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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 2015 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 Polyfills for ES6 Object functions.
 */

$jscomp.object = $jscomp.object || {};


/**
 * Polyfill for Object.assign() method:
 * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign
 *
 * Copies values of all enumerable own properties from one or more
 * sources to the given target object, and returns the target.
 * @param {!Object} target The target object onto which to copy.
 * @param {...?Object} sources The source objects.
 * @return {!Object} The target object is returned.
 */
$jscomp.object.assign = function(target, ...sources) {
  for (const source of sources) {
    if (!source) continue;
    for (const key in source) {
      // Note: it's possible that source.hasOwnPropery was overwritten,
      // so call the version on Object.prototype just to be sure.
      if (Object.prototype.hasOwnProperty.call(source, key)) {
        target[key] = source[key];
      }
    }
  }
  return target;
};


/**
 * 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}
 */
$jscomp.object.is = 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);
  }
};