selenium-webdriver/lib/webdriver.js

The official WebDriver JavaScript bindings from the Selenium project

// Licensed to the Software Freedom Conservancy (SFC) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The SFC licenses this file
// to you 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 The heart of the WebDriver JavaScript API.
 */

'use strict';

const actions = require('./actions');
const by = require('./by');
const Capabilities = require('./capabilities').Capabilities;
const command = require('./command');
const error = require('./error');
const input = require('./input');
const logging = require('./logging');
const {Session} = require('./session');
const Symbols = require('./symbols');
const promise = require('./promise');


/**
 * Defines a condition for use with WebDriver's {@linkplain WebDriver#wait wait
 * command}.
 *
 * @template OUT
 */
class Condition {
  /**
   * @param {string} message A descriptive error message. Should complete the
   *     sentence "Waiting [...]"
   * @param {function(!WebDriver): OUT} fn The condition function to
   *     evaluate on each iteration of the wait loop.
   */
  constructor(message, fn) {
    /** @private {string} */
    this.description_ = 'Waiting ' + message;

    /** @type {function(!WebDriver): OUT} */
    this.fn = fn;
  }

  /** @return {string} A description of this condition. */
  description() {
    return this.description_;
  }
}


/**
 * Defines a condition that will result in a {@link WebElement}.
 *
 * @extends {Condition<!(WebElement|IThenable<!WebElement>)>}
 */
class WebElementCondition extends Condition {
  /**
   * @param {string} message A descriptive error message. Should complete the
   *     sentence "Waiting [...]"
   * @param {function(!WebDriver): !(WebElement|IThenable<!WebElement>)}
   *     fn The condition function to evaluate on each iteration of the wait
   *     loop.
   */
  constructor(message, fn) {
    super(message, fn);
  }
}


//////////////////////////////////////////////////////////////////////////////
//
//  WebDriver
//
//////////////////////////////////////////////////////////////////////////////


/**
 * Translates a command to its wire-protocol representation before passing it
 * to the given `executor` for execution.
 * @param {!command.Executor} executor The executor to use.
 * @param {!command.Command} command The command to execute.
 * @return {!Promise} A promise that will resolve with the command response.
 */
function executeCommand(executor, command) {
  return toWireValue(command.getParameters()).
      then(function(parameters) {
        command.setParameters(parameters);
        return executor.execute(command);
      });
}


/**
 * Converts an object to its JSON representation in the WebDriver wire protocol.
 * When converting values of type object, the following steps will be taken:
 * <ol>
 * <li>if the object is a WebElement, the return value will be the element's
 *     server ID
 * <li>if the object defines a {@link Symbols.serialize} method, this algorithm
 *     will be recursively applied to the object's serialized representation
 * <li>if the object provides a "toJSON" function, this algorithm will
 *     recursively be applied to the result of that function
 * <li>otherwise, the value of each key will be recursively converted according
 *     to the rules above.
 * </ol>
 *
 * @param {*} obj The object to convert.
 * @return {!Promise<?>} A promise that will resolve to the input value's JSON
 *     representation.
 */
function toWireValue(obj) {
  if (promise.isPromise(obj)) {
    return Promise.resolve(obj).then(toWireValue);
  }
  return Promise.resolve(convertValue(obj));
}


function convertValue(value) {
  if (value === void 0 || value === null) {
    return value;
  }

  if (typeof value === 'boolean'
      || typeof value === 'number'
      || typeof value === 'string') {
    return value;
  }

  if (Array.isArray(value)) {
    return convertKeys(value);
  }

  if (typeof value === 'function') {
    return '' + value;
  }

  if (typeof value[Symbols.serialize] === 'function') {
    return toWireValue(value[Symbols.serialize]());
  } else if (typeof value.toJSON === 'function') {
    return toWireValue(value.toJSON());
  }
  return convertKeys(value);
}


function convertKeys(obj) {
  const isArray = Array.isArray(obj);
  const numKeys = isArray ? obj.length : Object.keys(obj).length;
  const ret = isArray ? new Array(numKeys) : {};
  if (!numKeys) {
    return Promise.resolve(ret);
  }

  let numResolved = 0;

  function forEachKey(obj, fn) {
    if (Array.isArray(obj)) {
      for (let i = 0, n = obj.length; i < n; i++) {
        fn(obj[i], i);
      }
    } else {
      for (let key in obj) {
        fn(obj[key], key);
      }
    }
  }

  return new Promise(function(done, reject) {
    forEachKey(obj, function(value, key) {
      if (promise.isPromise(value)) {
        value.then(toWireValue).then(setValue, reject);
      } else {
        value = convertValue(value);
        if (promise.isPromise(value)) {
          value.then(toWireValue).then(setValue, reject);
        } else {
          setValue(value);
        }
      }

      function setValue(value) {
        ret[key] = value;
        maybeFulfill();
      }
    });

    function maybeFulfill() {
      if (++numResolved === numKeys) {
        done(ret);
      }
    }
  });
}


/**
 * Converts a value from its JSON representation according to the WebDriver wire
 * protocol. Any JSON object that defines a WebElement ID will be decoded to a
 * {@link WebElement} object. All other values will be passed through as is.
 *
 * @param {!WebDriver} driver The driver to use as the parent of any unwrapped
 *     {@link WebElement} values.
 * @param {*} value The value to convert.
 * @return {*} The converted value.
 */
function fromWireValue(driver, value) {
  if (Array.isArray(value)) {
    value = value.map(v => fromWireValue(driver, v));
  } else if (WebElement.isId(value)) {
    let id = WebElement.extractId(value);
    value = new WebElement(driver, id);
  } else if (value && typeof value === 'object') {
    let result = {};
    for (let key in value) {
      if (value.hasOwnProperty(key)) {
        result[key] = fromWireValue(driver, value[key]);
      }
    }
    value = result;
  }
  return value;
}


