pure javascript draggable element

Question

I know there're example of doing this on the web but every example is different and so is my own implementation. I'm trying to figure what's wrong with my implementation since it doesn't work as expected.

code snippet:

var mousePressX = -1;
var mousePressY = -1;

document.getElementById("contextMenu").addEventListener("mousedown", function(event) {
    mousePressX = event.clientX;
    mousePressY = event.clientY;
}, false);

document.getElementById("contextMenu").addEventListener("mouseup", function(event) {
    mousePressX = -1;
    mousePressY = -1;
}, false);

document.getElementById("contextMenu").addEventListener("mousemove", function(event) {
    if (mousePressX != -1 && mousePressY != -1) {
        repositionElement(event.target, event.clientX, event.clientY);
    }
}, false);

function repositionElement(element, currentMouseX, currentMouseY) {
    element.style.left = element.offsetLeft + (currentMouseX - mousePressX) + 'px';
    element.style.top = element.offsetTop + (currentMouseY - mousePressY) + 'px';
}

jsfiddle : http://jsfiddle.net/4rLctko8/

can't really figure out what's wrong but I've noticed that if I change the following lines:

element.style.left = element.offsetLeft + (currentMouseX - mousePressX) + 'px';
element.style.top = element.offsetTop + (currentMouseY - mousePressY) + 'px';

to :

element.style.left = currentMouseX - mousePressX + 'px';
element.style.top = currentMouseY - mousePressY + 'px';

The element will be properly dragged only when it's being dragged towards the positive x-axis (to the right) and towards the positive y-axis (to the bottom) and when mouseup is triggered it'll be positioned in somewhere near the left-most top-most corner area (around 0,0)

Answer 1

Quite interesting, had so far only done it with jQuery. Rewrote it a bit and made sure that every mousemove has it's event listener removed afterwards - that would be a memory leak that you might start to notice otherwise :

http://jsfiddle.net/8wtq17L8/

var contextmenu = document.getElementById('contextMenu');
var initX, initY, mousePressX, mousePressY;

contextmenu.addEventListener('mousedown', function(event) {

    initX = this.offsetLeft;
    initY = this.offsetTop;
    mousePressX = event.clientX;
    mousePressY = event.clientY;

    this.addEventListener('mousemove', repositionElement, false);

    window.addEventListener('mouseup', function() {
      contextmenu.removeEventListener('mousemove', repositionElement, false);
    }, false);

}, false);

function repositionElement(event) {
    this.style.left = initX + event.clientX - mousePressX + 'px';
    this.style.top = initY + event.clientY - mousePressY + 'px';
}

Seems to be working well. :-)

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Edit - thought I'd add a version with touch support too (although newer devices seem to emulate mouse events). Not like Jquery where you can select multiple event listeners so it's mostly a repetition, just with touch events :

http://codepen.io/Shikkediel/pen/VLZKor?editors=011

var object = document.getElementById('element'),
initX, initY, firstX, firstY;

object.addEventListener('mousedown', function(e) {

    e.preventDefault();
    initX = this.offsetLeft;
    initY = this.offsetTop;
    firstX = e.pageX;
    firstY = e.pageY;

    this.addEventListener('mousemove', dragIt, false);

    window.addEventListener('mouseup', function() {
        object.removeEventListener('mousemove', dragIt, false);
    }, false);

}, false);

object.addEventListener('touchstart', function(e) {

    e.preventDefault();
    initX = this.offsetLeft;
    initY = this.offsetTop;
    var touch = e.touches;
    firstX = touch[0].pageX;
    firstY = touch[0].pageY;

    this.addEventListener('touchmove', swipeIt, false);

    window.addEventListener('touchend', function(e) {
        e.preventDefault();
        object.removeEventListener('touchmove', swipeIt, false);
    }, false);

}, false);

function dragIt(e) {
    this.style.left = initX+e.pageX-firstX + 'px';
    this.style.top = initY+e.pageY-firstY + 'px';
}

function swipeIt(e) {
    var contact = e.touches;
    this.style.left = initX+contact[0].pageX-firstX + 'px';
    this.style.top = initY+contact[0].pageY-firstY + 'px';
}