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Editing: react.js

/** @license React v16.6.1
 * react.development.js
 *
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

'use strict';

(function (global, factory) {
	typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory() :
	typeof define === 'function' && define.amd ? define(factory) :
	(global.React = factory());
}(this, (function () { 'use strict';

// TODO: this is special because it gets imported during build.

var ReactVersion = '16.6.3';

// The Symbol used to tag the ReactElement-like types. If there is no native Symbol
// nor polyfill, then a plain number is used for performance.
var hasSymbol = typeof Symbol === 'function' && Symbol.for;

var REACT_ELEMENT_TYPE = hasSymbol ? Symbol.for('react.element') : 0xeac7;
var REACT_PORTAL_TYPE = hasSymbol ? Symbol.for('react.portal') : 0xeaca;
var REACT_FRAGMENT_TYPE = hasSymbol ? Symbol.for('react.fragment') : 0xeacb;
var REACT_STRICT_MODE_TYPE = hasSymbol ? Symbol.for('react.strict_mode') : 0xeacc;
var REACT_PROFILER_TYPE = hasSymbol ? Symbol.for('react.profiler') : 0xead2;
var REACT_PROVIDER_TYPE = hasSymbol ? Symbol.for('react.provider') : 0xeacd;
var REACT_CONTEXT_TYPE = hasSymbol ? Symbol.for('react.context') : 0xeace;

var REACT_CONCURRENT_MODE_TYPE = hasSymbol ? Symbol.for('react.concurrent_mode') : 0xeacf;
var REACT_FORWARD_REF_TYPE = hasSymbol ? Symbol.for('react.forward_ref') : 0xead0;
var REACT_SUSPENSE_TYPE = hasSymbol ? Symbol.for('react.suspense') : 0xead1;
var REACT_MEMO_TYPE = hasSymbol ? Symbol.for('react.memo') : 0xead3;
var REACT_LAZY_TYPE = hasSymbol ? Symbol.for('react.lazy') : 0xead4;

var MAYBE_ITERATOR_SYMBOL = typeof Symbol === 'function' && Symbol.iterator;
var FAUX_ITERATOR_SYMBOL = '@@iterator';

var enableHooks = false;
// Helps identify side effects in begin-phase lifecycle hooks and setState reducers:

// In some cases, StrictMode should also double-render lifecycles.
// This can be confusing for tests though,
// And it can be bad for performance in production.
// This feature flag can be used to control the behavior:

// To preserve the "Pause on caught exceptions" behavior of the debugger, we
// replay the begin phase of a failed component inside invokeGuardedCallback.

// Warn about deprecated, async-unsafe lifecycles; relates to RFC #6:

// Gather advanced timing metrics for Profiler subtrees.

// Trace which interactions trigger each commit.
var enableSchedulerTracing = true;

// Only used in www builds.

// React Fire: prevent the value and checked attributes from syncing
// with their related DOM properties

// These APIs will no longer be "unstable" in the upcoming 16.7 release,
// Control this behavior with a flag to support 16.6 minor releases in the meanwhile.
var enableStableConcurrentModeAPIs = false;

/*
object-assign
(c) Sindre Sorhus
@license MIT
*/

/* eslint-disable no-unused-vars */
var getOwnPropertySymbols = Object.getOwnPropertySymbols;
var hasOwnProperty = Object.prototype.hasOwnProperty;
var propIsEnumerable = Object.prototype.propertyIsEnumerable;

function toObject(val) {
	if (val === null || val === undefined) {
		throw new TypeError('Object.assign cannot be called with null or undefined');
	}

return Object(val);
}

function shouldUseNative() {
	try {
		if (!Object.assign) {
			return false;
		}

// Detect buggy property enumeration order in older V8 versions.

// https://bugs.chromium.org/p/v8/issues/detail?id=4118
		var test1 = new String('abc');  // eslint-disable-line no-new-wrappers
		test1[5] = 'de';
		if (Object.getOwnPropertyNames(test1)[0] === '5') {
			return false;
		}

// https://bugs.chromium.org/p/v8/issues/detail?id=3056
		var test2 = {};
		for (var i = 0; i < 10; i++) {
			test2['_' + String.fromCharCode(i)] = i;
		}
		var order2 = Object.getOwnPropertyNames(test2).map(function (n) {
			return test2[n];
		});
		if (order2.join('') !== '0123456789') {
			return false;
		}

