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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 2018 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 {uselessCode} */
'require util/polyfill';

$jscomp.polyfill('Array.prototype.flatMap', function(orig) {
  if (orig) return orig;

  /**
   * Polyfills Array.prototype.flatMap.
   *
   * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flatMap
   *
   * @param {function(this: THIS, T, number, !IArrayLike): !Array} callback
   * @param {THIS=} thisArg
   * @return {!Array}
   * @this {!IArrayLike}
   * @template T, THIS, S
   * @suppress {reportUnknownTypes}
   */
  var flatMap = function(callback, thisArg) {
    var mapped = [];
    var mapAndFlattenElementIntoArray = function(element, index) {
      var result = callback.call(thisArg, element, index, this);
      if (Array.isArray(result)) {
        mapped.push.apply(mapped, result);
      } else {
        // NOTE: The specification says the callback can return a non-Array.
        // We intentionally don't include that in the type information on
        // this function or the corresponding extern in order to encourage
        // more readable code and avoid complex TTL in the type annotations,
        // but we still want to behave correctly if the callback gives us a
        // non-Array.
        mapped.push(result);
      }
    };
    // Use Array.prototype explicitly since IE11 doesn't support forEach
    // on array-like NodeList
    Array.prototype.forEach.call(this, mapAndFlattenElementIntoArray);
    return mapped;
  };

  return flatMap;
}, 'es9', 'es5');