getElementById and null - why? [duplicate]

Answer 1

The needed DOM is not loaded when the script is executed. Either move it down (below the href) or define it like this:

window.onload = function () {
    document.getElementById("someID").onclick = function(){
        alert("Yahooo");
    }
}

window.onload will be called when the page is completely loaded.

Answer 2

Because the element doesn't yet exist when the script runs - the document hasn't been rendered yet. Either run the script in a script block after the related HTML, or use a "document on ready" event handler - preferably from something like jQuery's .ready() event, or the native window.onload.