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Last active Apr 12, 2022
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A Very Fast Javascript thread yield (see blog posting)
// See
// This is a very fast "asynchronous" flow control - i.e. it yields the thread and executes later,
// but not much later. It is far faster and lighter than using setTimeout(fn,0) for yielding threads.
// Its also faster than other setImmediate shims, as it uses Mutation Observer and "mainlines" successive
// calls internally.
// WARNING: This does not yield to the browser UI loop, so by using this repeatedly
// you can starve the UI and be unresponsive to the user.
// Note: For an even faster version, see
var soon = (function() {
var fq = []; // function queue;
function callQueue()
while(fq.length) // this approach allows new yields to pile on during the execution of these
var fe = fq[0];
fe.f.apply(fe.m,fe.a) // call our fn with the args and preserve context
fq.shift(); // remove element just processed... do this after processing so we don't go 0 and trigger soon again
// run the callQueue function asyncrhonously, as fast as possible
var cqYield = (function() {
// This is the fastest way browsers have to yield processing
if(typeof MutationObserver !== "undefined")
// first, create a div not attached to DOM to "observe"
var dd = document.createElement("div");
var mo = new MutationObserver(callQueue);
mo.observe(dd, { attributes: true });
return function(fn) { dd.setAttribute("a",0); } // trigger callback to
// if No MutationObserver - this is the next best thing - handles Node and MSIE
if(typeof setImmediate !== "undefined")
return function() { setImmediate(callQueue) }
// final fallback - shouldn't be used for much except very old browsers
return function() { setTimeout(callQueue,0) }
// this is the function that will be assigned to soon
// it takes the function to call and examines all arguments
return function(fn) {
// push the function and any remaining arguments along with context
if(fq.length == 1) // upon adding our first entry, kick off the callback
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Developer27149 commented May 18, 2021

It's cool.But why not use "process.nextTick" before the "setImmediate"? @bluejava

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bluejava commented May 18, 2021

Thanks - I think there are some speed improvements in the version included in Zousan. I would recommend including Zousan and calling Zousan.soon rather than pasting this gist (though you are welcome to of course!) Zousan is tiny and maintained. I've been using it in production for many years - and Zouson.soon for a high performance centralized message bus used heavily in production for a few years as well. If I get a chance I will publish the message bus as well.


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Developer27149 commented May 18, 2021

Happy to see your comment.😁
I am learning how to create a simple Promise implement,and glad to see your articles on your blog.
Let's keep going.

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