document is not declared

Question

I have a problem when serching elements from HTML, in line
var x = document.getElementById ("btn");
it says document is not declared
why is this happening?

Answer 1

window.onload = function {
//your code
}

Answer 2

Try posting your whole code, but I think it’s running before the page is loaded

Answer 3

How can I fix it?

Answer 4

I'll try