node/doc/guides/timers-in-node.md

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Timers in Node.js and beyond

The Timers module in Node.js contains functions that execute code after a set period of time. Timers do not need to be imported via require(), since all the methods are available globally to emulate the browser JavaScript API. To fully understand when timer functions will be executed, it's a good idea to read up on the the Node.js Event Loop.

Controlling the Time Continuum with Node.js

The Node.js API provides several ways of scheduling code to execute at some point after the present moment. The functions below may seem familiar, since they are available in most browsers, but Node.js actually provides its own implementation of these methods. Timers integrate very closely with the system, and despite the fact that the API mirrors the browser API, there are some differences in implementation.

"When I say so" Execution ~ setTimeout()

setTimeout() can be used to schedule code execution after a designated amount of milliseconds. This function is similar to window.setTimeout() from the browser JavaScript API, however a string of code cannot be passed to be executed.

setTimeout() accepts a function to execute as its first argument and the millisecond delay defined as a number as the second argument. Additional arguments may also be included and these will be passed on to the function. Here is an example of that:

function myFunc(arg) {
  console.log('arg was => ' + arg);
}

setTimeout(myFunc, 1500, 'funky');

The above function myFunc() will execute as close to 1500 milliseconds (or 1.5 seconds) as possible due to the call of setTimeout().

The timeout interval that is set cannot be relied upon to execute after that exact number of milliseconds. This is because other executing code that blocks or holds onto the event loop will push the execution of the timeout back. The only guarantee is that the timeout will not execute sooner than the declared timeout interval.

setTimeout() returns a Timeout object that can be used to reference the timeout that was set. This returned object can be used to cancel the timeout ( see clearTimeout() below) as well as change the execution behavior (see unref() below).

"Right after this" Execution ~ setImmediate()

setImmediate() will execute code at the end of the current event loop cycle. This code will execute after any I/O operations in the current event loop and before any timers scheduled for the next event loop. This code execution could be thought of as happening "right after this", meaning any code following the setImmediate() function call will execute before the setImmediate() function argument.

The first argument to setImmediate() will be the function to execute. Any subsequent arguments will be passed to the function when it is executed. Here's an example:

console.log('before immediate');

setImmediate((arg) => {
  console.log(`executing immediate: ${arg}`);
}, 'so immediate');

console.log('after immediate');

The above function passed to setImmediate() will execute after all runnable code has executed, and the console output will be:

before immediate
after immediate
executing immediate: so immediate

setImmediate() returns and Immediate object, which can be used to cancel the scheduled immediate (see clearImmediate() below).

Note: Don't get setImmediate() confused with process.nextTick(). There are some major ways they differ. The first is that process.nextTick() will run before any Immediates that are set as well as before any scheduled I/O. The second is that process.nextTick() is non-clearable, meaning once code has been scheduled to execute with process.nextTick(), the execution cannot be stopped, just like with a normal function. Refer to this guide to better understand the operation of process.nextTick().

"Infinite Loop" Execution ~ setInterval()

If there is a block of code that should execute multiple times, setInterval() can be used to execute that code. setInterval() takes a function argument that will run an infinite number of times with a given millisecond delay as the second argument. Just like setTimeout(), additional arguments can be added beyond the delay, and these will be passed on to the function call. Also like setTimeout(), the delay cannot be guaranteed because of operations that may hold on to the event loop, and therefore should be treated as an approximate delay. See the below example:

function intervalFunc() {
  console.log('Cant stop me now!');
}

setInterval(intervalFunc, 1500);

In the above example, intervalFunc() will execute about every 1500 milliseconds, or 1.5 seconds, until it is stopped (see below).

Just like setTimeout(), setInterval() also returns a Timeout object which can be used to reference and modify the interval that was set.

Clearing the Future

What can be done if a Timeout or Immediate object needs to be cancelled? setTimeout(), setImmediate(), and setInterval() return a timer object that can be used to reference the set Timeout or Immediate object. By passing said object into the respective clear function, execution of that object will be halted completely. The respective functions are clearTimeout(), clearImmediate(), and clearInterval(). See the example below for an example of each:

const timeoutObj = setTimeout(() => {
  console.log('timeout beyond time');
}, 1500);

const immediateObj = setImmediate(() => {
  console.log('immediately executing immediate');
});

const intervalObj = setInterval(() => {
  console.log('interviewing the interval');
}, 500);

clearTimeout(timeoutObj);
clearImmediate(immediateObj);
clearInterval(intervalObj);

Leaving Timeouts Behind

Remember that Timeout objects are returned by setTimeout and setInterval. The Timeout object provides two functions intended to augment Timeout behavior with unref() and ref(). If there is a Timeout object scheduled using a set function, unref() can be called on that object. This will change the behavior slightly, and not call the Timeout object if it is the last code to execute. The Timeout object will not keep the process alive, waiting to execute.

In similar fashion, a Timeout object that has had unref() called on it can remove that behavior by calling ref() on that same Timeout object, which will then ensure its execution. Be aware, however, that this does not exactly restore the initial behavior for performance reasons. See below for examples of both:

const timerObj = setTimeout(() => {
  console.log('will i run?');
}, 1);

// if left alone, this statement will keep the above
// timeout from running, since the timeout will be the only
// thing keeping the program from exiting
timerObj.unref();

// we can bring it back to life by calling ref() inside
// an immediate
setImmediate(() => {
  timerObj.ref();
});

Further Down the Event Loop

There's much more to the Event Loop and Timers than this guide has covered. To learn more about the internals of the Node.js Event Loop and how Timers operate during execution, check out this Node.js guide: The Node.js Event Loop, Timers, and process.nextTick().