// https://bugs.chromium.org/p/v8/issues/detail?id=3056
		var test3 = {};
		'abcdefghijklmnopqrst'.split('').forEach(function (letter) {
			test3[letter] = letter;
		});
		if (Object.keys(Object.assign({}, test3)).join('') !==
				'abcdefghijklmnopqrst') {
			return false;
		}

return true;
	} catch (err) {
		// We don't expect any of the above to throw, but better to be safe.
		return false;
	}
}

var objectAssign = shouldUseNative() ? Object.assign : function (target, source) {
	var from;
	var to = toObject(target);
	var symbols;

for (var s = 1; s < arguments.length; s++) {
		from = Object(arguments[s]);

for (var key in from) {
			if (hasOwnProperty.call(from, key)) {
				to[key] = from[key];
			}
		}

if (getOwnPropertySymbols) {
			symbols = getOwnPropertySymbols(from);
			for (var i = 0; i < symbols.length; i++) {
				if (propIsEnumerable.call(from, symbols[i])) {
					to[symbols[i]] = from[symbols[i]];
				}
			}
		}
	}

return to;
};

/**
 * Use invariant() to assert state which your program assumes to be true.
 *
 * Provide sprintf-style format (only %s is supported) and arguments
 * to provide information about what broke and what you were
 * expecting.
 *
 * The invariant message will be stripped in production, but the invariant
 * will remain to ensure logic does not differ in production.
 */

var validateFormat = function () {};

{
  validateFormat = function (format) {
    if (format === undefined) {
      throw new Error('invariant requires an error message argument');
    }
  };
}

function invariant(condition, format, a, b, c, d, e, f) {
  validateFormat(format);

if (!condition) {
    var error = void 0;
    if (format === undefined) {
      error = new Error('Minified exception occurred; use the non-minified dev environment ' + 'for the full error message and additional helpful warnings.');
    } else {
      var args = [a, b, c, d, e, f];
      var argIndex = 0;
      error = new Error(format.replace(/%s/g, function () {
        return args[argIndex++];
      }));
      error.name = 'Invariant Violation';
    }

error.framesToPop = 1; // we don't care about invariant's own frame
    throw error;
  }
}

// Relying on the `invariant()` implementation lets us
// preserve the format and params in the www builds.

/**
 * Forked from fbjs/warning:
 * https://github.com/facebook/fbjs/blob/e66ba20ad5be433eb54423f2b097d829324d9de6/packages/fbjs/src/__forks__/warning.js
 *
 * Only change is we use console.warn instead of console.error,
 * and do nothing when 'console' is not supported.
 * This really simplifies the code.
 * ---
 * Similar to invariant but only logs a warning if the condition is not met.
 * This can be used to log issues in development environments in critical
 * paths. Removing the logging code for production environments will keep the
 * same logic and follow the same code paths.
 */

var lowPriorityWarning = function () {};

{
  var printWarning = function (format) {
    for (var _len = arguments.length, args = Array(_len > 1 ? _len - 1 : 0), _key = 1; _key < _len; _key++) {
      args[_key - 1] = arguments[_key];
    }

lowPriorityWarning = function (condition, format) {
    if (format === undefined) {
      throw new Error('`lowPriorityWarning(condition, format, ...args)` requires a warning ' + 'message argument');
    }
    if (!condition) {
      for (var _len2 = arguments.length, args = Array(_len2 > 2 ? _len2 - 2 : 0), _key2 = 2; _key2 < _len2; _key2++) {
        args[_key2 - 2] = arguments[_key2];
      }

printWarning.apply(undefined, [format].concat(args));
    }
  };
}

var lowPriorityWarning$1 = lowPriorityWarning;

/**
 * Similar to invariant but only logs a warning if the condition is not met.
 * This can be used to log issues in development environments in critical
 * paths. Removing the logging code for production environments will keep the
 * same logic and follow the same code paths.
 */

var warningWithoutStack = function () {};

{
  warningWithoutStack = function (condition, format) {
    for (var _len = arguments.length, args = Array(_len > 2 ? _len - 2 : 0), _key = 2; _key < _len; _key++) {
      args[_key - 2] = arguments[_key];
    }

if (format === undefined) {
      throw new Error('`warningWithoutStack(condition, format, ...args)` requires a warning ' + 'message argument');
    }
    if (args.length > 8) {
      // Check before the condition to catch violations early.
      throw new Error('warningWithoutStack() currently supports at most 8 arguments.');
    }
    if (condition) {
      return;
    }
    if (typeof console !== 'undefined') {
      var argsWithFormat = args.map(function (item) {
        return '' + item;
      });
      argsWithFormat.unshift('Warning: ' + format);

// We intentionally don't use spread (or .apply) directly because it
      // breaks IE9: https://github.com/facebook/react/issues/13610
      Function.prototype.apply.call(console.error, console, argsWithFormat);
    }
    try {
      // --- Welcome to debugging React ---
      // This error was thrown as a convenience so that you can use this stack
      // to find the callsite that caused this warning to fire.
      var argIndex = 0;
      var message = 'Warning: ' + format.replace(/%s/g, function () {
        return args[argIndex++];
      });
      throw new Error(message);
    } catch (x) {}
  };
}

var warningWithoutStack$1 = warningWithoutStack;

var didWarnStateUpdateForUnmountedComponent = {};

