I’m not a FrontEnd Web Developer. So it’s kind of a note for me.

I have this simple list, or array.

var arr = [23, 12, 1992];

I always use list or array to store data, and sometimes i need to check if some value is already exists on the list. Sometimes i need to check if an index has already exists on the list.

How to check if some value has already exists?

if (list.indexOf(value) !== -1) {
    // it did exist
}

How to check if some index has already exists?

if (typeof list[index] !== 'undefined') {
    // it did exist
}

That’s it. Well, it’s not the only way of course. You may have another way to check or playing with an array in JavaScript.

One thing, i used === so it compared not only the value but the type of the data too.

Some people might say that using === are not wise, but hey you can use == on above example. ;)