/**
 * Structural interface for a WebDriver client.
 *
 * @record
 */
class IWebDriver {

  /** @return {!promise.ControlFlow} The control flow used by this instance. */
  controlFlow() {}

  /**
   * Schedules a {@link command.Command} to be executed by this driver's
   * {@link command.Executor}.
   *
   * @param {!command.Command} command The command to schedule.
   * @param {string} description A description of the command for debugging.
   * @return {!promise.Thenable<T>} A promise that will be resolved
   *     with the command result.
   * @template T
   */
  schedule(command, description) {}

  /**
   * Sets the {@linkplain input.FileDetector file detector} that should be
   * used with this instance.
   * @param {input.FileDetector} detector The detector to use or {@code null}.
   */
  setFileDetector(detector) {}

  /**
   * @return {!command.Executor} The command executor used by this instance.
   */
  getExecutor() {}

  /**
   * @return {!promise.Thenable<!Session>} A promise for this client's session.
   */
  getSession() {}

  /**
   * @return {!promise.Thenable<!Capabilities>} A promise that will resolve with
   *     the this instance's capabilities.
   */
  getCapabilities() {}

  /**
   * Terminates the browser session. After calling quit, this instance will be
   * invalidated and may no longer be used to issue commands against the
   * browser.
   *
   * @return {!promise.Thenable<void>} A promise that will be resolved when the
   *     command has completed.
   */
  quit() {}

  /**
   * Creates a new action sequence using this driver. The sequence will not be
   * scheduled for execution until {@link actions.ActionSequence#perform} is
   * called. Example:
   *
   *     driver.actions().
   *         mouseDown(element1).
   *         mouseMove(element2).
   *         mouseUp().
   *         perform();
   *
   * @return {!actions.ActionSequence} A new action sequence for this instance.
   */
  actions() {}

  /**
   * Creates a new touch sequence using this driver. The sequence will not be
   * scheduled for execution until {@link actions.TouchSequence#perform} is
   * called. Example:
   *
   *     driver.touchActions().
   *         tap(element1).
   *         doubleTap(element2).
   *         perform();
   *
   * @return {!actions.TouchSequence} A new touch sequence for this instance.
   */
  touchActions() {}

  /**
   * Schedules a command to execute JavaScript in the context of the currently
   * selected frame or window. The script fragment will be executed as the body
   * of an anonymous function. If the script is provided as a function object,
   * that function will be converted to a string for injection into the target
   * window.
   *
   * Any arguments provided in addition to the script will be included as script
   * arguments and may be referenced using the {@code arguments} object.
   * Arguments may be a boolean, number, string, or {@linkplain WebElement}.
   * Arrays and objects may also be used as script arguments as long as each item
   * adheres to the types previously mentioned.
   *
   * The script may refer to any variables accessible from the current window.
   * Furthermore, the script will execute in the window's context, thus
   * {@code document} may be used to refer to the current document. Any local
   * variables will not be available once the script has finished executing,
   * though global variables will persist.
   *
   * If the script has a return value (i.e. if the script contains a return
   * statement), then the following steps will be taken for resolving this
   * functions return value:
   *
   * - For a HTML element, the value will resolve to a {@linkplain WebElement}
   * - Null and undefined return values will resolve to null</li>
   * - Booleans, numbers, and strings will resolve as is</li>
   * - Functions will resolve to their string representation</li>
   * - For arrays and objects, each member item will be converted according to
   *     the rules above
   *
   * @param {!(string|Function)} script The script to execute.
   * @param {...*} var_args The arguments to pass to the script.
   * @return {!promise.Thenable<T>} A promise that will resolve to the
   *    scripts return value.
   * @template T
   */
  executeScript(script, var_args) {}

  /**
   * Schedules a command to execute asynchronous JavaScript in the context of the
   * currently selected frame or window. The script fragment will be executed as
   * the body of an anonymous function. If the script is provided as a function
   * object, that function will be converted to a string for injection into the
   * target window.
   *
   * Any arguments provided in addition to the script will be included as script
   * arguments and may be referenced using the {@code arguments} object.
   * Arguments may be a boolean, number, string, or {@code WebElement}.
   * Arrays and objects may also be used as script arguments as long as each item
   * adheres to the types previously mentioned.
   *
   * Unlike executing synchronous JavaScript with {@link #executeScript},
   * scripts executed with this function must explicitly signal they are finished
   * by invoking the provided callback. This callback will always be injected
   * into the executed function as the last argument, and thus may be referenced
   * with {@code arguments[arguments.length - 1]}. The following steps will be
   * taken for resolving this functions return value against the first argument
   * to the script's callback function:
   *
   * - For a HTML element, the value will resolve to a
   *     {@link WebElement}
   * - Null and undefined return values will resolve to null
   * - Booleans, numbers, and strings will resolve as is
   * - Functions will resolve to their string representation
   * - For arrays and objects, each member item will be converted according to
   *     the rules above
   *
   * __Example #1:__ Performing a sleep that is synchronized with the currently
   * selected window:
   *
   *     var start = new Date().getTime();
   *     driver.executeAsyncScript(
   *         'window.setTimeout(arguments[arguments.length - 1], 500);').
   *         then(function() {
   *           console.log(
   *               'Elapsed time: ' + (new Date().getTime() - start) + ' ms');
   *         });
   *
   * __Example #2:__ Synchronizing a test with an AJAX application:
   *
   *     var button = driver.findElement(By.id('compose-button'));
   *     button.click();
   *     driver.executeAsyncScript(
   *         'var callback = arguments[arguments.length - 1];' +
   *         'mailClient.getComposeWindowWidget().onload(callback);');
   *     driver.switchTo().frame('composeWidget');
   *     driver.findElement(By.id('to')).sendKeys('dog@example.com');
   *
   * __Example #3:__ Injecting a XMLHttpRequest and waiting for the result. In
   * this example, the inject script is specified with a function literal. When
   * using this format, the function is converted to a string for injection, so it
   * should not reference any symbols not defined in the scope of the page under
   * test.
   *
   *     driver.executeAsyncScript(function() {
   *       var callback = arguments[arguments.length - 1];
   *       var xhr = new XMLHttpRequest();
   *       xhr.open("GET", "/resource/data.json", true);
   *       xhr.onreadystatechange = function() {
   *         if (xhr.readyState == 4) {
   *           callback(xhr.responseText);
   *         }
   *       };
   *       xhr.send('');
   *     }).then(function(str) {
   *       console.log(JSON.parse(str)['food']);
   *     });
   *
   * @param {!(string|Function)} script The script to execute.
   * @param {...*} var_args The arguments to pass to the script.
   * @return {!promise.Thenable<T>} A promise that will resolve to the
   *    scripts return value.
   * @template T
   */
  executeAsyncScript(script, var_args) {}

