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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
 * @suppress {uselessCode}
 */
'require es6/symbol';
'require util/polyfill';

$jscomp.polyfill('String.prototype.matchAll', function(orig) {
  if (orig) return orig;

  // We depend on Symbol.iterator, so ensure it's loaded.
  $jscomp.initSymbolIterator();

  /**
   * Returns an iterator of all results matching a string against a
   * regular expression, including capturing groups.
   *
   * Polyfills the instance method String.prototype.matchAll().
   *
   * The spec calls for any non-RegExp object to be automatically converted,
   * so we accept anything here, even though our externs only allow
   * RegExp|string.
   *
   * @this {string}
   * @param {*} regexp
   * A regular expression object. If a non-RegExp object obj is passed,
   * it is implicitly converted to a RegExp with a global tag by using
   * new RegExp(obj).
   * @return {!IteratorIterable}
   */
  var polyfill = function(regexp) {
    if (regexp instanceof RegExp && !regexp.global) {
      throw new TypeError('RegExp passed into String.prototype.matchAll() must have global tag.');
    }
    var /** !RegExp */ regexCopy =
        new RegExp(regexp, regexp instanceof RegExp ? undefined : 'g');
    var matchString = this;
    var /** boolean */ finished = false;
    var matchAllIterator = {
      next: function() {
        var result = {};
        var previousIndex = regexCopy.lastIndex;
        if (finished) {
          return {value: undefined, done: true};
        }
        var match = regexCopy.exec(matchString);
        if (!match) {
          finished = true;
          return {value: undefined, done: true};
        }
        if (regexCopy.lastIndex === previousIndex) {
          // matchAll() is not allowed to get "stuck" returning an empty
          // string match infinitely, so we must make sure lastIndex always
          // increases.
          regexCopy.lastIndex += 1;
        }
        result.value = match;
        result.done = false;
        return result;
      }
    };
    matchAllIterator[Symbol.iterator] = function() { return matchAllIterator; };
    return /**@type {!IteratorIterable}> */ (matchAllIterator);
  };
  return polyfill;
}, 'es_2020', 'es3');