node/doc/api/timers.md

Timers

Stability: 2 - Stable

The timer module exposes a global API for scheduling functions to be called at some future period of time. Because the timer functions are globals, there is no need to call require('timers') to use the API.

The timer functions within Node.js implement a similar API as the timers API provided by Web Browsers but use a different internal implementation that is built around the Node.js Event Loop.

Class: Immediate

This object is created internally and is returned from setImmediate(). It can be passed to clearImmediate() in order to cancel the scheduled actions.

Class: Timeout

This object is created internally and is returned from setTimeout() and setInterval(). It can be passed to clearTimeout() or clearInterval() (respectively) in order to cancel the scheduled actions.

By default, when a timer is scheduled using either setTimeout() or setInterval(), the Node.js event loop will continue running as long as the timer is active. Each of the Timeout objects returned by these functions export both timeout.ref() and timeout.unref() functions that can be used to control this default behavior.

timeout.ref()

When called, requests that the Node.js event loop not exit so long as the Timeout is active. Calling timeout.ref() multiple times will have no effect.

Note: By default, all Timeout objects are "ref'd", making it normally unnecessary to call timeout.ref() unless timeout.unref() had been called previously.

Returns a reference to the Timeout.

timeout.unref()

When called, the active Timeout object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the Timeout object's callback is invoked. Calling timeout.unref() multiple times will have no effect.

Note: Calling timeout.unref() creates an internal timer that will wake the Node.js event loop. Creating too many of these can adversely impact performance of the Node.js application.

Returns a reference to the Timeout.

Scheduling Timers

A timer in Node.js is an internal construct that calls a given function after a certain period of time. When a timer's function is called varies depending on which method was used to create the timer and what other work the Node.js event loop is doing.

setImmediate(callback[, ...args])

• callback {Function} The function to call at the end of this turn of the Node.js Event Loop
• ...args {any} Optional arguments to pass when the callback is called.

Schedules the "immediate" execution of the callback after I/O events' callbacks. Returns an Immediate for use with clearImmediate().

When multiple calls to setImmediate() are made, the callback functions are queued for execution in the order in which they are created. The entire callback queue is processed every event loop iteration. If an immediate timer is queued from inside an executing callback, that timer will not be triggered until the next event loop iteration.

If callback is not a function, a TypeError will be thrown.

Note: This method has a custom variant for promises that is available using util.promisify():

const util = require('util');
const setImmediatePromise = util.promisify(setImmediate);

setImmediatePromise('foobar').then((value) => {
  // value === 'foobar' (passing values is optional)
  // This is executed after all I/O callbacks.
});

// or with async function
async function timerExample() {
  console.log('Before I/O callbacks');
  await setImmediatePromise();
  console.log('After I/O callbacks');
}
timerExample();

setInterval(callback, delay[, ...args])

• callback {Function} The function to call when the timer elapses.
• delay {number} The number of milliseconds to wait before calling the callback.
• ...args {any} Optional arguments to pass when the callback is called.

Schedules repeated execution of callback every delay milliseconds. Returns a Timeout for use with clearInterval().

When delay is larger than 2147483647 or less than 1, the delay will be set to 1.

If callback is not a function, a TypeError will be thrown.

setTimeout(callback, delay[, ...args])

• callback {Function} The function to call when the timer elapses.
• delay {number} The number of milliseconds to wait before calling the callback.
• ...args {any} Optional arguments to pass when the callback is called.

Schedules execution of a one-time callback after delay milliseconds. Returns a Timeout for use with clearTimeout().

The callback will likely not be invoked in precisely delay milliseconds. Node.js makes no guarantees about the exact timing of when callbacks will fire, nor of their ordering. The callback will be called as close as possible to the time specified.

Note: When delay is larger than 2147483647 or less than 1, the delay will be set to 1.

If callback is not a function, a TypeError will be thrown.

Note: This method has a custom variant for promises that is available using util.promisify():

const util = require('util');
const setTimeoutPromise = util.promisify(setTimeout);

setTimeoutPromise(40, 'foobar').then((value) => {
  // value === 'foobar' (passing values is optional)
  // This is executed after about 40 milliseconds.
});

Cancelling Timers

The setImmediate(), setInterval(), and setTimeout() methods each return objects that represent the scheduled timers. These can be used to cancel the timer and prevent it from triggering.

It is not possible to cancel timers that were created using the promisified variants of setImmediate(), setTimeout().

clearImmediate(immediate)

• immediate {Immediate} An Immediate object as returned by setImmediate().

Cancels an Immediate object created by setImmediate().

clearInterval(timeout)

• timeout {Timeout} A Timeout object as returned by setInterval().

Cancels a Timeout object created by setInterval().

clearTimeout(timeout)

• timeout {Timeout} A Timeout object as returned by setTimeout().

Cancels a Timeout object created by setTimeout().