JavaScript doesn't return value

Question

I have some simple code that returns whether I'm scrolling up or down. I want to use it as a function with return, like this:

var scripts = {
getScrollDirection: function() {
    var dir;
    $(document).on('mousewheel DOMMouseScroll', function(e) {
        if(e.originalEvent.wheelDelta / 120 > 0) {
            dir = 'up';
        } else {
            dir = 'down';
        }
    });
    return dir;
}
}

When I call scripts.getScrollDirection, I get undefined. What I am doing wrong?

Setting up and event handler doesn't mean the code inside is going to run, so dir will stay undefined. Maybe you want to place the code for the function in mousewheel DOMMouseScroll? — Spencer Wieczorek
– Spencer Wieczorek, Commented Jul 9, 2015 at 16:41
are your codes in different js files? or located at different parts of the html document? — Ji_in_coding
– Ji_in_coding, Commented Jul 9, 2015 at 16:42
@SpencerWieczorek it's odd since I used similar statements before. When I add console.log inside the if/else statements, it works. I really want to avoid placing function inside mousewheel event. — Tomek Buszewski
– Tomek Buszewski, Commented Jul 9, 2015 at 16:44
@TomekBuszewski It is not odd at all, because you have two different functions that are called at different times. Inside the callback (and after it is called) you set a value to dir. Outside the callback, and before it is called, you have not. — dsh
– dsh, Commented Jul 9, 2015 at 16:46

dsh · Accepted Answer · 2015-07-09 16:44:44Z

2

You declared dir, and it is automatically initialized to undefined. Then you registered a function as an event handler. Then you returned the value of dir, which is still undefined because no value has been assigned to it.

https://en.wikipedia.org/wiki/Event-driven_programming

http://cs.brown.edu/courses/cs168/f12/handouts/async.pdf

http://www.i-programmer.info/programming/theory/6040-what-is-asynchronous-programming.html

answered Jul 9, 2015 at 16:44

dsh

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1 Comment

Tomek Buszewski Over a year ago

You're right, I forgot that it will change after an event. Thanks for this!

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