  /**
   * Schedules a command to execute a custom function.
   * @param {function(...): (T|IThenable<T>)} fn The function to execute.
   * @param {Object=} opt_scope The object in whose scope to execute the function.
   * @param {...*} var_args Any arguments to pass to the function.
   * @return {!promise.Thenable<T>} A promise that will be resolved'
   *     with the function's result.
   * @template T
   */
  call(fn, opt_scope, var_args) {}

  /**
   * Schedules a command to wait for a condition to hold. The condition may be
   * specified by a {@link Condition}, as a custom function, or as any
   * promise-like thenable.
   *
   * For a {@link Condition} or function, the wait will repeatedly
   * evaluate the condition until it returns a truthy value. If any errors occur
   * while evaluating the condition, they will be allowed to propagate. In the
   * event a condition returns a {@link promise.Promise promise}, the polling
   * loop will wait for it to be resolved and use the resolved value for whether
   * the condition has been satisified. Note the resolution time for a promise
   * is factored into whether a wait has timed out.
   *
   * Note, if the provided condition is a {@link WebElementCondition}, then
   * the wait will return a {@link WebElementPromise} that will resolve to the
   * element that satisified the condition.
   *
   * _Example:_ waiting up to 10 seconds for an element to be present on the
   * page.
   *
   *     var button = driver.wait(until.elementLocated(By.id('foo')), 10000);
   *     button.click();
   *
   * This function may also be used to block the command flow on the resolution
   * of any thenable promise object. When given a promise, the command will
   * simply wait for its resolution before completing. A timeout may be provided
   * to fail the command if the promise does not resolve before the timeout
   * expires.
   *
   * _Example:_ Suppose you have a function, `startTestServer`, that returns a
   * promise for when a server is ready for requests. You can block a WebDriver
   * client on this promise with:
   *
   *     var started = startTestServer();
   *     driver.wait(started, 5 * 1000, 'Server should start within 5 seconds');
   *     driver.get(getServerUrl());
   *
   * @param {!(IThenable<T>|
   *           Condition<T>|
   *           function(!WebDriver): T)} condition The condition to
   *     wait on, defined as a promise, condition object, or  a function to
   *     evaluate as a condition.
   * @param {number=} opt_timeout How long to wait for the condition to be true.
   * @param {string=} opt_message An optional message to use if the wait times
   *     out.
   * @return {!(promise.Thenable<T>|WebElementPromise)} A promise that will be
   *     resolved with the first truthy value returned by the condition
   *     function, or rejected if the condition times out. If the input
   *     input condition is an instance of a {@link WebElementCondition},
   *     the returned value will be a {@link WebElementPromise}.
   * @throws {TypeError} if the provided `condition` is not a valid type.
   * @template T
   */
  wait(condition, opt_timeout, opt_message) {}

  /**
   * Schedules a command to make the driver sleep for the given amount of time.
   * @param {number} ms The amount of time, in milliseconds, to sleep.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when the sleep has finished.
   */
  sleep(ms) {}

  /**
   * Schedules a command to retrieve the current window handle.
   * @return {!promise.Thenable<string>} A promise that will be
   *     resolved with the current window handle.
   */
  getWindowHandle() {}

  /**
   * Schedules a command to retrieve the current list of available window handles.
   * @return {!promise.Thenable<!Array<string>>} A promise that will
   *     be resolved with an array of window handles.
   */
  getAllWindowHandles() {}

  /**
   * Schedules a command to retrieve the current page's source. The page source
   * returned is a representation of the underlying DOM: do not expect it to be
   * formatted or escaped in the same way as the response sent from the web
   * server.
   * @return {!promise.Thenable<string>} A promise that will be
   *     resolved with the current page source.
   */
  getPageSource() {}

  /**
   * Schedules a command to close the current window.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when this command has completed.
   */
  close() {}

  /**
   * Schedules a command to navigate to the given URL.
   * @param {string} url The fully qualified URL to open.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when the document has finished loading.
   */
  get(url) {}

  /**
   * Schedules a command to retrieve the URL of the current page.
   * @return {!promise.Thenable<string>} A promise that will be
   *     resolved with the current URL.
   */
  getCurrentUrl() {}

