text: /*
   * Copyright 2016 Google Inc. All rights reserved.
   *
   * 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.
   */
  
  module.exports = function proxyPolyfill() {
    let lastRevokeFn = null;
    let ProxyPolyfill;
  
    /**
     * @param {*} o
     * @return {boolean} whether this is probably a (non-null) Object
     */
    function isObject(o) {
      return o ? (typeof o === 'object' || typeof o === 'function') : false;
    }
  
    /**
     * @constructor
     * @param {!Object} target
     * @param {{apply, construct, get, set}} handler
     */
    ProxyPolyfill = function(target, handler) {
      if (!isObject(target) || !isObject(handler)) {
        throw new TypeError('Cannot create proxy with a non-object as target or handler');
      }
  
      // Construct revoke function, and set lastRevokeFn so that Proxy.revocable can steal it.
      // The caller might get the wrong revoke function if a user replaces or wraps scope.Proxy
      // to call itself, but that seems unlikely especially when using the polyfill.
      let throwRevoked = function() {};
      lastRevokeFn = function() {
        /** @suppress {checkTypes} */
        target = null;  // clear ref
        throwRevoked = function(trap) {
          throw new TypeError(`Cannot perform '${trap}' on a proxy that has been revoked`);
        };
      };
      setTimeout(function() {
        lastRevokeFn = null;
      }, 0);
  
      // Fail on unsupported traps: Chrome doesn't do this, but ensure that users of the polyfill
      // are a bit more careful. Copy the internal parts of handler to prevent user changes.
      const unsafeHandler = handler;
      handler = { 'get': null, 'set': null, 'apply': null, 'construct': null };
      for (let k in unsafeHandler) {
        if (!(k in handler)) {
          throw new TypeError(`Proxy polyfill does not support trap '${k}'`);
        }
        handler[k] = unsafeHandler[k];
      }
      if (typeof unsafeHandler === 'function') {
        // Allow handler to be a function (which has an 'apply' method). This matches what is
        // probably a bug in native versions. It treats the apply call as a trap to be configured.
        handler.apply = unsafeHandler.apply.bind(unsafeHandler);
      }
  
      // Define proxy as this, or a Function (if either it's callable, or apply is set).
      // TODO(samthor): Closure compiler doesn't know about 'construct', attempts to rename it.
      let proxy = this;
      let isMethod = false;
      let isArray = false;
      if (typeof target === 'function') {
        proxy = function ProxyPolyfill() {
          const usingNew = (this && this.constructor === proxy);
          const args = Array.prototype.slice.call(arguments);
          throwRevoked(usingNew ? 'construct' : 'apply');
  
          if (usingNew && handler['construct']) {
            return handler['construct'].call(this, target, args);
          } else if (!usingNew && handler.apply) {
            return handler['apply'](target, this, args);
          }
  
          // since the target was a function, fallback to calling it directly.
          if (usingNew) {
            // inspired by answers to https://stackoverflow.com/q/1606797
            args.unshift(target);  // pass class as first arg to constructor, although irrelevant
            // nb. cast to convince Closure compiler that this is a constructor
            const f = /** @type {!Function} */ (target.bind.apply(target, args));
            return new f();
          }
          return target.apply(this, args);
        };
        isMethod = true;
      } else if (target instanceof Array) {
        proxy = [];
        isArray = true;
      }
  
      // Create default getters/setters. Create different code paths as handler.get/handler.set can't
      // change after creation.
      const getter = handler.get ? function(prop) {
        throwRevoked('get');
        return handler.get(this, prop, proxy);
      } : function(prop) {
        throwRevoked('get');
        return this[prop];
      };
      const setter = handler.set ? function(prop, value) {
        throwRevoked('set');
        const status = handler.set(this, prop, value, proxy);
        // TODO(samthor): If the calling code is in strict mode, throw TypeError.
        // if (!status) {
          // It's (sometimes) possible to work this out, if this code isn't strict- try to load the
          // callee, and if it's available, that code is non-strict. However, this isn't exhaustive.
        // }
      } : function(prop, value) {
        throwRevoked('set');
        this[prop] = value;
      };
  
      // Clone direct properties (i.e., not part of a prototype).
      const propertyNames = Object.getOwnPropertyNames(target);
      const propertyMap = {};
      propertyNames.forEach(function(prop) {
        if ((isMethod || isArray) && prop in proxy) {
          return;  // ignore properties already here, e.g. 'bind', 'prototype' etc
        }
        const real = Object.getOwnPropertyDescriptor(target, prop);
        const desc = {
          enumerable: !!real.enumerable,
          get: getter.bind(target, prop),
          set: setter.bind(target, prop),
        };
        Object.defineProperty(proxy, prop, desc);
        propertyMap[prop] = true;
      });
  
      // Set the prototype, or clone all prototype methods (always required if a getter is provided).
      // TODO(samthor): We don't allow prototype methods to be set. It's (even more) awkward.
      // An alternative here would be to _just_ clone methods to keep behavior consistent.
      let prototypeOk = true;
      if (Object.setPrototypeOf) {
        Object.setPrototypeOf(proxy, Object.getPrototypeOf(target));
      } else if (proxy.__proto__) {
        proxy.__proto__ = target.__proto__;
      } else {
        prototypeOk = false;
      }
      if (handler.get || !prototypeOk) {
        for (let k in target) {
          if (propertyMap[k]) {
            continue;
          }
          Object.defineProperty(proxy, k, { get: getter.bind(target, k) });
        }
      }
  
      // The Proxy polyfill cannot handle adding new properties. Seal the target and proxy.
      Object.seal(target);
      Object.seal(proxy);
  
      return proxy;  // nb. if isMethod is true, proxy != this
    };
  
    ProxyPolyfill.revocable = function(target, handler) {
      const p = new ProxyPolyfill(target, handler);
      return { 'proxy': p, 'revoke': lastRevokeFn };
    };
  
    return ProxyPolyfill;
  }