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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 2017 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/global';


/**
 * @fileoverview Check one of the most obscure features of ES6 as a proxy
 * for full conformance.  If this is enabled, this test is used instead of
 * larger, more specific conformance tests.
 */

/**
 * Check ES6 conformance by checking an obscure detail of Proxy that
 * wasn't implemented correctly until after all other ES6 features in
 * most browsers.
 * @return {boolean} Whether Proxy works correctly.
 * @suppress {reportUnknownTypes}
 */
$jscomp.checkEs6ConformanceViaProxy = function() {
  try {
    var proxied = {};
    var proxy = Object.create(new $jscomp.global['Proxy'](proxied, {
      'get': function (target, key, receiver) {
        return target == proxied && key == 'q' && receiver == proxy;
      }
    }));
    return proxy['q'] === true;
  } catch (err) {
    return false;
  }
};

/**
 * If this is true, assume that a runtime which implements Proxy also
 * implements the rest of the ECMAScript 2015 spec.
 * @define {boolean}
 */
$jscomp.USE_PROXY_FOR_ES6_CONFORMANCE_CHECKS = false;

/**
 * Whether the runtime implements the entire ECMAScript 2015 spec.
 * @const {boolean}
 */
$jscomp.ES6_CONFORMANCE =
    $jscomp.USE_PROXY_FOR_ES6_CONFORMANCE_CHECKS &&
    $jscomp.checkEs6ConformanceViaProxy();