How do I solve this "Cannot read property 'appendChild' of null" error?

Question

I tried to use this code below, which adds buttons in slideshow on my site:

window.onload = function loadContIcons() {
    var elem = document.createElement("img");
    elem.src = "http://arno.agnian.com/sites/all/themes/agnian/images/up.png";
    elem.setAttribute("class", "up_icon");

    var id = "views_slideshow_controls_text_next_slideshow-block";
    if (id !== 0) {
        document.getElementById(id).appendChild(elem);
    } else console.log("aaaaa");

    var elem1 = document.createElement("img");
    elem1.src = "http://arno.agnian.com/sites/all/themes/agnian/images/down.png";
    elem1.setAttribute("class", "down_icon");

    var id1 = "views_slideshow_controls_text_previous_slideshow-block";
    if (id1 !== 0) {
        document.getElementById(id1).appendChild(elem1);
    } else console.log("aaaaa");
}

On the front page, where I have slideshow everything works good, but on the other pages the error Cannot read property 'appendChild' of null occurs.

Answer 1

The element hasn't been appended yet, therefore it is equal to null. The Id will never = 0. When you call getElementById(id), it is null since it is not a part of the dom yet unless your static id is already on the DOM. Do a call through the console to see what it returns.