The Node.js Event Loop
What is the Event Loop?
The event loop is what allows Node.js to perform non-blocking I/O operations — despite the fact that a single JavaScript thread is used by default — by offloading operations to the system kernel whenever possible.
Since most modern kernels are multi-threaded, they can handle multiple operations executing in the background. When one of these operations completes, the kernel tells Node.js so that the appropriate callback may be added to the poll queue to eventually be executed. We'll explain this in further detail later in this topic.
Event Loop Explained
When Node.js starts, it initializes the event loop, processes the
provided input script (or drops into the REPL, which is not covered in
this document) which may make async API calls, schedule timers, or call
process.nextTick(), then begins processing the event loop.
The following diagram shows a simplified overview of the event loop's order of operations.
┌───────────────────────────┐
┌─>│ timers │
│ └─────────────┬─────────────┘
│ ┌─────────────┴─────────────┐
│ │ pending callbacks │
│ └─────────────┬─────────────┘
│ ┌─────────────┴─────────────┐
│ │ idle, prepare │
│ └─────────────┬─────────────┘ ┌───────────────┐
│ ┌─────────────┴─────────────┐ │ incoming: │
│ │ poll │<─────┤ connections, │
│ └─────────────┬─────────────┘ │ data, etc. │
│ ┌─────────────┴─────────────┐ └───────────────┘
│ │ check │
│ └─────────────┬─────────────┘
│ ┌─────────────┴─────────────┐
└──┤ close callbacks │
└───────────────────────────┘
Each box will be referred to as a "phase" of the event loop.
Each phase has a FIFO queue of callbacks to execute. While each phase is special in its own way, generally, when the event loop enters a given phase, it will perform any operations specific to that phase, then execute callbacks in that phase's queue until the queue has been exhausted or the maximum number of callbacks has executed. When the queue has been exhausted or the callback limit is reached, the event loop will move to the next phase, and so on.
Since any of these operations may schedule more operations and new events processed in the poll phase are queued by the kernel, poll events can be queued while polling events are being processed. As a result, long running callbacks can allow the poll phase to run much longer than a timer's threshold. See the timers and poll sections for more details.
There is a slight discrepancy between the Windows and the Unix/Linux implementation, but that's not important for this demonstration. The most important parts are here. There are actually seven or eight steps, but the ones we care about — ones that Node.js actually uses - are those above.
Phases Overview
- timers: this phase executes callbacks scheduled by
setTimeout()andsetInterval(). - pending callbacks: executes I/O callbacks deferred to the next loop iteration.
- idle, prepare: only used internally.
- poll: retrieve new I/O events; execute I/O related callbacks (almost
all with the exception of close callbacks, the ones scheduled by timers,
and
setImmediate()); node will block here when appropriate. - check:
setImmediate()callbacks are invoked here. - close callbacks: some close callbacks, e.g.
socket.on('close', ...).
Between each run of the event loop, Node.js checks if it is waiting for any asynchronous I/O or timers and shuts down cleanly if there are not any.
Phases in Detail
timers
A timer specifies the threshold after which a provided callback may be executed rather than the exact time a person wants it to be executed. Timers callbacks will run as early as they can be scheduled after the specified amount of time has passed; however, Operating System scheduling or the running of other callbacks may delay them.
Technically, the poll phase controls when timers are executed.
For example, say you schedule a timeout to execute after a 100 ms threshold, then your script starts asynchronously reading a file which takes 95 ms:
const module "node:fs"fs = var require: NodeJS.Require
(id: string) => any
function someAsyncOperation(callback: any): voidsomeAsyncOperation(callback: anycallback) {
// Assume this takes 95ms to complete
module "node:fs"fs.function readFile(path: fs.PathOrFileDescriptor, callback: (err: NodeJS.ErrnoException | null, data: NonSharedBuffer) => void): void (+3 overloads)Asynchronously reads the entire contents of a file.readFile('/path/to/file', callback: anycallback);
}
const const timeoutScheduled: numbertimeoutScheduled = var Date: DateConstructorEnables basic storage and retrieval of dates and times.Date.DateConstructor.now(): numberReturns the number of milliseconds elapsed since midnight, January 1, 1970 Universal Coordinated Time (UTC).now();
function setTimeout<[]>(callback: () => void, delay?: number): NodeJS.Timeout (+2 overloads)Schedules execution of a one-time `callback` after `delay` milliseconds.
