Node | Execution order

What could be execution order

Case1: Code in main module (Normal case)

• setTimeout(() => console.log("timeout"), 0);
• setImmediate(() => console.log("immediate"));
• Promise.resolve().then(() => console.log("promise"));
• process.nextTick(() => console.log("nextTick"));

Output:

1. nextTick
2. promise
3. timeout
4. immediate

Explanation:

Microtasks run after every phase, before moving to next phase.

Microtasks are executed immediately after the current operation, before the event loop proceeds to the next phase

Execution priority:

1. process.nextTick
2. Promises (.then)
3. Then event loop phases
   
   

• nextTick will go in process.nextTick() queue
• Promise will go in Promise microtask queue

Based on above given priority nextTick priority next click is always on top and then promise.

1. process.nextTick (nextTick)
2. Promises (.then)  (Promise)
3. Event Loop (setimmediate, settimeout, setinterval)

Event loop phases in order

1. Timers
2. Pending Callbacks
3. Idle, Prepare (internal)
4. Poll (MOST IMPORTANT)
5. Check
6. Close Callbacks

settimeout, setinterval executes in Timers phase 

Setimmediate executes in Check phase 

In event loop first phase is Timers so timeout will execute first then setimmediate in Check phase.

This is how below order comes in output

1. nextTick
2. promise
3. timeout
4. immediate

                                                  ==========================

Case2: The callback runs inside the Poll phase (I/O phase)8 (Special case)

const fs = require("fs");

fs.readFile(__filename, () => {
setTimeout(() => console.log("timeout"), 0);
setImmediate(() => console.log("immediate"));
Promise.resolve().then(() => console.log("promise"));
process.nextTick(() => console.log("nextTick"));
});

Output:

1. nextTick
2. promise
3. immediate
4. timeout

immediate and timeout sequence changed compare to normal case  

Explanation:

Above code will run inside  the Poll phase (I/O phase) because its I/O file read operation.
All I/O operations/API calls goes in Poll phase 

Inside that callback, we schedule:

API	Goes to
setTimeout	Timers queue
setImmediate	Check queue
Promise.then	Microtask queue
process.nextTick	NextTick queue

nextTick registers in process.nextTick() queue
Promise registers in Promise microtask queue

• next click is top priority so its at no.1
• microtask queue executed between each phase of Event Loop. currently execution is at poll phase as execution move to next phase first it will check and execute if anything in microtask queue, so Promise is at no.2
• next phase of event loop is check phase in which immediate get registered, so immediate is at no.3
• Execution finishes this cycle an re-iterate next cycle, in next cycle first phase is Timers, so timeout is at no.4