Why is setImmediate () executed before fs.readFile () in Nodejs Event Loop works?

Question

I read a lot of related documents. But I still can’t understand how it works.

const fs = require('fs')
const now = Date.now();

setTimeout(() => console.log('timer'), 10);
fs.readFile(__filename, () => console.log('readfile'));
setImmediate(() => console.log('immediate'));
while(Date.now() - now < 1000) {
}

---

const now = Date.now();

setImmediate(() => console.log('immediate'));
setTimeout(() => console.log('timer'), 10);
while(Date.now() - now < 1000) {
}

I think the first part of the code should be logged:

readfile
immediate

And the second part of the code logs.

timer
immediate

I think that everything is in order.

Problem: I do not understand why the first part of the code logs

immediate
readfile

I think the file has been completely read and its callback function enqueue I / O accesses the phase of the queue after 1 second.

And then I think the cycle of events will move to timers(none), I/O callbacks(fs.readFile callback), idle/prepare(none), poll(none), check(setImmediate callback)and, finally, close callbacks(none)in order, but the result is that setImmediate()still running first.

Answer 1

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setImmediate() , readFile().

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, while :

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• I/O. readFile() - ( ).
• checkHandlers, setImmediate().
• . , - , , readFile().

---

, , , , . ( ):

const fs = require('fs')

let begin = 0;
function log(msg) {
    if (!begin) {
        begin = Date.now();
    }
    let t = ((Date.now() - begin) / 1000).toFixed(3);
    console.log("" + t + ": " + msg);
}

log('start program');

setTimeout(() => log('timer'), 10);
setImmediate(() => log('immediate'));
fs.readFile(__filename, () => log('readfile'));

const now = Date.now();
log('start loop');
while(Date.now() - now < 1000) {}
log('done loop');

:

0.000: start program
0.004: start loop
1.004: done loop
1.005: timer
1.006: immediate
1.008: readfile

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---

readFile() setImmediate() setTimeout(). , :

const fs = require('fs')

let begin = 0;
function log(msg) {
    if (!begin) {
        begin = Date.now();
    }
    let t = ((Date.now() - begin) / 1000).toFixed(3);
    console.log("" + t + ": " + msg);
}

log('start program');

setImmediate(() => log('immediate'));
setTimeout(() => log('timer'), 10);

const now = Date.now();
log('start loop');
while(Date.now() - now < 1000) {}
log('done loop');

:

0.000: start program
0.003: start loop
1.003: done loop
1.005: timer
1.008: immediate

( , ).

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• setTimeout() .
• while 1000
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• , , setImmediate(), immediate.

---

-, :

// timeout_vs_immediate.js
const fs = require('fs');

fs.readFile(__filename, () => {
  setTimeout(() => {
    console.log('timeout');
  }, 0);
  setImmediate(() => {
    console.log('immediate');
  });
});

, setTimeout() setImmediate() , . setImmediate() , :

 immediate
 timeout

- , " -". setTimeout() setImmediate() -, " -" . , setImmediate() , .

setImmediate() setTimeout() , setImmediate(), . , , , .

Answer 2

setTimeout(() => console.log('timer'), 10);                
fs.readFile(__filename, () => console.log('readfile'));    
setImmediate(() => console.log('immediate'));               

while(Date.now() - now < 1000) {
}

• setTimeout, 10 .

• .

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• . .

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• , timer 10 .

• ( ) . O - .

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• - . , . .

• setImmediate .