Why wont this work?

Question

Why wont this code change the h1 tag, it just stays at its default text entered in the html?

var x = 1;
var y = 1;
var fib = document.getElementsByTagName("h1");

function increase() {
x += y;
y = x-y;
fib.innerHTML = y ;
}

setInterval(increase, 1000);

Answer 1

When you call document.getElementsByTagName("h1"), it returns an array of h1 tags; even if you have just one h1 tag.
You must use indexes, e.g. fib[0].innerHTML.

Answer 2

Oh, youre right, thank you @Mohammad Amin Chitgarha