So i got this code and what's the difference between using the variable speed and not i mean if i remove speed it does not make any difference but if i use speed it does not show up any difference?

The one using the variable speed:

(function() {


var speed = 10,

    moveBox = function() {

        var el = document.getElementById("box"),

        left = el.offsetLeft,

        moveBy = 1;


    el.style.left = left + moveBy + "px";


    if (left > 399) {

        clearTimeout(timer);

    }


    };


var timer = setInterval(moveBox, speed);



}());

The one not using the variable speed:

(function() {


var     moveBox = function() {

        var el = document.getElementById("box"),

        left = el.offsetLeft,

        moveBy = 1;


    el.style.left = left + moveBy + "px";


    if (left > 399) {

        clearTimeout(timer);

    }


    };


var timer = setInterval(moveBox);



}());

setInterval requires a delay as it's second parameter, without it the code may not run properly in some browsers.

You might want to try something like this:

 

function movebox(speed) {
    setInterval(function(){
        var el = document.getElementById("box"),
        left = el.offsetLeft,
        moveBy = 1;
        el.style.left = left + moveBy + "px";        
    }, speed);

    if (left > 399) {
        clearTimeout(timer);
    }

}

 

...then call the function as you need it. Haven't checked to see if this works....