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.
When `delay` is larger than `2147483647` or less than `1` or `NaN`, the `delay`
will be set to `1`. Non-integer delays are truncated to an integer.
If `callback` is not a function, a `TypeError` will be thrown.
This method has a custom variant for promises that is available using
`timersPromises.setTimeout()`.setTimeout(() => {
const const delay: numberdelay = var Date: DateConstructorEnables basic storage and retrieval of dates and times.Date.DateConstructor.now(): numberReturns the number of milliseconds elapsed since midnight, January 1, 1970 Universal Coordinated Time (UTC).now() - const timeoutScheduled: numbertimeoutScheduled;
var console: ConsoleThe `console` module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:
* A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream.
* A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and
[`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module.
_**Warning**_: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for
more information.
Example using the global `console`:
```js
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
// Error: Whoops, something bad happened
// at [eval]:5:15
// at Script.runInThisContext (node:vm:132:18)
// at Object.runInThisContext (node:vm:309:38)
// at node:internal/process/execution:77:19
// at [eval]-wrapper:6:22
// at evalScript (node:internal/process/execution:76:60)
// at node:internal/main/eval_string:23:3
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
```
Example using the `Console` class:
```js
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
```console.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to `stdout` with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html)
(the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)).
```js
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
```
See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.log(`${const delay: numberdelay}ms have passed since I was scheduled`);
}, 100);
// do someAsyncOperation which takes 95 ms to complete
function someAsyncOperation(callback: any): voidsomeAsyncOperation(() => {
const const startCallback: numberstartCallback = var Date: DateConstructorEnables basic storage and retrieval of dates and times.Date.DateConstructor.now(): numberReturns the number of milliseconds elapsed since midnight, January 1, 1970 Universal Coordinated Time (UTC).now();
// do something that will take 10ms...
while (var Date: DateConstructorEnables basic storage and retrieval of dates and times.Date.DateConstructor.now(): numberReturns the number of milliseconds elapsed since midnight, January 1, 1970 Universal Coordinated Time (UTC).now() - const startCallback: numberstartCallback < 10) {
// do nothing
}
});
When the event loop enters the poll phase, it has an empty queue
(fs.readFile() has not completed), so it will wait for the number of ms
remaining until the soonest timer's threshold is reached. While it is
waiting 95 ms pass, fs.readFile() finishes reading the file and its
callback which takes 10 ms to complete is added to the poll queue and
executed. When the callback finishes, there are no more callbacks in the
queue, so the event loop will see that the threshold of the soonest
timer has been reached then wrap back to the timers phase to execute
the timer's callback. In this example, you will see that the total delay
between the timer being scheduled and its callback being executed will
be 105ms.
To prevent the poll phase from starving the event loop, libuv (the C library that implements the Node.js event loop and all of the asynchronous behaviors of the platform) also has a hard maximum (system dependent) before it stops polling for more events.
pending callbacks
This phase executes callbacks for some system operations such as types
of TCP errors. For example if a TCP socket receives ECONNREFUSED when
attempting to connect, some *nix systems want to wait to report the
error. This will be queued to execute in the pending callbacks phase.
poll
The poll phase has two main functions:
- Calculating how long it should block and poll for I/O, then
- Processing events in the poll queue.
When the event loop enters the poll phase and there are no timers scheduled, one of two things will happen:
-
If the poll queue is not empty, the event loop will iterate through its queue of callbacks executing them synchronously until either the queue has been exhausted, or the system-dependent hard limit is reached.
-
If the poll queue is empty, one of two more things will happen:
-
If scripts have been scheduled by
setImmediate(), the event loop will end the poll phase and continue to the check phase to execute those scheduled scripts. -
If scripts have not been scheduled by
setImmediate(), the event loop will wait for callbacks to be added to the queue, then execute them immediately.
-
Once the poll queue is empty the event loop will check for timers whose time thresholds have been reached. If one or more timers are ready, the event loop will wrap back to the timers phase to execute those timers' callbacks.
check
This phase allows the event loop to execute callbacks immediately after the
poll phase has completed. If the poll phase becomes idle and
scripts have been queued with setImmediate(), the event loop may
continue to the check phase rather than waiting.
setImmediate() is actually a special timer that runs in a separate
phase of the event loop. It uses a libuv API that schedules callbacks to
execute after the poll phase has completed.
