invokeGuardedCallback.js

/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */

import invariant from 'fbjs/lib/invariant';

let invokeGuardedCallback = function<A, B, C, D, E, F, Context>(
  name: string | null,
  func: (a: A, b: B, c: C, d: D, e: E, f: F) => void,
  context: Context,
  a: A,
  b: B,
  c: C,
  d: D,
  e: E,
  f: F,
) {
  this._hasCaughtError = false;
  this._caughtError = null;
  const funcArgs = Array.prototype.slice.call(arguments, 3);
  try {
    func.apply(context, funcArgs);
  } catch (error) {
    this._caughtError = error;
    this._hasCaughtError = true;
  }
};

if (__DEV__) {
  // In DEV mode, we swap out invokeGuardedCallback for a special version
  // that plays more nicely with the browser's DevTools. The idea is to preserve
  // "Pause on exceptions" behavior. Because React wraps all user-provided
  // functions in invokeGuardedCallback, and the production version of
  // invokeGuardedCallback uses a try-catch, all user exceptions are treated
  // like caught exceptions, and the DevTools won't pause unless the developer
  // takes the extra step of enabling pause on caught exceptions. This is
  // untintuitive, though, because even though React has caught the error, from
  // the developer's perspective, the error is uncaught.
  //
  // To preserve the expected "Pause on exceptions" behavior, we don't use a
  // try-catch in DEV. Instead, we synchronously dispatch a fake event to a fake
  // DOM node, and call the user-provided callback from inside an event handler
  // for that fake event. If the callback throws, the error is "captured" using
  // a global event handler. But because the error happens in a different
  // event loop context, it does not interrupt the normal program flow.
  // Effectively, this gives us try-catch behavior without actually using
  // try-catch. Neat!

  // Check that the browser supports the APIs we need to implement our special
  // DEV version of invokeGuardedCallback
  if (
    typeof window !== 'undefined' &&
    typeof window.dispatchEvent === 'function' &&
    typeof document !== 'undefined' &&
    typeof document.createEvent === 'function'
  ) {
    const fakeNode = document.createElement('react');

    const invokeGuardedCallbackDev = function<A, B, C, D, E, F, Context>(
      name: string | null,
      func: (a: A, b: B, c: C, d: D, e: E, f: F) => void,
      context: Context,
      a: A,
      b: B,
      c: C,
      d: D,
      e: E,
      f: F,
    ) {
      // If document doesn't exist we know for sure we will crash in this method
      // when we call document.createEvent(). However this can cause confusing
      // errors: https://github.com/facebookincubator/create-react-app/issues/3482
      // So we preemptively throw with a better message instead.
      invariant(
        typeof document !== 'undefined',
        'The `document` global was defined when React was initialized, but is not ' +
          'defined anymore. This can happen in a test environment if a component ' +
          'schedules an update from an asynchronous callback, but the test has already ' +
          'finished running. To solve this, you can either unmount the component at ' +
          'the end of your test (and ensure that any asynchronous operations get ' +
          'canceled in `componentWillUnmount`), or you can change the test itself ' +
          'to be asynchronous.',
      );
      const evt = document.createEvent('Event');

      // Keeps track of whether the user-provided callback threw an error. We
      // set this to true at the beginning, then set it to false right after
      // calling the function. If the function errors, `didError` will never be
      // set to false. This strategy works even if the browser is flaky and
      // fails to call our global error handler, because it doesn't rely on
      // the error event at all.
      let didError = true;

      // Create an event handler for our fake event. We will synchronously
      // dispatch our fake event using `dispatchEvent`. Inside the handler, we
      // call the user-provided callback.
      const funcArgs = Array.prototype.slice.call(arguments, 3);
      function callCallback() {
        // We immediately remove the callback from event listeners so that
        // nested `invokeGuardedCallback` calls do not clash. Otherwise, a
        // nested call would trigger the fake event handlers of any call higher
        // in the stack.
        fakeNode.removeEventListener(evtType, callCallback, false);
        func.apply(context, funcArgs);
        didError = false;
      }

      // Create a global error event handler. We use this to capture the value
      // that was thrown. It's possible that this error handler will fire more
      // than once; for example, if non-React code also calls `dispatchEvent`
      // and a handler for that event throws. We should be resilient to most of
      // those cases. Even if our error event handler fires more than once, the
      // last error event is always used. If the callback actually does error,
      // we know that the last error event is the correct one, because it's not
      // possible for anything else to have happened in between our callback
      // erroring and the code that follows the `dispatchEvent` call below. If
      // the callback doesn't error, but the error event was fired, we know to
      // ignore it because `didError` will be false, as described above.
      let error;
      // Use this to track whether the error event is ever called.
      let didSetError = false;
      let isCrossOriginError = false;

      function onError(event) {
        error = event.error;
        didSetError = true;
        if (error === null && event.colno === 0 && event.lineno === 0) {
          isCrossOriginError = true;
        }
      }

      // Create a fake event type.
      const evtType = `react-${name ? name : 'invokeguardedcallback'}`;

      // Attach our event handlers
      window.addEventListener('error', onError);
      fakeNode.addEventListener(evtType, callCallback, false);

      // Synchronously dispatch our fake event. If the user-provided function
      // errors, it will trigger our global error handler.
      evt.initEvent(evtType, false, false);
      fakeNode.dispatchEvent(evt);

      if (didError) {
        if (!didSetError) {
          // The callback errored, but the error event never fired.
          error = new Error(
            'An error was thrown inside one of your components, but React ' +
              "doesn't know what it was. This is likely due to browser " +
              'flakiness. React does its best to preserve the "Pause on ' +
              'exceptions" behavior of the DevTools, which requires some ' +
              "DEV-mode only tricks. It's possible that these don't work in " +
              'your browser. Try triggering the error in production mode, ' +
              'or switching to a modern browser. If you suspect that this is ' +
              'actually an issue with React, please file an issue.',
          );
        } else if (isCrossOriginError) {
          error = new Error(
            "A cross-origin error was thrown. React doesn't have access to " +
              'the actual error object in development. ' +
              'See https://fb.me/react-crossorigin-error for more information.',
          );
        }
        this._hasCaughtError = true;
        this._caughtError = error;
      } else {
        this._hasCaughtError = false;
        this._caughtError = null;
      }

      // Remove our event listeners
      window.removeEventListener('error', onError);
    };

    invokeGuardedCallback = invokeGuardedCallbackDev;
  }
}

export default invokeGuardedCallback;