Question

So on my new website in google chrome, I am trying to make the images switch every 5 seconds using a setInterval function. This seems to work, however I have the problem cannot set property src of null.

var images = new Array();
images[0] = "/images/grilled-chicken-large.png";
images[1] = "/images/sizzly-steak.png";
var img = document.getElementById("img");

function changeImage() {
     for (var i = 0; i < 3; i++) {
        img.src = images[i];
        if (i == 2){i = i - 2;};
     }
}

window.onload=setInterval("changeImage()", 5000);

I know the problem is that I am trying to get the element with an id of img before the page is done loading, but I’ve already got a window.onload, so i don’t know what to do. Also, if i make an init() function containing the setInterval and the img variable, the page freezes.

Any ideas?

Answer 1

The problem is that you access element 2 of an array with only two elements (0 and 1).

But there is also a logical error in your script: Where do you store the index of the currently visible image? You need a global variable which holds that index.

var currentIndex = 0;
function changeImage() {
    // Switch currentIndex in a loop
    currentIndex++;
    if (currentIndex >= images.length)
        currentIndex = 0;

    var img = document.getElementById("img");
    img.src = images[currentIndex];
}

document.onload = function() {
    setInterval(changeImage, 5000);
}

I’ve moved the img variable into the function so that it is assigned after five seconds when the document has loaded.

Just to clear things up: JavaScript is an event-based programming language. That means that the slideshow-change code is executed every time the interval fires, does some work (switch to the next slide) and then is done, so that the browser can render the page. If you iterate through the slides in a loop, there is no way for the browser to show the new slide because the script is still executing between the slides. The solution is what I’ve posted: Define a global variable that replaces the loop variable, and increment it every time the next slide is to be shown.