JavaScript How to Dynamically Move Div by Clicking and Dragging

Okay it would seem like it should be simple. I need to take an already existing div and move it according to mouse position within the window. I have searched everywhere and it has led me to over-complicated ways of doing the same thing and involves the use of j-query. I need to strictly use javascript for what I am trying to do.

Method :

var mousePosition;
var div;

(function createDiv(){

    div = document.createElement("div");
    div.style.position = "absolute";
    div.style.left = "0px";
    div.style.top = "0px";
    div.style.width = "100px";
    div.style.height = "100px";
    div.style.background = "red";
    div.style.color = "blue";

    div.addEventListener('mousedown', handleKeyPressed, true);

    document.body.appendChild(div);


})();

function handleKeyPressed(event) {

    event.preventDefault();

    mousePosition = {

        x : event.clientX,
        y : event.clientY

    };

    div.style.left = mousePosition.x;
    div.style.top = mousePosition.y;

    //alert("whoa!");

}

I think you're looking for something more like this

var mousePosition;
var offset = [0,0];
var div;
var isDown = false;

div = document.createElement("div");
div.style.position = "absolute";
div.style.left = "0px";
div.style.top = "0px";
div.style.width = "100px";
div.style.height = "100px";
div.style.background = "red";
div.style.color = "blue";

document.body.appendChild(div);

div.addEventListener('mousedown', function(e) {
    isDown = true;
    offset = [
        div.offsetLeft - e.clientX,
        div.offsetTop - e.clientY
    ];
}, true);

document.addEventListener('mouseup', function() {
    isDown = false;
}, true);

document.addEventListener('mousemove', function(event) {
    event.preventDefault();
    if (isDown) {
        mousePosition = {

            x : event.clientX,
            y : event.clientY

        };
        div.style.left = (mousePosition.x + offset[0]) + 'px';
        div.style.top  = (mousePosition.y + offset[1]) + 'px';
    }
}, true);

FIDDLE

Support for touch inputs

All other answers (including the accepted one) do not work with touch inputs. Touch inputs have events different than that of mouse inputs. See Using Touch Events on MDN.

The following code snippet works even with touch inputs. I have highlighted all lines of code that need to be added to support touch devices.
The basic idea here is that every element containing draggable in the class list should be draggable. This concept is easier to follow when you have a big number of elements that need to be dragged.

See this Glitch page and following for a demo.

const d = document.getElementsByClassName("draggable");

for (let i = 0; i < d.length; i++) {
  d[i].style.position = "relative";
}

function filter(e) {
  let target = e.target;

  if (!target.classList.contains("draggable")) {
    return;
  }

  target.moving = true;

  //NOTICE THIS 👇 Check if Mouse events exist on users' device
  if (e.clientX) {
    target.oldX = e.clientX; // If they exist then use Mouse input
    target.oldY = e.clientY;
  } else {
    target.oldX = e.touches[0].clientX; // Otherwise use touch input
    target.oldY = e.touches[0].clientY;
  }
  //NOTICE THIS 👆 Since there can be multiple touches, you need to mention which touch to look for, we are using the first touch only in this case

  target.oldLeft = window.getComputedStyle(target).getPropertyValue('left').split('px')[0] * 1;
  target.oldTop = window.getComputedStyle(target).getPropertyValue('top').split('px')[0] * 1;

  document.onmousemove = dr;
  //NOTICE THIS 👇
  document.ontouchmove = dr;
  //NOTICE THIS 👆

  function dr(event) {
    event.preventDefault();

    if (!target.moving) {
      return;
    }
    //NOTICE THIS 👇
    if (event.clientX) {
      target.distX = event.clientX - target.oldX;
      target.distY = event.clientY - target.oldY;
    } else {
      target.distX = event.touches[0].clientX - target.oldX;
      target.distY = event.touches[0].clientY - target.oldY;
    }
    //NOTICE THIS 👆

    target.style.left = target.oldLeft + target.distX + "px";
    target.style.top = target.oldTop + target.distY + "px";
  }

  function endDrag() {
    target.moving = false;
  }
  target.onmouseup = endDrag;
  //NOTICE THIS 👇
  target.ontouchend = endDrag;
  //NOTICE THIS 👆
}
document.onmousedown = filter;
//NOTICE THIS 👇
document.ontouchstart = filter;
//NOTICE THIS 👆

.draggable {
  width: 100px;
  height: 100px;
  background: red;
}

<div class="draggable"></div>