How to a data from the element that i clicked using "This" in JS?

I want to get a data from the element that i clicked using this in JavaScript?

but insisted of the data it's return undefined.

function jj(){
    let yty = this.num;
    console.log(yty);
}

li {
width:200px;
}

<button num="2" onclick="jj()">3</button>