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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.
 */

/**
 * @fileoverview Provides methods to polyfill native objects.
 * @suppress {uselessCode}
 */
'require util/defineproperty';
'require util/defines';
'require util/global';
'require util/shouldpolyfill';

/** @const {!Object} map from classes (Map) to polyfills */
$jscomp.polyfills = {};

/**
 * Maps a property to an obfuscated look-up string for that property. Only used
 * when `$jscomp.ISOLATE_POLYFILLS is enabled.
 *
 * When compiling with polyfill isolation enabled, to polyfill an
 * `owner.prototype.method` the `$jscomp.polyfill` library function:
 * 
 *  creates an obfuscated string for `method`
 * 
 adds the obfuscated string as a new property on the `owner` object
 * 
 assigns the polyfill implementation of `method` to this new obfuscated
 * property on the `owner` object. 
 *
 *
 * This maps the original property name `method` to the new obfuscated property
 * name.
 * @const {!Object}
 */
$jscomp.propertyToPolyfillSymbol = {};

/** @const {string} */
$jscomp.POLYFILL_PREFIX = '$jscp$';

/**
 * Returns the polyfill function of the given property if present, otherwise
 * returns the actual property.
 *
 * This is a variable instead of a $jscomp.* property to make it simpler for
 * the compiler to avoid prematurely deleting it during optimizations.
 *
 * @param {*} target the receiver for the property access, e.g. `my.str` in
 *     `my.str.includes`
 * @param {string} property the name of the property, e.g. `includes` in
 *     `my.str.includes`
 * @param {boolean=} isOptionalAccess whether the access to property is via the
 *     optional chaining operator, e.g. `my.str?.includes`
 * @return {?} if an obfuscated symbol for the property was added onto the
 *     target, then this function returns the polyfill for the property.
 *     Otherwise simply looks up the property on the target and returns it as
 *     `target[key]`.
 * @noinline prevent inlining so IsolatePolyfills can find this declaration.
 * @suppress {reportUnknownTypes}
 */
var $jscomp$lookupPolyfilledValue = function(
    target, property, isOptionalAccess) {
  if (isOptionalAccess && target == null) {
    // For optional chain accesses such as `my.str?.includes`, we should not
    // crash and instead return undefined.
    return undefined;
  }

  /** @const */
  var obfuscatedName = $jscomp.propertyToPolyfillSymbol[property];
  if (obfuscatedName == null) {
    return target[property];
  }

  // NOTE: this will throw if `target` is null or undefined.
  /** @const */
  var polyfill = target[obfuscatedName];

  // Checking `polyfill !== undefined` is sufficient because $jscomp.polyfill
  // never installs nullish polyfills. Otherwise we'd have to check something
  // like `key in target`.
  return polyfill !== undefined ? polyfill : target[property];
};

/**
 * @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`.
 * @noinline
 * NOTE: We prevent inlining so RemoveUnusedPolyfills can always recognize this
 * call.
 */
$jscomp.polyfill = function(target, polyfill, fromLang, toLang) {
  if (!polyfill) return;
  if ($jscomp.ISOLATE_POLYFILLS) {
    $jscomp.polyfillIsolated(target, polyfill, fromLang, toLang);
  } else {
    $jscomp.polyfillUnisolated(target, polyfill, fromLang, toLang);
  }
};

/**
 * @param {string} target Qualified name of the class or method to polyfill,
 * @param {function(*): *} polyfill A function that takes the current browser
 *     implementation of the target and returns an optional new polyfill
 *     implementation.
 * @param {string=} fromLang The language level in which the target is expected
 *     to already be present in the browser.
 * @param {string=} toLang The language level required by the polyfill
 *     implementation.
 * @suppress {reportUnknownTypes}
 * @see $jscomp.polyfill for more documentation
 */
$jscomp.polyfillUnisolated = function(target, polyfill, fromLang, toLang) {
  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)) return;
    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});
};