/**
 * This is the abstract API for an update queue.
 */
var ReactNoopUpdateQueue = {
  /**
   * Checks whether or not this composite component is mounted.
   * @param {ReactClass} publicInstance The instance we want to test.
   * @return {boolean} True if mounted, false otherwise.
   * @protected
   * @final
   */
  isMounted: function (publicInstance) {
    return false;
  },

/**
   * Forces an update. This should only be invoked when it is known with
   * certainty that we are **not** in a DOM transaction.
   *
   * You may want to call this when you know that some deeper aspect of the
   * component's state has changed but `setState` was not called.
   *
   * This will not invoke `shouldComponentUpdate`, but it will invoke
   * `componentWillUpdate` and `componentDidUpdate`.
   *
   * @param {ReactClass} publicInstance The instance that should rerender.
   * @param {?function} callback Called after component is updated.
   * @param {?string} callerName name of the calling function in the public API.
   * @internal
   */
  enqueueForceUpdate: function (publicInstance, callback, callerName) {
    warnNoop(publicInstance, 'forceUpdate');
  },

/**
   * Replaces all of the state. Always use this or `setState` to mutate state.
   * You should treat `this.state` as immutable.
   *
   * There is no guarantee that `this.state` will be immediately updated, so
   * accessing `this.state` after calling this method may return the old value.
   *
   * @param {ReactClass} publicInstance The instance that should rerender.
   * @param {object} completeState Next state.
   * @param {?function} callback Called after component is updated.
   * @param {?string} callerName name of the calling function in the public API.
   * @internal
   */
  enqueueReplaceState: function (publicInstance, completeState, callback, callerName) {
    warnNoop(publicInstance, 'replaceState');
  },

/**
   * Sets a subset of the state. This only exists because _pendingState is
   * internal. This provides a merging strategy that is not available to deep
   * properties which is confusing. TODO: Expose pendingState or don't use it
   * during the merge.
   *
   * @param {ReactClass} publicInstance The instance that should rerender.
   * @param {object} partialState Next partial state to be merged with state.
   * @param {?function} callback Called after component is updated.
   * @param {?string} Name of the calling function in the public API.
   * @internal
   */
  enqueueSetState: function (publicInstance, partialState, callback, callerName) {
    warnNoop(publicInstance, 'setState');
  }
};

var emptyObject = {};
{
  Object.freeze(emptyObject);
}

/**
 * Base class helpers for the updating state of a component.
 */
function Component(props, context, updater) {
  this.props = props;
  this.context = context;
  // If a component has string refs, we will assign a different object later.
  this.refs = emptyObject;
  // We initialize the default updater but the real one gets injected by the
  // renderer.
  this.updater = updater || ReactNoopUpdateQueue;
}

Component.prototype.isReactComponent = {};

/**
 * Sets a subset of the state. Always use this to mutate
 * state. You should treat `this.state` as immutable.
 *
 * There is no guarantee that `this.state` will be immediately updated, so
 * accessing `this.state` after calling this method may return the old value.
 *
 * There is no guarantee that calls to `setState` will run synchronously,
 * as they may eventually be batched together.  You can provide an optional
 * callback that will be executed when the call to setState is actually
 * completed.
 *
 * When a function is provided to setState, it will be called at some point in
 * the future (not synchronously). It will be called with the up to date
 * component arguments (state, props, context). These values can be different
 * from this.* because your function may be called after receiveProps but before
 * shouldComponentUpdate, and this new state, props, and context will not yet be
 * assigned to this.
 *
 * @param {object|function} partialState Next partial state or function to
 *        produce next partial state to be merged with current state.
 * @param {?function} callback Called after state is updated.
 * @final
 * @protected
 */
Component.prototype.setState = function (partialState, callback) {
  !(typeof partialState === 'object' || typeof partialState === 'function' || partialState == null) ? invariant(false, 'setState(...): takes an object of state variables to update or a function which returns an object of state variables.') : void 0;
  this.updater.enqueueSetState(this, partialState, callback, 'setState');
};

/**
 * Forces an update. This should only be invoked when it is known with
 * certainty that we are **not** in a DOM transaction.
 *
 * You may want to call this when you know that some deeper aspect of the
 * component's state has changed but `setState` was not called.
 *
 * This will not invoke `shouldComponentUpdate`, but it will invoke
 * `componentWillUpdate` and `componentDidUpdate`.
 *
 * @param {?function} callback Called after update is complete.
 * @final
 * @protected
 */
Component.prototype.forceUpdate = function (callback) {
  this.updater.enqueueForceUpdate(this, callback, 'forceUpdate');
};

