InputEvent.isComposing
  The InputEvent.isComposing read-only property returns a
  boolean value indicating if the event is fired after
  compositionstart and before compositionend.
Syntax
var bool = event.isComposing;
Example
var inputEvent = new InputEvent('syntheticInput', false);
console.log(inputEvent.isComposing); // return false
Specifications
| Specification | 
|---|
| UI Events # dom-inputevent-iscomposing | 
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