Javascript event handling and flow control

event-handling, events, javascript, loops

Solution

In JavaScript, you give up control of the main loop. The browser runs the main loop and calls back down into your code when an event or timeout/interval occurs. You have to handle the event and then return so that the browser can get on with doing other things, firing events, and so on.

So you cannot have a ‘listening’ loop. The browser does that for you, giving you the event and letting you deal with it, but once you've finished handling the event you must return. You can't fall back into a different loop. This means you can't write step-by-step procedural code; if you have state that persists between event calls you must store it, eg. in a variable.

This approach cannot work:

<input type="text" readonly="readonly" value="" id="status" />

var s= document.getElementById('status');
s.value= 'Press A now';
while (true) {
    var e= eventLoop.nextKeyEvent(); // THERE IS NO SUCH THING AS THIS
    if (e.which=='a')
        break
}
s.value= 'Press Y or N';
while (true) {
    var e= eventLoop.nextKeyEvent();
    if (e.which=='y') ...

Step-by-step code has to be turned inside out so that the browser calls down to you, instead of you calling up to the browser:

var state= 0;
function keypressed(event) {
    var key= String.fromCharCode(event? event.which : window.event.keyCode); // IE compatibility
    switch (state) {
        case 0:
            if (key=='a') {
                s.value= 'Press Y or N';
                state++;
            }
            break;
        case 1:
            if (key=='y') ...
            break;
    }
}

s.value= 'Press A now';
document.onkeypress= keypressed;

You can also make code look a little more linear and clean up some of the state stuff by using nested anonymous functions:

s.value= 'Press A now';
document.onkeypress= function(event) {
    var key= String.fromCharCode(event? event.which : window.event.keyCode);
    if (key=='a') {
        s.value= 'Press Y or N';
        document.onkeypress= function(event) {
            var key= String.fromCharCode(event? event.which : window.event.keyCode);
            if (key=='y') ...
        };
    }
};

Problem

I'm attempting to build a webpage that loads depending on the input provided. I'm having some trouble wrapping my head around event handling in javascript, basically. Coming from python, if I wanted to wait for a specific keyboard input before moving on to the next object to display, I would create a while loop and put a key listener inside it. Python: ``` def getInput(): while 1: for event in pygame.event.get(): #returns a list of events from the keyboard/mouse if event.type == KEYDOWN: if event.key == "enter": # for example do function() return elif event.key == "up": do function2() continue else: continue # for clarity ``` In trying to find a way to implement this in DOM/javascript, I seem to just crash the page (I assume due to the While Loop), but I presume this is because my event handling is poorly written. Also, registering event handlers with "element.onkeydown = function;" difficult for me to wrap my head around, and setInterval(foo(), interval] hasn't brought me much success. Basically, I want a "listening" loop to do a certain behavior for key X, but to break when key Y is hit.

Original source