/**
 * Deprecated APIs. These APIs used to exist on classic React classes but since
 * we would like to deprecate them, we're not going to move them over to this
 * modern base class. Instead, we define a getter that warns if it's accessed.
 */
{
  var deprecatedAPIs = {
    isMounted: ['isMounted', 'Instead, make sure to clean up subscriptions and pending requests in ' + 'componentWillUnmount to prevent memory leaks.'],
    replaceState: ['replaceState', 'Refactor your code to use setState instead (see ' + 'https://github.com/facebook/react/issues/3236).']
  };
  var defineDeprecationWarning = function (methodName, info) {
    Object.defineProperty(Component.prototype, methodName, {
      get: function () {
        lowPriorityWarning$1(false, '%s(...) is deprecated in plain JavaScript React classes. %s', info[0], info[1]);
        return undefined;
      }
    });
  };
  for (var fnName in deprecatedAPIs) {
    if (deprecatedAPIs.hasOwnProperty(fnName)) {
      defineDeprecationWarning(fnName, deprecatedAPIs[fnName]);
    }
  }
}

function ComponentDummy() {}
ComponentDummy.prototype = Component.prototype;

/**
 * Convenience component with default shallow equality check for sCU.
 */
function PureComponent(props, context, updater) {
  this.props = props;
  this.context = context;
  // If a component has string refs, we will assign a different object later.
  this.refs = emptyObject;
  this.updater = updater || ReactNoopUpdateQueue;
}

var pureComponentPrototype = PureComponent.prototype = new ComponentDummy();
pureComponentPrototype.constructor = PureComponent;
// Avoid an extra prototype jump for these methods.
objectAssign(pureComponentPrototype, Component.prototype);
pureComponentPrototype.isPureReactComponent = true;

// an immutable object with a single mutable value
function createRef() {
  var refObject = {
    current: null
  };
  {
    Object.seal(refObject);
  }
  return refObject;
}

/* eslint-disable no-var */

// TODO: Use symbols?
var ImmediatePriority = 1;
var UserBlockingPriority = 2;
var NormalPriority = 3;
var LowPriority = 4;
var IdlePriority = 5;

// Max 31 bit integer. The max integer size in V8 for 32-bit systems.
// Math.pow(2, 30) - 1
// 0b111111111111111111111111111111
var maxSigned31BitInt = 1073741823;

// Times out immediately
var IMMEDIATE_PRIORITY_TIMEOUT = -1;
// Eventually times out
var USER_BLOCKING_PRIORITY = 250;
var NORMAL_PRIORITY_TIMEOUT = 5000;
var LOW_PRIORITY_TIMEOUT = 10000;
// Never times out
var IDLE_PRIORITY = maxSigned31BitInt;

// Callbacks are stored as a circular, doubly linked list.
var firstCallbackNode = null;

var currentDidTimeout = false;
var currentPriorityLevel = NormalPriority;
var currentEventStartTime = -1;
var currentExpirationTime = -1;

// This is set when a callback is being executed, to prevent re-entrancy.
var isExecutingCallback = false;

var isHostCallbackScheduled = false;

var hasNativePerformanceNow = typeof performance === 'object' && typeof performance.now === 'function';

function flushFirstCallback() {
  var flushedNode = firstCallbackNode;

// Remove the node from the list before calling the callback. That way the
  // list is in a consistent state even if the callback throws.
  var next = firstCallbackNode.next;
  if (firstCallbackNode === next) {
    // This is the last callback in the list.
    firstCallbackNode = null;
    next = null;
  } else {
    var lastCallbackNode = firstCallbackNode.previous;
    firstCallbackNode = lastCallbackNode.next = next;
    next.previous = lastCallbackNode;
  }

flushedNode.next = flushedNode.previous = null;

// Now it's safe to call the callback.
  var callback = flushedNode.callback;
  var expirationTime = flushedNode.expirationTime;
  var priorityLevel = flushedNode.priorityLevel;
  var previousPriorityLevel = currentPriorityLevel;
  var previousExpirationTime = currentExpirationTime;
  currentPriorityLevel = priorityLevel;
  currentExpirationTime = expirationTime;
  var continuationCallback;
  try {
    continuationCallback = callback();
  } finally {
    currentPriorityLevel = previousPriorityLevel;
    currentExpirationTime = previousExpirationTime;
  }

// A callback may return a continuation. The continuation should be scheduled
  // with the same priority and expiration as the just-finished callback.
  if (typeof continuationCallback === 'function') {
    var continuationNode = {
      callback: continuationCallback,
      priorityLevel: priorityLevel,
      expirationTime: expirationTime,
      next: null,
      previous: null
    };

