Difference between process.nextTick() and setImmediate() Methods

process.nextTick() runs code immediately after the current operation, before I/O tasks. setImmediate() schedules code to run after the current event loop phase, following I/O tasks, impacting execution timing.

To understand the difference between process.nextTick() and setImmediate() methods, we first need to understand the working of Node.js Event Loop. 

What is Node.js Event Loop?

Due to the fact that JavaScript is single-threaded i.e. it performs only a single process at a time, so it is the Event Loop that allows Node.js to perform non-blocking I/O operations. 

Working of Event Loop?

At the start of a JavaScript program, an event loop is initialized. There are several operations that execute in an event loop. Below is the sequence in which different they are executed in a single iteration of the event loop. These operations are processed in a queue.

timers-->pending callbacks-->idle,prepare-->connections(poll,data,etc)-->check-->close callbacks

process.nextTick() method

Whenever a new queue of operations is initialized we can think of it as a new tick. The process.nextTick() method adds the callback function to the start of the next event queue. It is to be noted that, at the start of the program process.nextTick() method is called for the first time before the event loop is processed. 

Syntax:

process.nextTick(callback);

setImmediate() method

Whenever we call setImmediate() method, it's callback function is placed in the check phase of the next event queue. There is slight detail to be noted here that setImmediate() method is called in the poll phase and it's callback functions are invoked in the check phase.

Syntax:

setImmediate(callback);

Example: This example illustrates that process.nextTick() callbacks execute before setImmediate() callbacks, even though both are scheduled in the same event loop iteration.

setImmediate(function A() {
	console.log("1st immediate");
});

setImmediate(function B() {
	console.log("2nd immediate");
});

process.nextTick(function C() {
	console.log("1st process");
});

process.nextTick(function D() {
	console.log("2nd process");
});

// First event queue ends here
console.log("program started");

Explanation:

For the above program, event queues are initialized in the following manner:

1. In the first event queue only 'program started is printed'.
2. Then second event queue is started and function C i.e. callback of process.nextTick() method is placed at the start of the event queue. C is executed and the queue ends.
3. Then previous event queue ends and third event queue is initialized with callback D. Then callback function A of setImmdeiate() method is placed in the followed by B. Now, the third event queue looks like this,

D A B

Now functions D, A, B are executed in the order they are present in the queue. 

Output: 

Difference between process.nextTick() and setImmediate() Methods

Conclusion

process.nextTick() runs code immediately after the current operation, before any I/O tasks or timers, making it ideal for urgent tasks. setImmediate() schedules code to run after the current event loop phase, particularly after I/O tasks, ensuring smoother execution flow.