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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.
 */

/**
 * @fileoverview Provides methods to polyfill native objects.
 */
'require util/defineproperty';
'require util/global';


/**
 * @param {string} target Qualified name of the class or method to polyfill,
 *     e.g. 'Array.prototype.includes' or 'Map'.
 * @param {?function(*): *} polyfill A function that takes the current browser
 *     implementation of the target and returns an optional new polyfill
 *     implementation.  If null is returned, then no polyfill will be added.  A
 *     null argument for this parameter indicates that the function will not be
 *     polyfilled, and is only useful for `build_polyfill_table.js` bookkeeping.
 * @param {string} fromLang The language level in which the target is expected
 *     to already be present in the browser.  The compiler requires that
 *     `languageOut < fromLang` before injecting a polyfill (i.e. if the
 *     specified output language already includes the feature then there's no
 *     need to polyfill it).
 * @param {string} toLang The language level required by the polyfill
 *     implementation.  The compiler will issue an error if a polyfill is
 *     required, but `languageOut < toLang`.  Additionally, the
 *     `build_polyfill_table.js` script audits the polyfill dependency tree to
 *     ensure that no polyfill with a lower `toLang` depends on one with a
 *     higher `toLang`.
 * @suppress {reportUnknownTypes}
 * @noinline
 * NOTE: We prevent inlining so RemoveUnusedPolyfills can always recognize this
 * call.
 */
$jscomp.polyfill = function(target, polyfill, fromLang, toLang) {
  if (!polyfill) return;
  var obj = $jscomp.global;
  var split = target.split('.');
  for (var i = 0; i < split.length - 1; i++) {
    var key = split[i];
    if (!(key in obj)) obj[key] = {};  // Might want to be defineProperty.
    obj = obj[key];
  }
  var property = split[split.length - 1];
  var orig = obj[property];
  var impl = polyfill(orig);
  if (impl == orig || impl == null) return;
  $jscomp.defineProperty(
      obj, property, {configurable: true, writable: true, value: impl});
};