  /**
   * Schedules a command to retrieve the current page's title.
   * @return {!promise.Thenable<string>} A promise that will be
   *     resolved with the current page's title.
   */
  getTitle() {}

  /**
   * Schedule a command to find an element on the page. If the element cannot be
   * found, a {@link bot.ErrorCode.NO_SUCH_ELEMENT} result will be returned
   * by the driver. Unlike other commands, this error cannot be suppressed. In
   * other words, scheduling a command to find an element doubles as an assert
   * that the element is present on the page. To test whether an element is
   * present on the page, use {@link #findElements}:
   *
   *     driver.findElements(By.id('foo'))
   *         .then(found => console.log('Element found? %s', !!found.length));
   *
   * The search criteria for an element may be defined using one of the
   * factories in the {@link webdriver.By} namespace, or as a short-hand
   * {@link webdriver.By.Hash} object. For example, the following two statements
   * are equivalent:
   *
   *     var e1 = driver.findElement(By.id('foo'));
   *     var e2 = driver.findElement({id:'foo'});
   *
   * You may also provide a custom locator function, which takes as input this
   * instance and returns a {@link WebElement}, or a promise that will resolve
   * to a WebElement. If the returned promise resolves to an array of
   * WebElements, WebDriver will use the first element. For example, to find the
   * first visible link on a page, you could write:
   *
   *     var link = driver.findElement(firstVisibleLink);
   *
   *     function firstVisibleLink(driver) {
   *       var links = driver.findElements(By.tagName('a'));
   *       return promise.filter(links, function(link) {
   *         return link.isDisplayed();
   *       });
   *     }
   *
   * @param {!(by.By|Function)} locator The locator to use.
   * @return {!WebElementPromise} A WebElement that can be used to issue
   *     commands against the located element. If the element is not found, the
   *     element will be invalidated and all scheduled commands aborted.
   */
  findElement(locator) {}

  /**
   * Schedule a command to search for multiple elements on the page.
   *
   * @param {!(by.By|Function)} locator The locator to use.
   * @return {!promise.Thenable<!Array<!WebElement>>} A
   *     promise that will resolve to an array of WebElements.
   */
  findElements(locator) {}

  /**
   * Schedule a command to take a screenshot. The driver makes a best effort to
   * return a screenshot of the following, in order of preference:
   *
   * 1. Entire page
   * 2. Current window
   * 3. Visible portion of the current frame
   * 4. The entire display containing the browser
   *
   * @return {!promise.Thenable<string>} A promise that will be
   *     resolved to the screenshot as a base-64 encoded PNG.
   */
  takeScreenshot() {}

  /**
   * @return {!Options} The options interface for this instance.
   */
  manage() {}

  /**
   * @return {!Navigation} The navigation interface for this instance.
   */
  navigate() {}

  /**
   * @return {!TargetLocator} The target locator interface for this
   *     instance.
   */
  switchTo() {}
}


/**
 * Each WebDriver instance provides automated control over a browser session.
 *
 * @implements {IWebDriver}
 */
class WebDriver {
  /**
   * @param {!(Session|IThenable<!Session>)} session Either a known session or a
   *     promise that will be resolved to a session.
   * @param {!command.Executor} executor The executor to use when sending
   *     commands to the browser.
   * @param {promise.ControlFlow=} opt_flow The flow to
   *     schedule commands through. Defaults to the active flow object.
   * @param {(function(this: void): ?)=} opt_onQuit A function to call, if any,
   *     when the session is terminated.
   */
  constructor(session, executor, opt_flow, opt_onQuit) {
    /** @private {!promise.ControlFlow} */
    this.flow_ = opt_flow || promise.controlFlow();

    /** @private {!promise.Thenable<!Session>} */
    this.session_ = this.flow_.promise(resolve => resolve(session));

    /** @private {!command.Executor} */
    this.executor_ = executor;

    /** @private {input.FileDetector} */
    this.fileDetector_ = null;

    /** @private @const {(function(this: void): ?|undefined)} */
    this.onQuit_ = opt_onQuit;
  }

  /**
   * Creates a new WebDriver client for an existing session.
   * @param {!command.Executor} executor Command executor to use when querying
   *     for session details.
   * @param {string} sessionId ID of the session to attach to.
   * @param {promise.ControlFlow=} opt_flow The control flow all
   *     driver commands should execute under. Defaults to the
   *     {@link promise.controlFlow() currently active}  control flow.
   * @return {!WebDriver} A new client for the specified session.
   */
  static attachToSession(executor, sessionId, opt_flow) {
    let flow = opt_flow || promise.controlFlow();
    let cmd = new command.Command(command.Name.DESCRIBE_SESSION)
        .setParameter('sessionId', sessionId);
    let session = flow.execute(
        () => executeCommand(executor, cmd).catch(err => {
          // The DESCRIBE_SESSION command is not supported by the W3C spec, so
          // if we get back an unknown command, just return a session with
          // unknown capabilities.
          if (err instanceof error.UnknownCommandError) {
            return new Session(sessionId, new Capabilities);
          }
          throw err;
        }),
        'WebDriver.attachToSession()');
    return new WebDriver(session, executor, flow);
  }