/**
 * Defines a polyfill for the given class or method under an obfuscated name.
 *
 * The main differences between this method and $jscomp.polyfillUnisolated are:
 *  - classes are defined on $jscomp.polyfills, not window.
 *  - methods (Array.prototype.includes) are defined on Array.prototype under
 *    an obfuscated name Array.prototype.$jscp$907312$includes or a Symbol, if
 * native.
 *  - this method installs our polyfill even when an existing implementation is
 *    found, as it might be an untrusted polyfill. The exception is that if
 *    Symbol is detected to be native, we assume any implementations of ES6
 *    classes/methods are also native and not polyfills.
 *
 * @param {string} target Qualified name of the class or method to polyfill,
 * @param {function(*): *} polyfill A function that takes the current browser
 *     implementation of the target and returns an optional new polyfill
 *     implementation.
 * @param {string} fromLang The language level in which the target is expected
 *     to already be present in the browser.
 * @param {string} toLang The language level required by the polyfill
 *     implementation.
 * @suppress {reportUnknownTypes}
 * @see $jscomp.polyfill for more parameter documentation
 */
$jscomp.polyfillIsolated = function(target, polyfill, fromLang, toLang) {
  var split = target.split('.');
  var isSimpleName = split.length === 1;
  var root = split[0];

  // Look up the parent object. For classes this is just $jscomp.global.
  // For methods/properties this may be a polyfill (Promise) or may a native
  // object (Array.prototype).
  var ownerObject;
  if (!isSimpleName && root in $jscomp.polyfills) {
    // Handle Symbol.asyncIterator and Promise.prototype.finally
    ownerObject = $jscomp.polyfills;
  } else {
    // Handle Promise, Array.of, and String.prototype.startsWith
    ownerObject = $jscomp.global;
  }

  for (var i = 0; i < split.length - 1; i++) {
    var key = split[i];
    if (!(key in ownerObject)) return;
    ownerObject = ownerObject[key];
  }

  var property = split[split.length - 1];
  // If Symbol is native and the target is in the ES6 spec, use the native imp.
  // We assume the method/class was not polyfilled since polyfills typically
  // back off in the presence of an existing implementation.
  var nativeImpl = $jscomp.IS_SYMBOL_NATIVE && fromLang === 'es6' ?
      ownerObject[property] :
      null;
  var impl = polyfill(nativeImpl);

  if (impl == null) {
    return;
  }

  if (isSimpleName) {
    // Note: `impl` may be the actual native class instead of a
    // polyfill. Add it to $jscomp.polyfills anyway. The IsolatePolyfills pass
    // unconditionally replaces `Symbol` with `$jscomp.polyfills['Symbol']`.

    // Use $jscomp.polyfills instead of ownerObject. For simple names like
    // Promise and Symbol, ownerObject is the global object, and we want to
    // avoid adding new global variables.
    $jscomp.defineProperty(
        $jscomp.polyfills, property,
        {configurable: true, writable: true, value: impl});
  } else if (impl !== nativeImpl) {
    // Skip installing an obfuscated property if we have found a native version
    // of the method we're polyfilling. $jscomp$lookupPolyfilledValue will fall
    // back to the native version anyway.
    if ($jscomp.propertyToPolyfillSymbol[property] === undefined) {
      var BIN_ID = (Math.random() * 1e9) >>> 0;
      $jscomp.propertyToPolyfillSymbol[property] = $jscomp.IS_SYMBOL_NATIVE ?
          // use bracket access to avoid injecting the Symbol polyfill
          $jscomp.global['Symbol'](property) :
          $jscomp.POLYFILL_PREFIX + BIN_ID + '$' + property;
    }

    /** @const {symbol} */
    var obfuscatedName = $jscomp.propertyToPolyfillSymbol[property];

    // Define the polyfilled method on its owner but under an obfuscated
    // name to avoid collisions. The owner will be a native class like `Promise`
    // or a native class's prototype like `Array.prototype`.
    $jscomp.defineProperty(
        ownerObject, obfuscatedName,
        {configurable: true, writable: true, value: impl});
  }
};