// Insert the new callback into the list, sorted by its expiration. This is
    // almost the same as the code in `scheduleCallback`, except the callback
    // is inserted into the list *before* callbacks of equal expiration instead
    // of after.
    if (firstCallbackNode === null) {
      // This is the first callback in the list.
      firstCallbackNode = continuationNode.next = continuationNode.previous = continuationNode;
    } else {
      var nextAfterContinuation = null;
      var node = firstCallbackNode;
      do {
        if (node.expirationTime >= expirationTime) {
          // This callback expires at or after the continuation. We will insert
          // the continuation *before* this callback.
          nextAfterContinuation = node;
          break;
        }
        node = node.next;
      } while (node !== firstCallbackNode);

if (nextAfterContinuation === null) {
        // No equal or lower priority callback was found, which means the new
        // callback is the lowest priority callback in the list.
        nextAfterContinuation = firstCallbackNode;
      } else if (nextAfterContinuation === firstCallbackNode) {
        // The new callback is the highest priority callback in the list.
        firstCallbackNode = continuationNode;
        ensureHostCallbackIsScheduled();
      }

var previous = nextAfterContinuation.previous;
      previous.next = nextAfterContinuation.previous = continuationNode;
      continuationNode.next = nextAfterContinuation;
      continuationNode.previous = previous;
    }
  }
}

function unstable_runWithPriority(priorityLevel, eventHandler) {
  switch (priorityLevel) {
    case ImmediatePriority:
    case UserBlockingPriority:
    case NormalPriority:
    case LowPriority:
    case IdlePriority:
      break;
    default:
      priorityLevel = NormalPriority;
  }

var previousPriorityLevel = currentPriorityLevel;
  var previousEventStartTime = currentEventStartTime;
  currentPriorityLevel = priorityLevel;
  currentEventStartTime = getCurrentTime();

try {
    return eventHandler();
  } finally {
    currentPriorityLevel = previousPriorityLevel;
    currentEventStartTime = previousEventStartTime;

// Before exiting, flush all the immediate work that was scheduled.
    flushImmediateWork();
  }
}

function unstable_wrapCallback(callback) {
  var parentPriorityLevel = currentPriorityLevel;
  return function () {
    // This is a fork of runWithPriority, inlined for performance.
    var previousPriorityLevel = currentPriorityLevel;
    var previousEventStartTime = currentEventStartTime;
    currentPriorityLevel = parentPriorityLevel;
    currentEventStartTime = getCurrentTime();

try {
      return callback.apply(this, arguments);
    } finally {
      currentPriorityLevel = previousPriorityLevel;
      currentEventStartTime = previousEventStartTime;
      flushImmediateWork();
    }
  };
}

function unstable_scheduleCallback(callback, deprecated_options) {
  var startTime = currentEventStartTime !== -1 ? currentEventStartTime : getCurrentTime();

var expirationTime;
  if (typeof deprecated_options === 'object' && deprecated_options !== null && typeof deprecated_options.timeout === 'number') {
    // FIXME: Remove this branch once we lift expiration times out of React.
    expirationTime = startTime + deprecated_options.timeout;
  } else {
    switch (currentPriorityLevel) {
      case ImmediatePriority:
        expirationTime = startTime + IMMEDIATE_PRIORITY_TIMEOUT;
        break;
      case UserBlockingPriority:
        expirationTime = startTime + USER_BLOCKING_PRIORITY;
        break;
      case IdlePriority:
        expirationTime = startTime + IDLE_PRIORITY;
        break;
      case LowPriority:
        expirationTime = startTime + LOW_PRIORITY_TIMEOUT;
        break;
      case NormalPriority:
      default:
        expirationTime = startTime + NORMAL_PRIORITY_TIMEOUT;
    }
  }

var newNode = {
    callback: callback,
    priorityLevel: currentPriorityLevel,
    expirationTime: expirationTime,
    next: null,
    previous: null
  };

// Insert the new callback into the list, ordered first by expiration, then
  // by insertion. So the new callback is inserted any other callback with
  // equal expiration.
  if (firstCallbackNode === null) {
    // This is the first callback in the list.
    firstCallbackNode = newNode.next = newNode.previous = newNode;
    ensureHostCallbackIsScheduled();
  } else {
    var next = null;
    var node = firstCallbackNode;
    do {
      if (node.expirationTime > expirationTime) {
        // The new callback expires before this one.
        next = node;
        break;
      }
      node = node.next;
    } while (node !== firstCallbackNode);

if (next === null) {
      // No callback with a later expiration was found, which means the new
      // callback has the latest expiration in the list.
      next = firstCallbackNode;
    } else if (next === firstCallbackNode) {
      // The new callback has the earliest expiration in the entire list.
      firstCallbackNode = newNode;
      ensureHostCallbackIsScheduled();
    }

var previous = next.previous;
    previous.next = next.previous = newNode;
    newNode.next = next;
    newNode.previous = previous;
  }

