entry.key;
    });
  };


  /** @override */
  PolyfillMap.prototype.values = function() {
    return makeIterator(this, /** @return {VALUE} */ function(
        /** !$jscomp.MapEntry */ entry) {
      return entry.value;
    });
  };


  /** @override */
  PolyfillMap.prototype.forEach = function(callback, opt_thisArg) {
    var iter = this.entries();
    var item;
    while (!(item = iter.next()).done) {
      var entry = item.value;
      callback.call(
          /** @type {?} */ (opt_thisArg),
          /** @type {VALUE} */ (entry[1]),
          /** @type {KEY} */ (entry[0]),
          this);
    }
  };


  /** @type {?} */ (PolyfillMap.prototype)[Symbol.iterator] =
      PolyfillMap.prototype.entries;


  /**
   * Returns an entry or undefined.
   * @param {!PolyfillMap} map
   * @param {KEY} key
   * @return {{id: string,
   *           list: (!Array>|undefined),
   *           index: number,
   *           entry: (!$jscomp.MapEntry|undefined)}}
   * @template KEY, VALUE
   */
  var maybeGetEntry = function(map, key) {
    var id = getId(key);
    var list = map[DATA][id];
    if (list && $jscomp.owns(map[DATA], id)) {
      for (var index = 0; index < list.length; index++) {
        var entry = list[index];
        if ((key !== key && entry.key !== entry.key) || key === entry.key) {
          return {id: id, list: list, index: index, entry: entry};
        }
      }
    }
    return {id: id, list: list, index: -1, entry: undefined};
  };


  /**
   * Maps over the entries with the given function.
   * @param {!PolyfillMap} map
   * @param {function(!$jscomp.MapEntry): T} func
   * @return {!IteratorIterable}
   * @template KEY, VALUE, T
   * @private
   */
  var makeIterator = function(map, func) {
    var entry = map[HEAD];
    return $jscomp.iteratorPrototype(function() {
      if (entry) {
        while (entry.head != map[HEAD]) {
          entry = entry.previous;
        }
        while (entry.next != entry.head) {
          entry = entry.next;
          return {done: false, value: func(entry)};
        }
        entry = null; // make sure depletion is permanent
      }
      return {done: true, value: void 0};
    });
  };


  /**
   * Makes a new "head" element.
   * @return {!$jscomp.MapEntry}
   * @template KEY, VALUE
   * @suppress {checkTypes} ignore missing key/value for head only
   */
  var createHead = function() {
    var head = /** type {!$jscomp.MapEntry} */ ({});
    head.previous = head.next = head.head = head;
    return head;
  };


  /**
   * Counter for generating IDs.
   * @private {number}
   */
  var mapIndex = 0;


  /**
   * @param {*} obj An extensible object.
   * @return {string} A unique ID.
   */
  var getId = function(obj) {
    var type = obj && typeof obj;
    if (type == 'object' || type == 'function') {
      obj = /** @type {!Object} */ (obj);
      if (!idMap.has(obj)) {
        var id = '' + (++mapIndex);
        idMap.set(obj, id);
        return id;
      }
      return idMap.get(obj);
    }
    // Add a prefix since obj could be '__proto__';
    return 'p_' + obj;
  };


  return PolyfillMap;
}, 'es6', 'es3');