process.nexTick vs setImmediate

process.nextTick and setImmediate are two functions in Node.js that are used to schedule the execution of callback functions. They are similar but have different timing and use cases. Here's a breakdown of their differences along with examples:
process.nextTick

process.nextTick

Timing: process.nextTick schedules a callback function to be invoked in the same phase of the event loop, right after the current operation completes and before the event loop continues.

Use Case: It is useful when you want to execute a callback immediately after the current operation but before any I/O operations or timers.

setImmediate

Timing: setImmediate schedules a callback function to be invoked in the next iteration of the event loop, after I/O events' callbacks.

Use Case: It is useful when you want to defer the execution of a function until the current I/O events have been processed.

Example


const fs = require('fs');

console.log('Start');

// Schedule with process.nextTick
process.nextTick(() => {
    console.log('Next Tick');
});

// Schedule with setImmediate
setImmediate(() => {
    console.log('Immediate');
});

// Read a file (simulates I/O operation)
fs.readFile(__filename, () => {
    console.log('File Read');

    // Schedule another nextTick inside I/O
    process.nextTick(() => {
        console.log('Next Tick inside I/O');
    });

    // Schedule another setImmediate inside I/O
    setImmediate(() => {
        console.log('Immediate inside I/O');
    });
});

console.log('End');

Expected Output

Start
End
Next Tick
File Read
Next Tick inside I/O
Immediate inside I/O
Immediate

Explanation

Initial Phase:
    console.log('Start') is executed.
    process.nextTick(() => { console.log('Next Tick'); }) schedules a callback to be executed after the current phase.
    setImmediate(() => { console.log('Immediate'); }) schedules a callback for the next iteration of the event loop.
    fs.readFile(__filename, () => { ... }) schedules an I/O operation.

After Initial Phase:
    console.log('End') is executed.
    process.nextTick callback is executed: console.log('Next Tick').

I/O Phase:
    File read operation completes, console.log('File Read') is executed.
    Inside the I/O callback:
        process.nextTick(() => { console.log('Next Tick inside I/O'); }) schedules another callback for the current phase.
        setImmediate(() => { console.log('Immediate inside I/O'); }) schedules another callback for the next iteration of the event loop.
    process.nextTick callback inside I/O is executed: console.log('Next Tick inside I/O').

Next Event Loop Iteration:
    All setImmediate callbacks are executed in order: console.log('Immediate inside I/O') and console.log('Immediate').

Summary

process.nextTick executes the callback before the next I/O event.
setImmediate executes the callback after the current I/O events.

https://medium.com/@a35851150/interview-qustions-on-event-loop-in-js-901c567a1271
https://medium.com/@a35851150/interview-qustions-on-event-loop-in-js-901c567a1271