  /**
   * Creates a new WebDriver session.
   *
   * By default, the requested session `capabilities` are merely "desired" and
   * the remote end will still create a new session even if it cannot satisfy
   * all of the requested capabilities. You can query which capabilities a
   * session actually has using the
   * {@linkplain #getCapabilities() getCapabilities()} method on the returned
   * WebDriver instance.
   *
   * To define _required capabilities_, provide the `capabilities` as an object
   * literal with `required` and `desired` keys. The `desired` key may be
   * omitted if all capabilities are required, and vice versa. If the server
   * cannot create a session with all of the required capabilities, it will
   * return an {@linkplain error.SessionNotCreatedError}.
   *
   *     let required = new Capabilities().set('browserName', 'firefox');
   *     let desired = new Capabilities().set('version', '45');
   *     let driver = WebDriver.createSession(executor, {required, desired});
   *
   * This function will always return a WebDriver instance. If there is an error
   * creating the session, such as the aforementioned SessionNotCreatedError,
   * the driver will have a rejected {@linkplain #getSession session} promise.
   * It is recommended that this promise is left _unhandled_ so it will
   * propagate through the {@linkplain promise.ControlFlow control flow} and
   * cause subsequent commands to fail.
   *
   *     let required = Capabilities.firefox();
   *     let driver = WebDriver.createSession(executor, {required});
   *
   *     // If the createSession operation failed, then this command will also
   *     // also fail, propagating the creation failure.
   *     driver.get('http://www.google.com').catch(e => console.log(e));
   *
   * @param {!command.Executor} executor The executor to create the new session
   *     with.
   * @param {(!Capabilities|
   *          {desired: (Capabilities|undefined),
   *           required: (Capabilities|undefined)})} capabilities The desired
   *     capabilities for the new session.
   * @param {promise.ControlFlow=} opt_flow The control flow all driver
   *     commands should execute under, including the initial session creation.
   *     Defaults to the {@link promise.controlFlow() currently active}
   *     control flow.
   * @param {(function(new: WebDriver,
   *                   !IThenable<!Session>,
   *                   !command.Executor,
   *                   promise.ControlFlow=))=} opt_ctor
   *    A reference to the constructor of the specific type of WebDriver client
   *    to instantiate. Will create a vanilla {@linkplain WebDriver} instance
   *    if a constructor is not provided.
   * @param {(function(this: void): ?)=} opt_onQuit A callback to invoke when
   *    the newly created session is terminated. This should be used to clean
   *    up any resources associated with the session.
   * @return {!WebDriver} The driver for the newly created session.
   */
  static createSession(
        executor, capabilities, opt_flow, opt_ctor, opt_onQuit) {
    let flow = opt_flow || promise.controlFlow();
    let cmd = new command.Command(command.Name.NEW_SESSION);

    if (capabilities && (capabilities.desired || capabilities.required)) {
      cmd.setParameter('desiredCapabilities', capabilities.desired);
      cmd.setParameter('requiredCapabilities', capabilities.required);
    } else {
      cmd.setParameter('desiredCapabilities', capabilities);
    }

    let session = flow.execute(
        () => executeCommand(executor, cmd),
        'WebDriver.createSession()');
    if (typeof opt_onQuit === 'function') {
      session = session.catch(err => {
        return Promise.resolve(opt_onQuit.call(void 0)).then(_ => {throw err});
      });
    }
    const ctor = opt_ctor || WebDriver;
    return new ctor(session, executor, flow, opt_onQuit);
  }

  /** @override */
  controlFlow() {
    return this.flow_;
  }

  /** @override */
  schedule(command, description) {
    command.setParameter('sessionId', this.session_);

    // If any of the command parameters are rejected promises, those
    // rejections may be reported as unhandled before the control flow
    // attempts to execute the command. To ensure parameters errors
    // propagate through the command itself, we resolve all of the
    // command parameters now, but suppress any errors until the ControlFlow
    // actually executes the command. This addresses scenarios like catching
    // an element not found error in:
    //
    //     driver.findElement(By.id('foo')).click().catch(function(e) {
    //       if (e instanceof NoSuchElementError) {
    //         // Do something.
    //       }
    //     });
    var prepCommand = toWireValue(command.getParameters());
    prepCommand.catch(function() {});

    var flow = this.flow_;
    var executor = this.executor_;
    return flow.execute(() => {
      // Retrieve resolved command parameters; any previously suppressed errors
      // will now propagate up through the control flow as part of the command
      // execution.
      return prepCommand.then(function(parameters) {
        command.setParameters(parameters);
        return executor.execute(command);
      }).then(value => fromWireValue(this, value));
    }, description);
  }

  /** @override */
  setFileDetector(detector) {
    this.fileDetector_ = detector;
  }

  /** @override */
  getExecutor() {
    return this.executor_;
  }

  /** @override */
  getSession() {
    return this.session_;
  }

  /** @override */
  getCapabilities() {
    return this.session_.then(s => s.getCapabilities());
  }

  /** @override */
  quit() {
    var result = this.schedule(
        new command.Command(command.Name.QUIT),
        'WebDriver.quit()');
    // Delete our session ID when the quit command finishes; this will allow us
    // to throw an error when attemnpting to use a driver post-quit.
    return /** @type {!promise.Thenable} */(promise.finally(result, () => {
      this.session_ = this.flow_.promise((_, reject) => {
        reject(new error.NoSuchSessionError(
            'This driver instance does not have a valid session ID ' +
            '(did you call WebDriver.quit()?) and may no longer be used.'));
      });

      // Only want the session rejection to bubble if accessed.
      this.session_.catch(function() {});

      if (this.onQuit_) {
        return this.onQuit_.call(void 0);
      }
    }));
  }

  /** @override */
  actions() {
    return new actions.ActionSequence(this);
  }

  /** @override */
  touchActions() {
    return new actions.TouchSequence(this);
  }

  /** @override */
  executeScript(script, var_args) {
    if (typeof script === 'function') {
      script = 'return (' + script + ').apply(null, arguments);';
    }
    let args =
        arguments.length > 1 ? Array.prototype.slice.call(arguments, 1) : [];
   return this.schedule(
        new command.Command(command.Name.EXECUTE_SCRIPT).
            setParameter('script', script).
            setParameter('args', args),
        'WebDriver.executeScript()');
  }

