Accessing variables with javascript (no jQuery) .onmouseup event

Question

I am having some trouble understanding how to get a variable to work in my .onmouseup event.

I have a .onmousemove event that defines a local variable that is, for example the distance the mouse has moved since .onmousedown. I want to use that information in a function that executes .onmouseup, however, I can't get it there. Here are the relevant bits of code:

document.onmousedown = function(){
   var mouseStart = [event.pageX,event.pageY];
   document.onmousemove = function(){
       var dist = Math.sqrt(Math.pow(event.pageY-mouseStart[1],2)+Math.pow(event.pageX-mouseStart[0],2));
       document.onmouseup = function() {
          global_function(dist);
          document.onmousemove = null;
      }
   }
}

I don't understand why mouseStart is accessible but I get the get the error that dist is undefined.

I have other variables that also need to be passed which cannot be redefined during .onmouseup.

Why are you binding an event *inside* an event handler *inside* an event handler? — gen_Eric, Jan 30 '14 at 15:22
Works fine in Chrome [Fiddle](http://fiddle.jshell.net/Pisi2012/VQKrN/) — alexP, Jan 30 '14 at 15:24

score 0 · Accepted Answer · answered Jan 30 '14 at 15:56

I think you'll want add the onmouseup event handler inside the onmousedown handler. The mousemove event fires something like 50 times per second in some browsers, so you repeatedly add and remove the mouse up/down events. Then you'll want to declare dist inside the mousedown handler.

For browser compatibility, the event object gets passed in the event handler by many browsers, and in others it is a global variable. A simple event = window.event || event coupled with declaring an event argument to the handler does the trick.

document.onmousedown = function(event) {
    event = window.event || event;
    var mouseStart = [event.pageX,event.pageY];
    var dist;

    document.onmouseup = function() {
        global_function(dist);
        document.onmousemove = null;
    };

    document.onmousemove = function(ev) {
        ev = window.event || ev;
        dist = Math.sqrt(Math.pow(ev.pageY-mouseStart[1],2)+Math.pow(ev.pageX-mouseStart[0],2));
    };
};

Femi · Answer 2 · 2014-01-30T16:04:30.947

Make sure that the your function definitions aren't passively inheriting the event variable from their closure. You should explicitly accept the event variable in the function definition, i.e. function(event){}, not function(){}.

Also, you're making excessive calculations by computing dist in the mousemove event, it shouldn't be calculated until mouseup.

The following works for me in Firefox.

document.onmousedown = function(event){
    var mouseStart = [event.pageX,event.pageY];
    document.onmouseup = function(event) {
        var dist = Math.sqrt(Math.pow(event.pageY-mouseStart[1],2)+Math.pow(event.pageX-mouseStart[0],2));
        console.log(dist); //global_function(dist);
        document.onmouseup = null;
    }
}

Mehran Hatami · Answer 3 · 2014-01-30T16:06:08.817

I changed it like this and it worked:

document.onmousedown = function (evt) {
    var e = evt || window.event;
    var mouseStart = [e.pageX, e.pageY];
    document.onmouseup = function (ev) {
        var evnt = ev || window.event;
        var dist = Math.sqrt(Math.pow(evnt.pageY - mouseStart[1], 2) + Math.pow(evnt.pageX - mouseStart[0], 2));
        alert(dist);
        document.onmouseup = null;
    };
};

As you see I have removed the onmousemove event, but it still works with the same result.

Accessing variables with javascript (no jQuery) .onmouseup event

3 Answers3