Generally, as the code is executed, the event loop will eventually hit
the poll phase where it will wait for an incoming connection, request,
etc. However, if a callback has been scheduled with setImmediate()
and the poll phase becomes idle, it will end and continue to the
check phase rather than waiting for poll events.
close callbacks
If a socket or handle is closed abruptly (e.g. socket.destroy()), the
'close' event will be emitted in this phase. Otherwise it will be
emitted via process.nextTick().
setImmediate() vs setTimeout()
setImmediate() and setTimeout() are similar, but behave in different
ways depending on when they are called.
setImmediate()is designed to execute a script once the current poll phase completes.setTimeout()schedules a script to be run after a minimum threshold in ms has elapsed.
The order in which the timers are executed will vary depending on the context in which they are called. If both are called from within the main module, then timing will be bound by the performance of the process (which can be impacted by other applications running on the machine).
For example, if we run the following script which is not within an I/O cycle (i.e. the main module), the order in which the two timers are executed is non-deterministic, as it is bound by the performance of the process:
// timeout_vs_immediate.js
function setTimeout<[]>(callback: () => void, delay?: number): NodeJS.Timeout (+2 overloads)Schedules execution of a one-time `callback` after `delay` milliseconds.
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.
When `delay` is larger than `2147483647` or less than `1` or `NaN`, the `delay`
will be set to `1`. Non-integer delays are truncated to an integer.
If `callback` is not a function, a `TypeError` will be thrown.
This method has a custom variant for promises that is available using
`timersPromises.setTimeout()`.setTimeout(() => {
var console: ConsoleThe `console` module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:
* A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream.
* A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and
[`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module.
_**Warning**_: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for
more information.
Example using the global `console`:
```js
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
// Error: Whoops, something bad happened
// at [eval]:5:15
// at Script.runInThisContext (node:vm:132:18)
// at Object.runInThisContext (node:vm:309:38)
// at node:internal/process/execution:77:19
// at [eval]-wrapper:6:22
// at evalScript (node:internal/process/execution:76:60)
// at node:internal/main/eval_string:23:3
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
```
Example using the `Console` class:
```js
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
```console.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to `stdout` with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html)
(the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)).
```js
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
```
See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.log('timeout');
}, 0);
function setImmediate<[]>(callback: () => void): NodeJS.Immediate (+1 overload)Schedules the "immediate" execution of the `callback` after I/O events'
callbacks.
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.
This method has a custom variant for promises that is available using
`timersPromises.setImmediate()`.setImmediate(() => {
var console: ConsoleThe `console` module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:
* A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream.
* A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and
[`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module.
_**Warning**_: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for
more information.
Example using the global `console`:
```js
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
// Error: Whoops, something bad happened
// at [eval]:5:15
// at Script.runInThisContext (node:vm:132:18)
// at Object.runInThisContext (node:vm:309:38)
// at node:internal/process/execution:77:19
// at [eval]-wrapper:6:22
// at evalScript (node:internal/process/execution:76:60)
// at node:internal/main/eval_string:23:3
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
```
Example using the `Console` class:
```js
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
```console.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to `stdout` with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html)
(the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)).
```js
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
```
See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.log('immediate');
});
However, if you move the two calls within an I/O cycle, the immediate callback is always executed first:
// timeout_vs_immediate.js
const module "node:fs"fs = var require: NodeJS.Require
(id: string) => any
module "node:fs"fs.function readFile(path: fs.PathOrFileDescriptor, callback: (err: NodeJS.ErrnoException | null, data: NonSharedBuffer) => void): void (+3 overloads)Asynchronously reads the entire contents of a file.readFile(var __filename: stringThe file name of the current module. This is the current module file's absolute
path with symlinks resolved.
For a main program this is not necessarily the same as the file name used in the
command line.__filename, () => {
function setTimeout<[]>(callback: () => void, delay?: number): NodeJS.Timeout (+2 overloads)Schedules execution of a one-time `callback` after `delay` milliseconds.
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.
When `delay` is larger than `2147483647` or less than `1` or `NaN`, the `delay`
will be set to `1`. Non-integer delays are truncated to an integer.
If `callback` is not a function, a `TypeError` will be thrown.
This method has a custom variant for promises that is available using
`timersPromises.setTimeout()`.setTimeout(() => {
var console: ConsoleThe `console` module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:
* A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream.
