How to rate-limit ajax requests?

The excellent Underscore.js has a throttle function. You pass in the handler that you want to throttle and get back a rate-limited version of the same function.

var throttled = _.throttle(someHandler, 100);
$(div).click(throttled);

http://documentcloud.github.com/underscore/#throttle

Here’s a simplified version that I’ve used in my own code:

function throttle(func, wait) {
    var timeout;
    return function() {
        var context = this, args = arguments;
        if (!timeout) {
            // the first time the event fires, we setup a timer, which 
            // is used as a guard to block subsequent calls; once the 
            // timer's handler fires, we reset it and create a new one
            timeout = setTimeout(function() {
                timeout = null;
                func.apply(context, args);
            }, wait);
        }
    }
}

A good way to test it is by firing off a bunch of scroll events and watching your handler log to the Firebug console:

document.addEventListener("scroll", throttle(function() {
    console.log("test");
}, 2000), false); 

Here’s a version that limits click-events on divs to once every 30 seconds, as requested (requires jQuery):

$("div").click(throttle(function() {
    // ajax here
}, 30000));