return newNode;
}

function unstable_cancelCallback(callbackNode) {
  var next = callbackNode.next;
  if (next === null) {
    // Already cancelled.
    return;
  }

if (next === callbackNode) {
    // This is the only scheduled callback. Clear the list.
    firstCallbackNode = null;
  } else {
    // Remove the callback from its position in the list.
    if (callbackNode === firstCallbackNode) {
      firstCallbackNode = next;
    }
    var previous = callbackNode.previous;
    previous.next = next;
    next.previous = previous;
  }

callbackNode.next = callbackNode.previous = null;
}

function unstable_getCurrentPriorityLevel() {
  return currentPriorityLevel;
}

function unstable_shouldYield() {
  return !currentDidTimeout && (firstCallbackNode !== null && firstCallbackNode.expirationTime < currentExpirationTime || shouldYieldToHost());
}

// The remaining code is essentially a polyfill for requestIdleCallback. It
// works by scheduling a requestAnimationFrame, storing the time for the start
// of the frame, then scheduling a postMessage which gets scheduled after paint.
// Within the postMessage handler do as much work as possible until time + frame
// rate. By separating the idle call into a separate event tick we ensure that
// layout, paint and other browser work is counted against the available time.
// The frame rate is dynamically adjusted.

// We capture a local reference to any global, in case it gets polyfilled after
// this module is initially evaluated. We want to be using a
// consistent implementation.
var localDate = Date;

// This initialization code may run even on server environments if a component
// just imports ReactDOM (e.g. for findDOMNode). Some environments might not
// have setTimeout or clearTimeout. However, we always expect them to be defined
// on the client. https://github.com/facebook/react/pull/13088
var localSetTimeout = typeof setTimeout === 'function' ? setTimeout : undefined;
var localClearTimeout = typeof clearTimeout === 'function' ? clearTimeout : undefined;

// We don't expect either of these to necessarily be defined, but we will error
// later if they are missing on the client.
var localRequestAnimationFrame = typeof requestAnimationFrame === 'function' ? requestAnimationFrame : undefined;
var localCancelAnimationFrame = typeof cancelAnimationFrame === 'function' ? cancelAnimationFrame : undefined;

var getCurrentTime;

// requestAnimationFrame does not run when the tab is in the background. If
// we're backgrounded we prefer for that work to happen so that the page
// continues to load in the background. So we also schedule a 'setTimeout' as
// a fallback.
// TODO: Need a better heuristic for backgrounded work.
var ANIMATION_FRAME_TIMEOUT = 100;
var rAFID;
var rAFTimeoutID;
var requestAnimationFrameWithTimeout = function (callback) {
  // schedule rAF and also a setTimeout
  rAFID = localRequestAnimationFrame(function (timestamp) {
    // cancel the setTimeout
    localClearTimeout(rAFTimeoutID);
    callback(timestamp);
  });
  rAFTimeoutID = localSetTimeout(function () {
    // cancel the requestAnimationFrame
    localCancelAnimationFrame(rAFID);
    callback(getCurrentTime());
  }, ANIMATION_FRAME_TIMEOUT);
};

if (hasNativePerformanceNow) {
  var Performance = performance;
  getCurrentTime = function () {
    return Performance.now();
  };
} else {
  getCurrentTime = function () {
    return localDate.now();
  };
}

var requestHostCallback;
var cancelHostCallback;
var shouldYieldToHost;

if (typeof window !== 'undefined' && window._schedMock) {
  // Dynamic injection, only for testing purposes.
  var impl = window._schedMock;
  requestHostCallback = impl[0];
  cancelHostCallback = impl[1];
  shouldYieldToHost = impl[2];
} else if (
// If Scheduler runs in a non-DOM environment, it falls back to a naive
// implementation using setTimeout.
typeof window === 'undefined' ||
// "addEventListener" might not be available on the window object
// if this is a mocked "window" object. So we need to validate that too.
typeof window.addEventListener !== 'function') {
  var _callback = null;
  var _currentTime = -1;
  var _flushCallback = function (didTimeout, ms) {
    if (_callback !== null) {
      var cb = _callback;
      _callback = null;
      try {
        _currentTime = ms;
        cb(didTimeout);
      } finally {
        _currentTime = -1;
      }
    }
  };
  requestHostCallback = function (cb, ms) {
    if (_currentTime !== -1) {
      // Protect against re-entrancy.
      setTimeout(requestHostCallback, 0, cb, ms);
    } else {
      _callback = cb;
      setTimeout(_flushCallback, ms, true, ms);
      setTimeout(_flushCallback, maxSigned31BitInt, false, maxSigned31BitInt);
    }
  };
  cancelHostCallback = function () {
    _callback = null;
  };
  shouldYieldToHost = function () {
    return false;
  };
  getCurrentTime = function () {
    return _currentTime === -1 ? 0 : _currentTime;
  };
} else {
  if (typeof console !== 'undefined') {
    // TODO: Remove fb.me link
    if (typeof localRequestAnimationFrame !== 'function') {
      console.error("This browser doesn't support requestAnimationFrame. " + 'Make sure that you load a ' + 'polyfill in older browsers. https://fb.me/react-polyfills');
    }
    if (typeof localCancelAnimationFrame !== 'function') {
      console.error("This browser doesn't support cancelAnimationFrame. " + 'Make sure that you load a ' + 'polyfill in older browsers. https://fb.me/react-polyfills');
    }
  }