  /** @override */
  executeAsyncScript(script, var_args) {
    if (typeof script === 'function') {
      script = 'return (' + script + ').apply(null, arguments);';
    }
    let args = Array.prototype.slice.call(arguments, 1);
    return this.schedule(
        new command.Command(command.Name.EXECUTE_ASYNC_SCRIPT).
            setParameter('script', script).
            setParameter('args', args),
        'WebDriver.executeScript()');
  }

  /** @override */
  call(fn, opt_scope, var_args) {
    let args = Array.prototype.slice.call(arguments, 2);
    return this.flow_.execute(function() {
      return promise.fullyResolved(args).then(function(args) {
        if (promise.isGenerator(fn)) {
          args.unshift(fn, opt_scope);
          return promise.consume.apply(null, args);
        }
        return fn.apply(opt_scope, args);
      });
    }, 'WebDriver.call(' + (fn.name || 'function') + ')');
  }

  /** @override */
  wait(condition, opt_timeout, opt_message) {
    if (promise.isPromise(condition)) {
      return this.flow_.wait(
          /** @type {!IThenable} */(condition),
          opt_timeout, opt_message);
    }

    var message = opt_message;
    var fn = /** @type {!Function} */(condition);
    if (condition instanceof Condition) {
      message = message || condition.description();
      fn = condition.fn;
    }

    if (typeof fn !== 'function') {
      throw TypeError(
          'Wait condition must be a promise-like object, function, or a '
              + 'Condition object');
    }

    var driver = this;
    var result = this.flow_.wait(function() {
      if (promise.isGenerator(fn)) {
        return promise.consume(fn, null, [driver]);
      }
      return fn(driver);
    }, opt_timeout, message);

    if (condition instanceof WebElementCondition) {
      result = new WebElementPromise(this, result.then(function(value) {
        if (!(value instanceof WebElement)) {
          throw TypeError(
              'WebElementCondition did not resolve to a WebElement: '
                  + Object.prototype.toString.call(value));
        }
        return value;
      }));
    }
    return result;
  }

  /** @override */
  sleep(ms) {
    return this.flow_.timeout(ms, 'WebDriver.sleep(' + ms + ')');
  }

  /** @override */
  getWindowHandle() {
    return this.schedule(
        new command.Command(command.Name.GET_CURRENT_WINDOW_HANDLE),
        'WebDriver.getWindowHandle()');
  }

  /** @override */
  getAllWindowHandles() {
    return this.schedule(
        new command.Command(command.Name.GET_WINDOW_HANDLES),
        'WebDriver.getAllWindowHandles()');
  }

  /** @override */
  getPageSource() {
    return this.schedule(
        new command.Command(command.Name.GET_PAGE_SOURCE),
        'WebDriver.getPageSource()');
  }

  /** @override */
  close() {
    return this.schedule(new command.Command(command.Name.CLOSE),
                         'WebDriver.close()');
  }

  /** @override */
  get(url) {
    return this.navigate().to(url);
  }

  /** @override */
  getCurrentUrl() {
    return this.schedule(
        new command.Command(command.Name.GET_CURRENT_URL),
        'WebDriver.getCurrentUrl()');
  }

  /** @override */
  getTitle() {
    return this.schedule(new command.Command(command.Name.GET_TITLE),
                         'WebDriver.getTitle()');
  }

  /** @override */
  findElement(locator) {
    let id;
    locator = by.checkedLocator(locator);
    if (typeof locator === 'function') {
      id = this.findElementInternal_(locator, this);
    } else {
      let cmd = new command.Command(command.Name.FIND_ELEMENT).
          setParameter('using', locator.using).
          setParameter('value', locator.value);
      id = this.schedule(cmd, 'WebDriver.findElement(' + locator + ')');
    }
    return new WebElementPromise(this, id);
  }

  /**
   * @param {!Function} locatorFn The locator function to use.
   * @param {!(WebDriver|WebElement)} context The search
   *     context.
   * @return {!promise.Thenable<!WebElement>} A
   *     promise that will resolve to a list of WebElements.
   * @private
   */
  findElementInternal_(locatorFn, context) {
    return this.call(() => locatorFn(context)).then(function(result) {
      if (Array.isArray(result)) {
        result = result[0];
      }
      if (!(result instanceof WebElement)) {
        throw new TypeError('Custom locator did not return a WebElement');
      }
      return result;
    });
  }

  /** @override */
  findElements(locator) {
    locator = by.checkedLocator(locator);
    if (typeof locator === 'function') {
      return this.findElementsInternal_(locator, this);
    } else {
      let cmd = new command.Command(command.Name.FIND_ELEMENTS).
          setParameter('using', locator.using).
          setParameter('value', locator.value);
      let res = this.schedule(cmd, 'WebDriver.findElements(' + locator + ')');
      return res.catch(function(e) {
        if (e instanceof error.NoSuchElementError) {
          return [];
        }
        throw e;
      });
    }
  }

  /**
   * @param {!Function} locatorFn The locator function to use.
   * @param {!(WebDriver|WebElement)} context The search context.
   * @return {!promise.Thenable<!Array<!WebElement>>} A promise that
   *     will resolve to an array of WebElements.
   * @private
   */
  findElementsInternal_(locatorFn, context) {
    return this.call(() => locatorFn(context)).then(function(result) {
      if (result instanceof WebElement) {
        return [result];
      }

      if (!Array.isArray(result)) {
        return [];
      }

      return result.filter(function(item) {
        return item instanceof WebElement;
      });
    });
  }