* A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and
[`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module.
_**Warning**_: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for
more information.
Example using the global `console`:
```js
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
// Error: Whoops, something bad happened
// at [eval]:5:15
// at Script.runInThisContext (node:vm:132:18)
// at Object.runInThisContext (node:vm:309:38)
// at node:internal/process/execution:77:19
// at [eval]-wrapper:6:22
// at evalScript (node:internal/process/execution:76:60)
// at node:internal/main/eval_string:23:3
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
```
Example using the `Console` class:
```js
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
```console.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to `stdout` with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html)
(the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)).
```js
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
```
See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.log('timeout');
}, 0);
function setImmediate<[]>(callback: () => void): NodeJS.Immediate (+1 overload)Schedules the "immediate" execution of the `callback` after I/O events'
callbacks.
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.
This method has a custom variant for promises that is available using
`timersPromises.setImmediate()`.setImmediate(() => {
var console: ConsoleThe `console` module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:
* A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream.
* A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and
[`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module.
_**Warning**_: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for
more information.
Example using the global `console`:
```js
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
// Error: Whoops, something bad happened
// at [eval]:5:15
// at Script.runInThisContext (node:vm:132:18)
// at Object.runInThisContext (node:vm:309:38)
// at node:internal/process/execution:77:19
// at [eval]-wrapper:6:22
// at evalScript (node:internal/process/execution:76:60)
// at node:internal/main/eval_string:23:3
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
```
Example using the `Console` class:
```js
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
```console.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to `stdout` with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html)
(the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)).
```js
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
```
See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.log('immediate');
});
});
The main advantage to using setImmediate() over setTimeout() is
setImmediate() will always be executed before any timers if scheduled
within an I/O cycle, independently of how many timers are present.
process.nextTick()
Understanding process.nextTick()
You may have noticed that process.nextTick() was not displayed in the
diagram, even though it's a part of the asynchronous API. This is because
process.nextTick() is not technically part of the event loop. Instead,
the nextTickQueue will be processed after the current operation is
completed, regardless of the current phase of the event loop. Here,
an operation is defined as a transition from the
underlying C/C++ handler, and handling the JavaScript that needs to be
executed.
Looking back at our diagram, any time you call process.nextTick() in a
given phase, all callbacks passed to process.nextTick() will be
resolved before the event loop continues. This can create some bad
situations because it allows you to "starve" your I/O by making
recursive process.nextTick() calls, which prevents the event loop
from reaching the poll phase.
Why would that be allowed?
Why would something like this be included in Node.js? Part of it is a design philosophy where an API should always be asynchronous even where it doesn't have to be. Take this code snippet for example:
function function apiCall(arg: any, callback: any): voidapiCall(arg: anyarg, callback: anycallback) {
if (typeof arg: anyarg !== 'string') {
return var process: NodeJS.Processprocess.NodeJS.Process.nextTick(callback: Function, ...args: any[]): void`process.nextTick()` adds `callback` to the "next tick queue". This queue is
fully drained after the current operation on the JavaScript stack runs to
completion and before the event loop is allowed to continue. It's possible to
create an infinite loop if one were to recursively call `process.nextTick()`.
See the [Event Loop](https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick/#process-nexttick) guide for more background.
```js
import { nextTick } from 'node:process';
console.log('start');
nextTick(() => {
console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback
```
This is important when developing APIs in order to give users the opportunity
to assign event handlers _after_ an object has been constructed but before any
I/O has occurred:
```js
import { nextTick } from 'node:process';
function MyThing(options) {
this.setupOptions(options);
nextTick(() => {
this.startDoingStuff();
});
}
const thing = new MyThing();
thing.getReadyForStuff();
// thing.startDoingStuff() gets called now, not before.
```
It is very important for APIs to be either 100% synchronous or 100%
asynchronous. Consider this example:
```js
// WARNING! DO NOT USE! BAD UNSAFE HAZARD!
function maybeSync(arg, cb) {
if (arg) {
cb();
return;
}
fs.stat('file', cb);
}
```
This API is hazardous because in the following case:
```js
const maybeTrue = Math.random() > 0.5;
maybeSync(maybeTrue, () => {
foo();
});
bar();
```
It is not clear whether `foo()` or `bar()` will be called first.
