sleep.js

function createSleepPromise(timeout) {
    return new Promise(function(resolve) {
        setTimeout(resolve, timeout);
    });
};

function sleep(timeout) {
    // Pass value through, if used in a promise chain
    function promiseFunction(value) {
        return createSleepPromise(timeout).then(function() {
            return value;
        });
    };

    // Normal promise
    promiseFunction.then = function() {
        var sleepPromise = createSleepPromise(timeout);

        return sleepPromise.then.apply(sleepPromise, arguments);
    };
    promiseFunction.catch = Promise.resolve().catch;

    return promiseFunction;
}