  /** @override */
  takeScreenshot() {
    return this.schedule(new command.Command(command.Name.SCREENSHOT),
        'WebDriver.takeScreenshot()');
  }

  /** @override */
  manage() {
    return new Options(this);
  }

  /** @override */
  navigate() {
    return new Navigation(this);
  }

  /** @override */
  switchTo() {
    return new TargetLocator(this);
  }
}


/**
 * Interface for navigating back and forth in the browser history.
 *
 * This class should never be instantiated directly. Instead, obtain an instance
 * with
 *
 *    webdriver.navigate()
 *
 * @see WebDriver#navigate()
 */
class Navigation {
  /**
   * @param {!WebDriver} driver The parent driver.
   * @private
   */
  constructor(driver) {
    /** @private {!WebDriver} */
    this.driver_ = driver;
  }

  /**
   * Schedules a command to navigate to a new URL.
   * @param {string} url The URL to navigate to.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when the URL has been loaded.
   */
  to(url) {
    return this.driver_.schedule(
        new command.Command(command.Name.GET).
            setParameter('url', url),
        'WebDriver.navigate().to(' + url + ')');
  }

  /**
   * Schedules a command to move backwards in the browser history.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when the navigation event has completed.
   */
  back() {
    return this.driver_.schedule(
        new command.Command(command.Name.GO_BACK),
        'WebDriver.navigate().back()');
  }

  /**
   * Schedules a command to move forwards in the browser history.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when the navigation event has completed.
   */
  forward() {
    return this.driver_.schedule(
        new command.Command(command.Name.GO_FORWARD),
        'WebDriver.navigate().forward()');
  }

  /**
   * Schedules a command to refresh the current page.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when the navigation event has completed.
   */
  refresh() {
    return this.driver_.schedule(
        new command.Command(command.Name.REFRESH),
        'WebDriver.navigate().refresh()');
  }
}


/**
 * Provides methods for managing browser and driver state.
 *
 * This class should never be instantiated directly. Insead, obtain an instance
 * with {@linkplain WebDriver#manage() webdriver.manage()}.
 */
class Options {
  /**
   * @param {!WebDriver} driver The parent driver.
   * @private
   */
  constructor(driver) {
    /** @private {!WebDriver} */
    this.driver_ = driver;
  }

  /**
   * Schedules a command to add a cookie.
   *
   * __Sample Usage:__
   *
   *     // Set a basic cookie.
   *     driver.options().addCookie({name: 'foo', value: 'bar'});
   *
   *     // Set a cookie that expires in 10 minutes.
   *     let expiry = new Date(Date.now() + (10 * 60 * 1000));
   *     driver.options().addCookie({name: 'foo', value: 'bar', expiry});
   *
   *     // The cookie expiration may also be specified in seconds since epoch.
   *     driver.options().addCookie({
   *       name: 'foo',
   *       value: 'bar',
   *       expiry: Math.floor(Date.now() / 1000)
   *     });
   *
   * @param {!Options.Cookie} spec Defines the cookie to add.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when the cookie has been added to the page.
   * @throws {error.InvalidArgumentError} if any of the cookie parameters are
   *     invalid.
   * @throws {TypeError} if `spec` is not a cookie object.
   */
  addCookie(spec) {
    if (!spec || typeof spec !== 'object') {
      throw TypeError('addCookie called with non-cookie parameter');
    }

    // We do not allow '=' or ';' in the name.
    let name = spec.name;
    if (/[;=]/.test(name)) {
      throw new error.InvalidArgumentError(
          'Invalid cookie name "' + name + '"');
    }

    // We do not allow ';' in value.
    let value = spec.value;
    if (/;/.test(value)) {
      throw new error.InvalidArgumentError(
          'Invalid cookie value "' + value + '"');
    }

    let cookieString = name + '=' + value +
        (spec.domain ? ';domain=' + spec.domain : '') +
        (spec.path ? ';path=' + spec.path : '') +
        (spec.secure ? ';secure' : '');

    let expiry;
    if (typeof spec.expiry === 'number') {
      expiry = Math.floor(spec.expiry);
      cookieString += ';expires=' + new Date(spec.expiry * 1000).toUTCString();
    } else if (spec.expiry instanceof Date) {
      let date = /** @type {!Date} */(spec.expiry);
      expiry = Math.floor(date.getTime() / 1000);
      cookieString += ';expires=' + date.toUTCString();
    }

    return this.driver_.schedule(
        new command.Command(command.Name.ADD_COOKIE).
            setParameter('cookie', {
              'name': name,
              'value': value,
              'path': spec.path,
              'domain': spec.domain,
              'secure': !!spec.secure,
              'expiry': expiry
            }),
        'WebDriver.manage().addCookie(' + cookieString + ')');
  }

  /**
   * Schedules a command to delete all cookies visible to the current page.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when all cookies have been deleted.
   */
  deleteAllCookies() {
    return this.driver_.schedule(
        new command.Command(command.Name.DELETE_ALL_COOKIES),
        'WebDriver.manage().deleteAllCookies()');
  }

  /**
   * Schedules a command to delete the cookie with the given name. This command
   * is a no-op if there is no cookie with the given name visible to the current
   * page.
   * @param {string} name The name of the cookie to delete.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when the cookie has been deleted.
   */
  deleteCookie(name) {
    return this.driver_.schedule(
        new command.Command(command.Name.DELETE_COOKIE).
            setParameter('name', name),
        'WebDriver.manage().deleteCookie(' + name + ')');
  }

