Published Oct 03, 2019
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setImmediate() is equivalent to setTimeout(fn, 0), but with some slight advantages.
setTimeout(fn, delay) calls the give callbacks after the given delay has ellapsed (in
milliseconds). However, the callback is not executed immediately at this time, but added
to the function queue so that it is executed as soon as possible, after all the
currently executing and currently queued event handlers have completed. Setting the delay
to 0 adds the callback to the queue immediately so that it is executed as soon as all
currently queued functions are finished.
setImmediate achieves the same effect, except that it doesn’t use the queue of
functions. Instead, it checks the queue of I/O event handlers. If an I/O events in the
current snapshot are processed, it executes the callback. It queue them immediately after
the last I/O handler, somewhat like process.nextTick. This is faster than
setTimeout(fn, 0).
console.log("first");
setTimeout(() => {
console.log("second");
}, 0);
console.log("third");
console.log("first");
setImmediate(() => {
console.log("second");
});
console.log("third");
References: