function getFallbackBeforeInputChars(topLevelType, nativeEvent) {
  // If we are currently composing (IME) and using a fallback to do so,
  // try to extract the composed characters from the fallback object.
  // If composition event is available, we extract a string only at
  // compositionevent, otherwise extract it at fallback events.
  if (isComposing) {
    if (topLevelType === 'topCompositionEnd' || !canUseCompositionEvent && isFallbackCompositionEnd(topLevelType, nativeEvent)) {
      var chars = FallbackCompositionState_1.getData();
      FallbackCompositionState_1.reset();
      isComposing = false;
      return chars;
    }
    return null;
  }

  switch (topLevelType) {
    case 'topPaste':
      // If a paste event occurs after a keypress, throw out the input
      // chars. Paste events should not lead to BeforeInput events.
      return null;
    case 'topKeyPress':
      /**
       * As of v27, Firefox may fire keypress events even when no character
       * will be inserted. A few possibilities:
       *
       * - `which` is `0`. Arrow keys, Esc key, etc.
       *
       * - `which` is the pressed key code, but no char is available.
       *   Ex: 'AltGr + d` in Polish. There is no modified character for
       *   this key combination and no character is inserted into the
       *   document, but FF fires the keypress for char code `100` anyway.
       *   No `input` event will occur.
       *
       * - `which` is the pressed key code, but a command combination is
       *   being used. Ex: `Cmd+C`. No character is inserted, and no
       *   `input` event will occur.
       */
      if (!isKeypressCommand(nativeEvent)) {
        // IE fires the `keypress` event when a user types an emoji via
        // Touch keyboard of Windows.  In such a case, the `char` property
        // holds an emoji character like `\uD83D\uDE0A`.  Because its length
        // is 2, the property `which` does not represent an emoji correctly.
        // In such a case, we directly return the `char` property instead of
        // using `which`.
        if (nativeEvent.char && nativeEvent.char.length > 1) {
          return nativeEvent.char;
        } else if (nativeEvent.which) {
          return String.fromCharCode(nativeEvent.which);
        }
      }
      return null;
    case 'topCompositionEnd':
      return useFallbackCompositionData ? null : nativeEvent.data;
    default:
      return null;
  }
}
For browsers that do not provide the `textInput` event, extract the
appropriate string to use for SyntheticInputEvent.

@param {string} topLevelType Record from `BrowserEventConstants`.
@param {object} nativeEvent Native browser event.
@return {?string} The fallback string for this `beforeInput` event.