The following approach is much better:
```js
import { nextTick } from 'node:process';
function definitelyAsync(arg, cb) {
if (arg) {
nextTick(cb);
return;
}
fs.stat('file', cb);
}
```nextTick(
callback: anycallback,
new var TypeError: TypeErrorConstructor
new (message?: string, options?: ErrorOptions) => TypeError (+3 overloads)
The snippet does an argument check and if it's not correct, it will pass
the error to the callback. The API updated fairly recently to allow
passing arguments to process.nextTick() allowing it to take any
arguments passed after the callback to be propagated as the arguments to
the callback so you don't have to nest functions.
What we're doing is passing an error back to the user but only after
we have allowed the rest of the user's code to execute. By using
process.nextTick() we guarantee that apiCall() always runs its
callback after the rest of the user's code and before the event loop
is allowed to proceed. To achieve this, the JS call stack is allowed to
unwind then immediately execute the provided callback which allows a
person to make recursive calls to process.nextTick() without reaching a
RangeError: Maximum call stack size exceeded from v8.
This philosophy can lead to some potentially problematic situations. Take this snippet for example:
let let bar: anybar = null;
// this has an asynchronous signature, but calls callback synchronously
function function someAsyncApiCall(callback: any): voidsomeAsyncApiCall(callback: anycallback) {
callback: anycallback();
}
// the callback is called before `someAsyncApiCall` completes.
function someAsyncApiCall(callback: any): voidsomeAsyncApiCall(() => {
// since someAsyncApiCall hasn't completed, bar hasn't been assigned any value
var console: ConsoleThe `console` module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:
* A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream.
* A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and
[`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module.
_**Warning**_: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for
more information.
Example using the global `console`:
```js
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
// Error: Whoops, something bad happened
// at [eval]:5:15
// at Script.runInThisContext (node:vm:132:18)
// at Object.runInThisContext (node:vm:309:38)
// at node:internal/process/execution:77:19
// at [eval]-wrapper:6:22
// at evalScript (node:internal/process/execution:76:60)
// at node:internal/main/eval_string:23:3
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
```
Example using the `Console` class:
```js
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
```console.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to `stdout` with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html)
(the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)).
```js
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
```
See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.log('bar', let bar: anybar); // null
});
let bar: anybar = 1;
The user defines someAsyncApiCall() to have an asynchronous signature,
but it actually operates synchronously. When it is called, the callback
provided to someAsyncApiCall() is called in the same phase of the
event loop because someAsyncApiCall() doesn't actually do anything
asynchronously. As a result, the callback tries to reference bar even
though it may not have that variable in scope yet, because the script has not
been able to run to completion.
By placing the callback in a process.nextTick(), the script still has the
ability to run to completion, allowing all the variables, functions,
etc., to be initialized prior to the callback being called. It also has
the advantage of not allowing the event loop to continue. It may be
useful for the user to be alerted to an error before the event loop is
allowed to continue. Here is the previous example using process.nextTick():
let let bar: anybar = null;
function function someAsyncApiCall(callback: any): voidsomeAsyncApiCall(callback: anycallback) {
var process: NodeJS.Processprocess.NodeJS.Process.nextTick(callback: Function, ...args: any[]): void`process.nextTick()` adds `callback` to the "next tick queue". This queue is
fully drained after the current operation on the JavaScript stack runs to
completion and before the event loop is allowed to continue. It's possible to
create an infinite loop if one were to recursively call `process.nextTick()`.
See the [Event Loop](https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick/#process-nexttick) guide for more background.
```js
import { nextTick } from 'node:process';
console.log('start');
nextTick(() => {
console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback
```
This is important when developing APIs in order to give users the opportunity
to assign event handlers _after_ an object has been constructed but before any
I/O has occurred:
```js
import { nextTick } from 'node:process';
function MyThing(options) {
this.setupOptions(options);
nextTick(() => {
this.startDoingStuff();
});
}
const thing = new MyThing();
thing.getReadyForStuff();
// thing.startDoingStuff() gets called now, not before.
```
It is very important for APIs to be either 100% synchronous or 100%
asynchronous. Consider this example:
```js
// WARNING! DO NOT USE! BAD UNSAFE HAZARD!
function maybeSync(arg, cb) {
if (arg) {
cb();
return;
}
fs.stat('file', cb);
}
```
This API is hazardous because in the following case:
```js
const maybeTrue = Math.random() > 0.5;
maybeSync(maybeTrue, () => {
foo();
});
bar();
```
It is not clear whether `foo()` or `bar()` will be called first.