var scheduledHostCallback = null;
  var isMessageEventScheduled = false;
  var timeoutTime = -1;

var isAnimationFrameScheduled = false;

var isFlushingHostCallback = false;

var frameDeadline = 0;
  // We start out assuming that we run at 30fps but then the heuristic tracking
  // will adjust this value to a faster fps if we get more frequent animation
  // frames.
  var previousFrameTime = 33;
  var activeFrameTime = 33;

shouldYieldToHost = function () {
    return frameDeadline <= getCurrentTime();
  };

// We use the postMessage trick to defer idle work until after the repaint.
  var messageKey = '__reactIdleCallback$' + Math.random().toString(36).slice(2);
  var idleTick = function (event) {
    if (event.source !== window || event.data !== messageKey) {
      return;
    }

isMessageEventScheduled = false;

var prevScheduledCallback = scheduledHostCallback;
    var prevTimeoutTime = timeoutTime;
    scheduledHostCallback = null;
    timeoutTime = -1;

var currentTime = getCurrentTime();

if (prevScheduledCallback !== null) {
      isFlushingHostCallback = true;
      try {
        prevScheduledCallback(didTimeout);
      } finally {
        isFlushingHostCallback = false;
      }
    }
  };
  // Assumes that we have addEventListener in this environment. Might need
  // something better for old IE.
  window.addEventListener('message', idleTick, false);

var animationTick = function (rafTime) {
    if (scheduledHostCallback !== null) {
      // Eagerly schedule the next animation callback at the beginning of the
      // frame. If the scheduler queue is not empty at the end of the frame, it
      // will continue flushing inside that callback. If the queue *is* empty,
      // then it will exit immediately. Posting the callback at the start of the
      // frame ensures it's fired within the earliest possible frame. If we
      // waited until the end of the frame to post the callback, we risk the
      // browser skipping a frame and not firing the callback until the frame
      // after that.
      requestAnimationFrameWithTimeout(animationTick);
    } else {
      // No pending work. Exit.
      isAnimationFrameScheduled = false;
      return;
    }

requestHostCallback = function (callback, absoluteTimeout) {
    scheduledHostCallback = callback;
    timeoutTime = absoluteTimeout;
    if (isFlushingHostCallback || absoluteTimeout < 0) {
      // Don't wait for the next frame. Continue working ASAP, in a new event.
      window.postMessage(messageKey, '*');
    } else if (!isAnimationFrameScheduled) {
      // If rAF didn't already schedule one, we need to schedule a frame.
      // TODO: If this rAF doesn't materialize because the browser throttles, we
      // might want to still have setTimeout trigger rIC as a backup to ensure
      // that we keep performing work.
      isAnimationFrameScheduled = true;
      requestAnimationFrameWithTimeout(animationTick);
    }
  };

cancelHostCallback = function () {
    scheduledHostCallback = null;
    isMessageEventScheduled = false;
    timeoutTime = -1;
  };
}

var DEFAULT_THREAD_ID = 0;

// Counters used to generate unique IDs.
var interactionIDCounter = 0;
var threadIDCounter = 0;

// Set of currently traced interactions.
// Interactions "stack"–
// Meaning that newly traced interactions are appended to the previously active set.
// When an interaction goes out of scope, the previous set (if any) is restored.
var interactionsRef = null;

// Listener(s) to notify when interactions begin and end.
var subscriberRef = null;

if (enableSchedulerTracing) {
  interactionsRef = {
    current: new Set()
  };
  subscriberRef = {
    current: null
  };
}

function unstable_clear(callback) {
  if (!enableSchedulerTracing) {
    return callback();
  }

var prevInteractions = interactionsRef.current;
  interactionsRef.current = new Set();

try {
    return callback();
  } finally {
    interactionsRef.current = prevInteractions;
  }
}

function unstable_getCurrent() {
  if (!enableSchedulerTracing) {
    return null;
  } else {
    return interactionsRef.current;
  }
}

function unstable_getThreadID() {
  return ++threadIDCounter;
}

function unstable_trace(name, timestamp, callback) {
  var threadID = arguments.length > 3 && arguments[3] !== undefined ? arguments[3] : DEFAULT_THREAD_ID;

if (!enableSchedulerTracing) {
    return callback();
  }

var interaction = {
    __count: 1,
    id: interactionIDCounter++,
    name: name,
    timestamp: timestamp
  };

var prevInteractions = interactionsRef.current;

