Lesson 10 (part 2): Style

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Style (also called Cascading Style Sheets or CSS) is a very useful feature, and when combined with JavaScript you can do some impressive things. It is a separate language - different from both Html and JavaScript. We don't need to learn much of it though - just a few special formulae.

Note that tricks with style work for all sorts of objects, such as images and text blocks.


Style can be used to set an object's position:

First we define an object with a position given by its style.

Position is given by the top and left properties. These give coordinates in pixels from the top and from the left. So myObject.style.top = 100; actually means '100 pixels down'. These coordinates can be relative (which means 'in the natural flow of the page'), absolute (which means 'from the top-left of the web page') or fixed (which means 'relative to the browser window' - try it out on a large web page to see what that means).

We also give the image an onClick event-handler so that it will respond to clicks:
 <img id="plane" src="aeroplane.gif"
      style="position:relative; top:0; left:0;"

Now we better define the movePlane() function that gets called when you click on the image:

 myplane = document.getElementById("plane");
// Define a variable to hold the position value
 x = 0;
// Define a function to increase x, and reset the image's position
 function movePlane()
      x += 50;
      myplane.style.left = x;

Click on the plane above to make it move right

Hide and Seek

Another useful style property is visibility. This lets you hide objects. visibility can be "visible" or "hidden". You can use this for many special effects. One common use is to make pop-up menus and drop-down menus. In the code below we will make another plane image appear and disappear.

First we need the plane image:

 <img id="plane2" src="aeroplane.gif"
      style="visibility:visible;" >

Now let's get hold of the image object and store it in a variable:
 myplane2 = document.getElementById("plane2");

Now we need a way of calling the function. So let's create two links with onClick event-handlers:
 <a href="#" onClick="myplane2.style.visibility = 'hidden'">Click here to make the
 plane disappear</a>
 <a href="#" onClick="myplane2.style.visibility = 'visible'">Click here to make the
 plane appear</a>

Click here to make the plane disappear
Click here to make the plane appear