  /**
   * Schedules a command to retrieve all cookies visible to the current page.
   * Each cookie will be returned as a JSON object as described by the WebDriver
   * wire protocol.
   * @return {!promise.Thenable<!Array<!Options.Cookie>>} A promise that will be
   *     resolved with the cookies visible to the current browsing context.
   */
  getCookies() {
    return this.driver_.schedule(
        new command.Command(command.Name.GET_ALL_COOKIES),
        'WebDriver.manage().getCookies()');
  }

  /**
   * Schedules a command to retrieve the cookie with the given name. Returns null
   * if there is no such cookie. The cookie will be returned as a JSON object as
   * described by the WebDriver wire protocol.
   *
   * @param {string} name The name of the cookie to retrieve.
   * @return {!promise.Thenable<?Options.Cookie>} A promise that will be resolved
   *     with the named cookie, or `null` if there is no such cookie.
   */
  getCookie(name) {
    return this.getCookies().then(function(cookies) {
      for (let cookie of cookies) {
        if (cookie && cookie['name'] === name) {
          return cookie;
        }
      }
      return null;
    });
  }

  /**
   * @return {!Logs} The interface for managing driver
   *     logs.
   */
  logs() {
    return new Logs(this.driver_);
  }

  /**
   * @return {!Timeouts} The interface for managing driver timeouts.
   */
  timeouts() {
    return new Timeouts(this.driver_);
  }

  /**
   * @return {!Window} The interface for managing the current window.
   */
  window() {
    return new Window(this.driver_);
  }
}


/**
 * A record object describing a browser cookie.
 *
 * @record
 */
Options.Cookie = function() {};


/**
 * The name of the cookie.
 *
 * @type {string}
 */
Options.Cookie.prototype.name;


/**
 * The cookie value.
 *
 * @type {string}
 */
Options.Cookie.prototype.value;


/**
 * The cookie path. Defaults to "/" when adding a cookie.
 *
 * @type {(string|undefined)}
 */
Options.Cookie.prototype.path;


/**
 * The domain the cookie is visible to. Defaults to the current browsing
 * context's document's URL when adding a cookie.
 *
 * @type {(string|undefined)}
 */
Options.Cookie.prototype.domain;


/**
 * Whether the cookie is a secure cookie. Defaults to false when adding a new
 * cookie.
 *
 * @type {(boolean|undefined)}
 */
Options.Cookie.prototype.secure;


/**
 * Whether the cookie is an HTTP only cookie. Defaults to false when adding a
 * new cookie.
 *
 * @type {(boolean|undefined)}
 */
Options.Cookie.prototype.httpOnly;


/**
 * When the cookie expires.
 *
 * When {@linkplain Options#addCookie() adding a cookie}, this may be specified
 * in _seconds_ since Unix epoch (January 1, 1970). The expiry will default to
 * 20 years in the future if omitted.
 *
 * The expiry is always returned in seconds since epoch when
 * {@linkplain Options#getCookies() retrieving cookies} from the browser.
 *
 * @type {(!Date|number|undefined)}
 */
Options.Cookie.prototype.expiry;


/**
 * An interface for managing timeout behavior for WebDriver instances.
 *
 * This class should never be instantiated directly. Insead, obtain an instance
 * with
 *
 *    webdriver.manage().timeouts()
 *
 * @see WebDriver#manage()
 * @see Options#timeouts()
 */
class Timeouts {
  /**
   * @param {!WebDriver} driver The parent driver.
   * @private
   */
  constructor(driver) {
    /** @private {!WebDriver} */
    this.driver_ = driver;
  }

  /**
   * Specifies the amount of time the driver should wait when searching for an
   * element if it is not immediately present.
   *
   * When searching for a single element, the driver should poll the page
   * until the element has been found, or this timeout expires before failing
   * with a {@link bot.ErrorCode.NO_SUCH_ELEMENT} error. When searching
   * for multiple elements, the driver should poll the page until at least one
   * element has been found or this timeout has expired.
   *
   * Setting the wait timeout to 0 (its default value), disables implicit
   * waiting.
   *
   * Increasing the implicit wait timeout should be used judiciously as it
   * will have an adverse effect on test run time, especially when used with
   * slower location strategies like XPath.
   *
   * @param {number} ms The amount of time to wait, in milliseconds.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when the implicit wait timeout has been set.
   */
  implicitlyWait(ms) {
    return this._scheduleCommand(ms, 'implicit', 'implicitlyWait');
  }

  /**
   * Sets the amount of time to wait, in milliseconds, for an asynchronous
   * script to finish execution before returning an error. If the timeout is
   * less than or equal to 0, the script will be allowed to run indefinitely.
   *
   * @param {number} ms The amount of time to wait, in milliseconds.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when the script timeout has been set.
   */
  setScriptTimeout(ms) {
    return this._scheduleCommand(ms, 'script', 'setScriptTimeout');
  }

  /**
   * Sets the amount of time to wait for a page load to complete before
   * returning an error.  If the timeout is negative, page loads may be
   * indefinite.
   *
   * @param {number} ms The amount of time to wait, in milliseconds.
   * @return {!promise.Thenable<void>} A promise that will be resolved
   *     when the timeout has been set.
   */
  pageLoadTimeout(ms) {
    return this._scheduleCommand(ms, 'page load', 'pageLoadTimeout');
  }

  _scheduleCommand(ms, timeoutIdentifier, timeoutName) {
    return this.driver_.schedule(
        new command.Command(command.Name.SET_TIMEOUT).
            setParameter('type', timeoutIdentifier).
            setParameter('ms', ms),
        `WebDriver.manage().timeouts().${timeoutName}(${ms})`);
  }
}