What reason is there to use null instead of undefined in JavaScript?

Question

I\'ve been writing JavaScript for quite a long time now, and I have never had a reason to use null. It seems that undefined is always preferable an

Answer 1

A useful property in null that undefined does not qualifies:

> null + 3
3
> undefined + 3
NaN

I use null when I want to 'turn off' a numeric value, or to initialize some. My last use was manipulating css transform:

const transforms = { perspective : null, rotateX : null };
// if already set, increase, if not, set to x
runTimeFunction((x) => { trasforms.perspective += x; });
// still useful, as setting perspective to 0 is different than turning it off
runTimeFunction2((x) => { transforms.perspective = null; });

// toCss will check for 'null' values and not set then at all
runTimeFunction3(() => { el.style.transform = toCss(transforms); });

Not sure if I should use this property thought...