// Traced interactions should stack/accumulate.
  // To do that, clone the current interactions.
  // The previous set will be restored upon completion.
  var interactions = new Set(prevInteractions);
  interactions.add(interaction);
  interactionsRef.current = interactions;

var subscriber = subscriberRef.current;
  var returnValue = void 0;

try {
    if (subscriber !== null) {
      subscriber.onInteractionTraced(interaction);
    }
  } finally {
    try {
      if (subscriber !== null) {
        subscriber.onWorkStarted(interactions, threadID);
      }
    } finally {
      try {
        returnValue = callback();
      } finally {
        interactionsRef.current = prevInteractions;

try {
          if (subscriber !== null) {
            subscriber.onWorkStopped(interactions, threadID);
          }
        } finally {
          interaction.__count--;

// If no async work was scheduled for this interaction,
          // Notify subscribers that it's completed.
          if (subscriber !== null && interaction.__count === 0) {
            subscriber.onInteractionScheduledWorkCompleted(interaction);
          }
        }
      }
    }
  }

return returnValue;
}

function unstable_wrap(callback) {
  var threadID = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : DEFAULT_THREAD_ID;

if (!enableSchedulerTracing) {
    return callback;
  }

var wrappedInteractions = interactionsRef.current;

var subscriber = subscriberRef.current;
  if (subscriber !== null) {
    subscriber.onWorkScheduled(wrappedInteractions, threadID);
  }

// Update the pending async work count for the current interactions.
  // Update after calling subscribers in case of error.
  wrappedInteractions.forEach(function (interaction) {
    interaction.__count++;
  });

var hasRun = false;

function wrapped() {
    var prevInteractions = interactionsRef.current;
    interactionsRef.current = wrappedInteractions;

subscriber = subscriberRef.current;

try {
      var returnValue = void 0;

try {
        if (subscriber !== null) {
          subscriber.onWorkStarted(wrappedInteractions, threadID);
        }
      } finally {
        try {
          returnValue = callback.apply(undefined, arguments);
        } finally {
          interactionsRef.current = prevInteractions;

if (subscriber !== null) {
            subscriber.onWorkStopped(wrappedInteractions, threadID);
          }
        }
      }

return returnValue;
    } finally {
      if (!hasRun) {
        // We only expect a wrapped function to be executed once,
        // But in the event that it's executed more than once–
        // Only decrement the outstanding interaction counts once.
        hasRun = true;

// Update pending async counts for all wrapped interactions.
        // If this was the last scheduled async work for any of them,
        // Mark them as completed.
        wrappedInteractions.forEach(function (interaction) {
          interaction.__count--;

if (subscriber !== null && interaction.__count === 0) {
            subscriber.onInteractionScheduledWorkCompleted(interaction);
          }
        });
      }
    }
  }

wrapped.cancel = function cancel() {
    subscriber = subscriberRef.current;

try {
      if (subscriber !== null) {
        subscriber.onWorkCanceled(wrappedInteractions, threadID);
      }
    } finally {
      // Update pending async counts for all wrapped interactions.
      // If this was the last scheduled async work for any of them,
      // Mark them as completed.
      wrappedInteractions.forEach(function (interaction) {
        interaction.__count--;

if (subscriber && interaction.__count === 0) {
          subscriber.onInteractionScheduledWorkCompleted(interaction);
        }
      });
    }
  };

return wrapped;
}

var subscribers = null;
if (enableSchedulerTracing) {
  subscribers = new Set();
}

function unstable_subscribe(subscriber) {
  if (enableSchedulerTracing) {
    subscribers.add(subscriber);

if (subscribers.size === 1) {
      subscriberRef.current = {
        onInteractionScheduledWorkCompleted: onInteractionScheduledWorkCompleted,
        onInteractionTraced: onInteractionTraced,
        onWorkCanceled: onWorkCanceled,
        onWorkScheduled: onWorkScheduled,
        onWorkStarted: onWorkStarted,
        onWorkStopped: onWorkStopped
      };
    }
  }
}

function unstable_unsubscribe(subscriber) {
  if (enableSchedulerTracing) {
    subscribers.delete(subscriber);

if (subscribers.size === 0) {
      subscriberRef.current = null;
    }
  }
}

function onInteractionTraced(interaction) {
  var didCatchError = false;
  var caughtError = null;

subscribers.forEach(function (subscriber) {
    try {
      subscriber.onInteractionTraced(interaction);
    } catch (error) {
      if (!didCatchError) {
        didCatchError = true;
        caughtError = error;
      }
    }
  });