javascript - Ask a small question about the touchstart event

Question

There is a piece of code that I am confused about

var clickEventType=((document.ontouchstart!==null)?'click':'touchstart');

If it says document.ontouchstart!==null, it means that touchstart exists, and it should look like this:

var clickEventType=((document.ontouchstart!==null)?'touchstart':'click')

Why is it written the way above?

Answer 1

If written as

var clickEventType=((typeof document.ontouchstart ==="undefined")?'click':'touchstart')

You will understand, ontoucstart is an attribute of the doucment object, but ontouchstart points to a reference to the function, that is, ontouchstart points to an object. When it is not pointed to, ontouchstart needs an initial value. This initial value is null. If the touchstart event is supported, then the initial value of ontouchstart is set to null, so it is written like this

Answer 2

document.ontouchstart == null;

This shows that the current browser supports ontouchstart, because if it does not support it, the value of document.ontouchstart is undefined
, and if document.ontouchstart is not bound to the event, it is equal to null, so you You can use it like thisdocument.ontouchstart = function(){};

The initial value is Null. If it is not equal to null, it can be said that it is not supported

document.ontouchstart !== null;