Throttle JavaScript function calls, but with queui

Question

How can a function rate-limit its calls? The calls should not be discarded if too frequent, but rather be queued up and spaced out in time, X milliseconds apart. I've looked at throttle and debounce, but they discard calls instead of queuing them up to be run in the future.

Any better solution than a queue with a process() method set on an X millisecond interval? Are there such standard implementations in JS frameworks? I've looked at underscore.js so far - nothing.

Answer 1

Here is an example which carries forward this (or lets you set a custom one)

function RateLimit(fn, delay, context) {
    var canInvoke = true,
        queue = [],
        timeout,
        limited = function () {
            queue.push({
                context: context || this,
                arguments: Array.prototype.slice.call(arguments)
            });
            if (canInvoke) {
                canInvoke = false;
                timeEnd();
            }
        };
    function run(context, args) {
        fn.apply(context, args);
    }
    function timeEnd() {
        var e;
        if (queue.length) {
            e = queue.splice(0, 1)[0];
            run(e.context, e.arguments);
            timeout = window.setTimeout(timeEnd, delay);
        } else
            canInvoke = true;
    }
    limited.reset = function () {
        window.clearTimeout(timeout);
        queue = [];
        canInvoke = true;
    };
    return limited;
}

Now

function foo(x) {
    console.log('hello world', x);
}
var bar = RateLimit(foo, 1e3);
bar(1); // logs: hello world 1
bar(2);
bar(3);
// undefined, bar is void
// ..
// logged: hello world 2
// ..
// logged: hello world 3

Answer 2

While the snippets offered by others do work (I've built a library based on them), for those who want to use well-supported modules, here are the top choices:

• the most popular rate limiter is limiter
• function-rate-limit has a simple API that works, and good usage stats on npmjs
• valvelet, a newer module, claims to do an even better job by supporting promises, but hasn't gained popularity yet

Answer 3

Should be rather simple without a library:

var stack = [], 
    timer = null;

function process() {
    var item = stack.shift();
    // process
    if (stack.length === 0) {
        clearInterval(timer);
        timer = null;
    }
}

function queue(item) {
    stack.push(item);
    if (timer === null) {
        timer = setInterval(process, 500);
    }
}

http://jsfiddle.net/6TPed/4/