Question

Ok so I made a page that can move an image with the arrow keys but only the down and right functions work. I made the functions from a hide show script because i’m very new to javascript.

Here is the code.

left

function left(id) { 
    var y = '5';
    var z =document.getElementById(id).style.left;
    var x = parseInt(y)+parseInt(z);
    var state = document.getElementById(id).style.left;
        if (state == '1') {
            document.getElementById(id).style.left = x;
        } else {
            document.getElementById(id).style.left = x;
        }
    }

right

function right(id) { 
    var y = '5';
    var z =document.getElementById(id).style.right;
    var x = parseInt(y)+parseInt(z);
    var state = document.getElementById(id).style.right;
        if (state == '1') {
            document.getElementById(id).style.right = x;
        } else {
            document.getElementById(id).style.right = x;
        }
    }

up

function up(id) { 
    var y = '5';
    var z =document.getElementById(id).style.bottom;
    var x = parseInt(y)+parseInt(z);
    var state = document.getElementById(id).style.bottom;
        if (state == '1') {
            document.getElementById(id).style.bottom = x;
        } else {
            document.getElementById(id).style.bottom = x;
        }
    }

down

function down(id) { 
    var y = '5';
    var z =document.getElementById(id).style.top;
    var x = parseInt(y)+parseInt(z);
    var state = document.getElementById(id).style.top;
        if (state == '1') {
            document.getElementById(id).style.top = x;
        } else {
            document.getElementById(id).style.top = x;
        }
    }

then the arrow key script

document.onkeyup = KeyCheck;       
function KeyCheck() {

 var KeyID = event.keyCode;

  switch(KeyID)
  {

  case 37:

  right('img');

  break;

  case 38:

  up('img')

  break

  case 39:

  left('img');

  break;

  case 40:

  down('img');

  break;
  }
  }

and then the html

<img id="img" src="http://trevorrudolph.com/logo.png" style="position:absolute; left:1; bottom:1; right:1; top:1;">

you can download the html at arrow.zip

the actual page is here

Answer 1

You should only be positioning it with top and left, because if you try to do it the way you’re doing it, you are going to end up with very messed up properties, such as a top of say 5 but a bottom of say 100.

Also, you should use a unit, preferable pixels, for the top and left properties.

So, all you really need to do is change your functions to look like:

function left(id) {
    document.getElementById(id).style.left.match(/^([0-9]+)/);
    var current = RegExp.$1; // get just the number and not the units
    document.getElementById(id).style.left = current - 1 + 'px'; // taking advantage of JavaScript's strange but sometimes useful type conversion. The subtraction converts it to an int and the addition converts it back to a string. 
}

function right(id) {
    document.getElementById(id).style.left.match(/^([0-9]+)/);
    var current = RegExp.$1;
    document.getElementById(id).style.left = parseInt(current) + 1 + 'px'; // here we can't use that trick
}

function up(id) {
    document.getElementById(id).style.top.match(/^([0-9]+)/);
    var current = RegExp.$1;
    document.getElementById(id).style.top = current - 1 + 'px';
}

function down(id) {
    document.getElementById(id).style.top.match(/^([0-9]+)/);
    var current = RegExp.$1;
    document.getElementById(id).style.top = parseInt(current) + 1 + 'px';
}

Also, you should be using <script type="text/javascript"> instead of <script language="JavaScript">. The latter form is deprecated.