Falsy code executed in fabricjs _fireOverOutEvents

Question

Good day, I'm using fabricjs, and it works well for me. I did however recently come across a JavaScript thing I cannot explain, and that I only detected on fabricjs (across all browsers).

Note: I am using 1.7.11 of fabricjs.

It occurs on mouse:out events. It's only side-effect is to bloat the browser console with:

Uncaught TypeError: Cannot read property 'fire' of null at klass._fireOverOutEvents (fabric.js:9717) ...

It goes like this:

This is vanilla fabric.js (i.e. not my code).

As you can see, target is undefined, so the if block is not executed. So far, that is expected. Now for the odd part.

this._hoveredTarget is null (see debugger view below). The else if block should not be executed, yet it is.

Debugging view of falsy "else if" being executed

I tried to fix it with variants like this._hoveredTarget !== null and this._hoveredTarget != null, but it still goes there when this._hoveredTarget is null.

I never saw this before. Any idea?

Answer 1

Ok this cannot happen just because a bug in the code. an IF is an IF. What can happen is that you have some event handling on mouse:out that is happening before that line that is erroring and that event handler is deleting the reference to this._hoverTarget.

This is still a bug in my opinion since if custom code is allowed the condition must be checked again before accessing the variable as an object.

Current version of fabricJS 1.7.17 has a different code in place:

this.hoveredTarget is saved as reference immediately so that whatever it happens to hoveredTarget you are safe.

You should upgrade maybe.