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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. This binary checks for style issues such as incorrect or missing JSDoc usage, and missing goog.require() statements. It does not do more advanced checks such as typechecking.

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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/polyfill';

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

  /**
   * Creates a new Array from an array-like or iterable object.
   *
   * Polyfills the static function Array.from().  Does not support
   * constructor inheritance (i.e. (subclass of Array).from), and
   * relies on the compiler to check the validity of inputs rather
   * than producing spec-compliant TypeErrors.
   *
   * @param {!IArrayLike|!Iterable} arrayLike
   *     An array-like or iterable.
   * @param {(function(this: THIS, INPUT, number): OUTPUT)=} opt_mapFn
   *     Function to call on each argument.
   * @param {THIS=} opt_thisArg
   *     Object to use as 'this' when calling mapFn.
   * @return {!Array}
   * @template INPUT, OUTPUT, THIS
   * @suppress {reportUnknownTypes}
   */
  var polyfill = function(arrayLike, opt_mapFn, opt_thisArg) {
    opt_mapFn = opt_mapFn != null ? opt_mapFn : function(x) { return x; };
    var result = [];
    // NOTE: this is cast to ? because [] on @struct is an error
    var iteratorFunction = typeof Symbol != 'undefined' && Symbol.iterator &&
        (/** @type {?} */ (arrayLike)[Symbol.iterator]);
    if (typeof iteratorFunction == 'function') {
      arrayLike = iteratorFunction.call(arrayLike);
      var next;
      var k = 0;
      while (!(next = arrayLike.next()).done) {
        result.push(
            opt_mapFn.call(/** @type {?} */ (opt_thisArg), next.value, k++));
      }
    } else {
      var len = arrayLike.length;  // need to support non-iterables
      for (var i = 0; i < len; i++) {
        result.push(
            opt_mapFn.call(/** @type {?} */ (opt_thisArg), arrayLike[i], i));
      }
    }
    return result;
  };

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