Question

I’m developing a simple web quiz and using javascript, I would like to create an effect that displays a small image (1UP) that wanders around the “game deck” when users reach a specific level or score; user could gain an extra life simply clicking on it in time.

Do you know any Jquery plugin or javascript snippet to achieve an effect like this?

Answer 1

It’s actually surprisingly easy to do this:

Create the element:

img = document.createElement('img');

Set its source:

img.src = "myimage.png";

Position it absolutely and such:

img.style.position = "absolute";
img.style.left = "50px";
img.style.top = "50px";
img.style.width = "50px";  // Make these match the image...
img.style.height = "50px"; // ...or leave them off

(Obviously, use whatever coordinates and size you want.)

You may want to make sure it appears above other things:

img.style.zIndex = 100; // Or whatever

Add it to the document:

document.body.appendChild(img);

Move it around

Use window.setInterval (or setTimeout depending on how you want to do it) to move it around by changing its style.left and style.top settings. You can use Math.random to get a random floating point number between 0 and 1, and multiply that and run it through Math.floor to get a whole number for changing your coordinates.

Example

This creates an image at 50,50 and animates it (in a very jittery random way; I didn’t spend any time making it look nifty) every fifth of a second for 10 seconds, then removes it:

function createWanderingDiv() {
    var img, left, top, counter, interval;

    img = document.createElement('img');

    img.src = "myimage.png";

    left = 200;
    top  = 200;
    img.style.position = "absolute";
    img.style.left = left + "px";
    img.style.top = top + "px";
    img.style.width = "200px";  // Make these match the image...
    img.style.height = "200px"; // ...or leave them out.

    img.style.zIndex = 100; // Or whatever

    document.body.appendChild(img);

    counter = 50;
    interval = 200; // ms
    window.setTimeout(wanderAround, interval);

    function wanderAround() {

        --counter;
        if (counter < 0)
        {
            // Done; remove it
            document.body.removeChild(img);
        }
        else
        {
            // Animate a bit more
            left += Math.floor(Math.random() * 20) - 10;
            if (left < 0)
            {
                left = 0;
            }
            top  += Math.floor(Math.random() * 10)  - 5;
            if (top < 0)
            {
                top = 0;
            }
            img.style.left = left + "px";
            img.style.top  = top  + "px";

            // Re-trigger ourselves
            window.setTimeout(wanderAround, interval);
        }
    }
}

(I prefer re-scheduling on each iteration via setTimeout [as above] to using setInterval, but it’s totally your call. If using setInterval, remember the interval handle [return value from setInterval and use window.clearTimeout to cancel it when you’re done.)

The above is raw DOM/JavaScript; jQuery offers some helpers to make it a bit simpler, but as you can see, it’s pretty straightforward even without.