Node, setImmeidate, setTimeout, nextTick Do you really know?

// test.js setTimeout(() => { console.log('setTimeout'); }, 1) setImmediate(() => { console.log('setImmediate'); }); // Assuming my T is 2, then the output is # setTimeout # setImmediateCopy the code

The choice of t depends entirely on the performance of the machine, and there is also a problem with node setTimeout minimum 1 ms

//lib/internal/timers.js~line#34 after *= 1; // coalesce to number or NaN if (! (after >= 1 && after <= TIMEOUT_MAX)) { if (after > TIMEOUT_MAX) { process.emitWarning(`${after} does not fit into` + ' a 32-bit signed integer.' + '\nTimeout duration was set to 1.', 'TimeoutOverflowWarning'); } after = 1; // schedule on next tick, follows browser behavior }Copy the code

That is, if you set 0 to 1, then you will never see setImmediate executing before setTimeout if your machine performs well and starts executing the Event loop 1 millisecond earlier.

Timeout For details, see the following

Timeout _idleStart is important and refers to the start time of the timer. For example, if a 40-second timer is set at the time of 10 seconds, check whether the now-_idlestart is longer than the specified 40 seconds when the timer expires. This time should be the number of milliseconds after the program starts. (Purely personal guess)

Timer objects in Node are organized by object and linked list. Timers of the same time will be placed in the same linked list. For example, if the timer is set to 40 milliseconds, but the time is set differently, they will be placed in the same list

/ / ╔ ═ ═ ═ ═ > Object Map / / ║ / / ╠ ═ ═ / / ║ refedLists: {' 40 ': {},' 320 ': {etc}} (keys of millisecond duration) / / ╚ ═ ═ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ / / │ / / ╔ ═ ═ │ / / ║ TimersList {_idleNext: {}, _idlePrev: (self), _timer: (TimerWrap)} / / ║ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ / / ║ ╔ ═ ═ │ ^ / / ║ ║ {_idleNext: {}, _idlePrev: {}, _onTimeout: (the callback)} / / ║ ║ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ / / ║ ║ │ ^ / / ║ ║ {_idleNext: {etc}, _idlePrev: {}, _onTimeout: (the callback)} / / ╠ ═ ═ ╠ ═ ═ / / ║ ║ / / ║ ╚ ═ ═ ═ ═ > Actual JavaScript timeouts / / ║ / / ╚ ═ ═ ═ ═ > Linked ListCopy the code

The insertion process is as follows

insert

Initialize the TimerList as follows

When the timer list initializes a TimersList, it sets a libuv timer at the expiration time of the TimersList. When the timer expires, it processes the node timer in the list. If there are any unexpired libuv timers, it continues to set the libuv timer

Now what happens when the timer expirestimer outdate

As you can see from the above, we have set the same timer specifically for setTimeout(0)(although we can’t have zero). What I want to say is, suppose we set two 1msecs timers, see the code

setTimeout(() => { // 1 console.log(1); setTimeout(() => { // 2 console.log(2); }); setImmediate(() => { // 3 console.log(3); })})Copy the code

The code 1, 2 above has an expiration time of 1, so they are in the same timerList, and we saw earlier that when we were processing the timerList while we knew that 3 must be output before 2, but we were processing the timerList while, Now is set before cb, and timer._idlestart is set at setTimeout. That means, The _idleStart of 2 must be larger than the now of 1, so it is obvious that 2 is out of date no matter what, even in the following code

setTimeout(() => { // 1 console.log(1); setTimeout(() => { // 2 console.log(2); }); for(let i = 0; i < 100e100; I ++) {// Let c = 0; } setImmediate(() => { // 3 console.log(3); })})Copy the code

The overall organization of timers is to facilitate management and reduce the use of real timers at the bottom.