throttle function for node.js

throttle.js

/**
 * 限制fn同时执行的个数，fn必须符合node函数规范，即参数最后一位是回调函数
 * 如果有this绑定需求，需要提前bind好，否则this会变null。
 *
 * @param fn
 * @param concurrency
 * @returns {Function}
 */
function throttle(fn, concurrency) {
    if (concurrency <= 0) {
        throw new Error("concurrency must large than zero");
    }
    var queue = [];
    var cnt = concurrency;

    function callFn(args) {
        var cb = args[args.length - 1];
        var argsWithNewCallback = Array.prototype.slice.call(args, 0, args.length - 1);
        argsWithNewCallback.push(function newCb() {
            var cbArgs = arguments;
            process.nextTick(function () {
                cb.apply(null, cbArgs);
            });
            cnt++;
            if (queue.length > 0) {
                var queuedArgs = queue.shift();
                cnt--;
                callFn(queuedArgs);
            }
        });
        fn.apply(null, argsWithNewCallback);
    }

    return function newFn() {
        if (cnt > 0) {
            cnt--;
            callFn(arguments);
        } else {
            queue.push(arguments);
        }
    };
}

module.exports = throttle;

if (!module.parent) {
    // 一段简单的自测试代码
    var echoThrottled = throttle(function echo(content, cb) {
        setTimeout(function () {
            console.log(content);
            cb(content);
        }, 1000);
    }, 2);
    for (var i = 0; i < 10; i++) {
        echoThrottled(i, function cb(i) {
            echoThrottled(i, cb);
        });
    }
}