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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/polyfill';

$jscomp.polyfill('Object.is', function(orig) {
  if (orig) return orig;

  /**
   * Polyfill for Object.is() method:
   * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is
   *
   * Determines whether two values are the same value (that is,
   * functionally equivalent).  This is the same as ===-equality,
   * except for two cases: 0 is not the same as -0, and NaN is
   * the same as NaN.
   *
   * @param {*} left
   * @param {*} right
   * @return {boolean}
   */
  var polyfill = function(left, right) {
    if (left === right) {
      // Handle the 0 === -0 exception
      return (left !== 0) || (1 / left === 1 / /** @type {number} */ (right));
    } else {
      // Handle the NaN !== NaN exception
      return (left !== left) && (right !== right);
    }
  };

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