setTimeout in loop to check for a change in bounds

This is my code:

var b;
while(!b){
    setTimeout(function(){
        alert('sss')
        b=1;
    }, 500);
}

and it will not alert 'sss'

What can i do?

Updated:

I want to get bounds on google maps v3:

function get_bounds(){
            var bounds_;
            while(!bounds_){
                setTimeout(function(){
                    bounds_=map.getBounds();
                    if(bounds_){
                        var leftBottom=[bounds_.getSouthWest().lat(),bounds_.getSouthWest().lng()]
                        var rightTop=[bounds_.getNorthEast().lat(),bounds_.getNorthEast().lng()]
                        return [leftBottom,rightTop];
                        }
                }, 500);
            }
        }

updated2:

hi patrick dw, i don't know why , b开发者_开发问答ut your code doesn't work:

var b;
function waitForB() {
    setTimeout(function(){
        if(!b)
            waitForB();
        else
            alert('sss');
    }, 500);
}
waitForB()

updated3:

it is ok now :

var b;
function waitForB() {
    setTimeout(function(){
        if(!b){
            waitForB();
            b='ss';
        }
        else{
            alert('sss')
        }
    }, 500);
}
waitForB()

JavaScript in web browsers is run in a single thread. When you call setTimeout(), it won't spawn a new thread. This means that setTimeout() will not execute until all of your main code has finished executing.

For this reason, you will end up with an infinite loop, because your loop condition is dependant on the execution of the setTimeout() callback.

Here's an interesting article on how JavaScript timers work:

• How JavaScript Timers Work by John Resig

UPDATE:

Further to the updated question, you may want to listen to the bounds_changed event instead. I am not sure how you are planning to use your get_bounds() function, but you may want to refactor your logic to use an event listener instead:

google.maps.event.addListener(map,'bounds_changed', function () {
   // The code here is triggered when the bounds change
});

That code is going to burn CPU time and memory by scheduling timeouts to happen. Think about it: you're loop condition is "b" becoming truthy. How is that going to happen? Only when a timer event fires. Will that happen? No, because you're eating the whole machine scheduling zillions more timeouts.

This sort of situation has as a tell-tale sign the effect of warming up the room you're sitting in.

I don't know what effect you're trying to get. Why not start by just the setTimeout() call and see how that goes. Maybe you could describe more about what it is you're trying to do.

Maybe you will want to use setInterval instead of setTimeout. When b is changed, alert shows up.

var b = false;
(function () {
    var intervalId;
    function wait() {
        if (b) {
            clearInterval(intervalId);
            alert('sss');
        }
    }
    intervalId = setInterval(wait, 500);
})();

It is more intuitive and it doesn't mess with global variables too much.

HINT: Put semicolon after every statement if you are not sure where to omit safely.

This problem can now be solved correctly using the idle rather than the bounds_changed event listener:

google.maps.event.addListener(map, 'idle', function() { 
  updateStuff();
});

This event is fired when the map becomes idle after panning or zooming. http://code.google.com/apis/maps/documentation/javascript/reference.html

It also fires after the map is first rendered, so this is probably the only event listener you need on your map in order to keep it up to date.