The setImmediate () function is called after the setTimeout () function.

Question

The official sites of nodejs ( https://nodejs.org/api/timers.html#timers_setimmediate_callback_arg ) say:

setImmediate () function schedules "immediate" callback execution after callbacks of I / O events and before the timers setTimeout and setInterval are started.

However, in the code below, the setTimeout () function is executed before setImmediate (). Why?

setImmediate(function A() { setImmediate(function B() { console.log(1); setImmediate(function D() { console.log(2); }); setImmediate(function E() { console.log(3); }); }); setImmediate(function C() { console.log(4); setImmediate(function F() { console.log(5); }); setImmediate(function G() { console.log(6); }); }); }); setTimeout(function timeout() { console.log('TIMEOUT FIRED'); }, 0) 

Result:

  TIMEOUT FIRED
 one
 4
 2
 3
 5
 6

I am writing another example, and setTimeout works up to setImmediate here too.

 setTimeout(function timeout() { console.log('TIMEOUT-1 FIRED'); }, 0) setTimeout(function timeout() { console.log('TIMEOUT-2 FIRED'); }, 0) setImmediate(function D() { console.log(1); }); setImmediate(function D() { console.log(2); }); setImmediate(function D() { console.log(3); }); setTimeout(function timeout() { console.log('TIMEOUT-1 FIRED'); }, 0) setTimeout(function timeout() { console.log('TIMEOUT-2 FIRED'); }, 0) 

Output:

  TIMEOUT-1 FIRED
 TIMEOUT-2 FIRED
 TIMEOUT-1 FIRED
 TIMEOUT-2 FIRED
 one
 2
 3

Answer 1

Let's write the above example as follows:

 var fs = require('fs') fs.readFile("readme.txt", function (){ setTimeout(function timeout() { console.log('TIMEOUT-1 FIRED'); }, 0) setTimeout(function timeout() { console.log('TIMEOUT-2 FIRED'); }, 0) setImmediate(function D() { console.log(1); }); setImmediate(function D() { console.log(2); }); setImmediate(function D() { console.log(3); }); setTimeout(function timeout() { console.log('TIMEOUT-1 FIRED'); }, 0) setTimeout(function timeout() { console.log('TIMEOUT-2 FIRED'); }, 0)}) 

Exit:

 1 2 3 TIMEOUT-1 FIRED TIMEOUT-2 FIRED TIMEOUT-1 FIRED TIMEOUT-2 FIRED 

Explanation:

The execution order of the timers will depend on the context in which they are called. If both are called from the main module, then synchronization will be related to the performance of the process (which can be affected by other applications running on the machine). For example, if we run the following script that is not in the input / output cycle (i.e., the main module), the order in which two timers are executed is non-deterministic, since it is related to the performance of the process:

 // timeout_vs_immediate.js setTimeout(function timeout () { console.log('timeout'); },0); setImmediate(function immediate () { console.log('immediate'); }); $ node timeout_vs_immediate.js timeout immediate $ node timeout_vs_immediate.js immediate timeout 

However, if you move two calls during an I / O cycle, the immediate callback is always executed first:

 // timeout_vs_immediate.js var fs = require('fs') fs.readFile(__filename, () => { setTimeout(() => { console.log('timeout') }, 0) setImmediate(() => { console.log('immediate') }) }) $ node timeout_vs_immediate.js immediate timeout $ node timeout_vs_immediate.js immediate timeout 

The main advantage of using setImmediate () over setTimeout () is that setImmediate () will always be executed before any timers if it is planned during an I / O cycle, regardless of the number of timers.

For more information, go to the following link: https://github.com/nodejs/node/blob/master/doc/topics/the-event-loop-timers-and-nexttick.md