The following approach is much better:
```js
import { nextTick } from 'node:process';
function definitelyAsync(arg, cb) {
if (arg) {
nextTick(cb);
return;
}
fs.stat('file', cb);
}
```nextTick(callback: anycallback);
}
function someAsyncApiCall(callback: any): voidsomeAsyncApiCall(() => {
var console: ConsoleThe `console` module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:
* A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream.
* A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and
[`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module.
_**Warning**_: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for
more information.
Example using the global `console`:
```js
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
// Error: Whoops, something bad happened
// at [eval]:5:15
// at Script.runInThisContext (node:vm:132:18)
// at Object.runInThisContext (node:vm:309:38)
// at node:internal/process/execution:77:19
// at [eval]-wrapper:6:22
// at evalScript (node:internal/process/execution:76:60)
// at node:internal/main/eval_string:23:3
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
```
Example using the `Console` class:
```js
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
```console.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to `stdout` with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html)
(the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)).
```js
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
```
See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.log('bar', let bar: anybar); // 1
});
let bar: anybar = 1;
Here's another real world example:
const const server: anyserver = net.createServer(() => {}).listen(8080);
const server: anyserver.on('listening', () => {});
When only a port is passed, the port is bound immediately. So, the
'listening' callback could be called immediately. The problem is that the
.on('listening') callback will not have been set by that time.
To get around this, the 'listening' event is queued in a nextTick()
to allow the script to run to completion. This allows the user to set
any event handlers they want.
process.nextTick() vs setImmediate()
We have two calls that are similar as far as users are concerned, but their names are confusing.
process.nextTick()fires immediately on the same phasesetImmediate()fires on the following iteration or 'tick' of the event loop
In essence, the names should be swapped. process.nextTick() fires more
immediately than setImmediate(), but this is an artifact of the past
which is unlikely to change. Making this switch would break a large
percentage of the packages on npm. Every day more new modules are being
added, which means every day we wait, more potential breakages occur.
While they are confusing, the names themselves won't change.
We recommend developers use
setImmediate()in all cases because it's easier to reason about.
Why use process.nextTick()?
There are two main reasons:
-
Allow users to handle errors, cleanup any then unneeded resources, or perhaps try the request again before the event loop continues.
-
At times it's necessary to allow a callback to run after the call stack has unwound but before the event loop continues.
One example is to match the user's expectations. Simple example:
const const server: anyserver = net.createServer();
const server: anyserver.on('connection', conn: anyconn => {});
const server: anyserver.listen(8080);
const server: anyserver.on('listening', () => {});
Say that listen() is run at the beginning of the event loop, but the
listening callback is placed in a setImmediate(). Unless a
hostname is passed, binding to the port will happen immediately. For
the event loop to proceed, it must hit the poll phase, which means
there is a non-zero chance that a connection could have been received
allowing the connection event to be fired before the listening event.
Another example is extending an EventEmitter and emitting an
event from within the constructor:
const class EventEmitter<T extends EventMap<T> = DefaultEventMap>The `EventEmitter` class is defined and exposed by the `node:events` module:
```js
import { EventEmitter } from 'node:events';
```
All `EventEmitter`s emit the event `'newListener'` when new listeners are
added and `'removeListener'` when existing listeners are removed.
It supports the following option:EventEmitter = var require: NodeJS.Require
(id: string) => any
class MyEmitterMyEmitter extends class EventEmitter<T extends EventMap<T> = DefaultEventMap>The `EventEmitter` class is defined and exposed by the `node:events` module:
```js
import { EventEmitter } from 'node:events';
```
All `EventEmitter`s emit the event `'newListener'` when new listeners are
added and `'removeListener'` when existing listeners are removed.
It supports the following option:EventEmitter {
constructor() {
super();
this.NodeJS.EventEmitter<DefaultEventMap>.emit<any>(eventName: string | symbol, ...args: AnyRest): booleanSynchronously calls each of the listeners registered for the event named `eventName`, in the order they were registered, passing the supplied arguments
to each.
Returns `true` if the event had listeners, `false` otherwise.
```js
import { EventEmitter } from 'node:events';
const myEmitter = new EventEmitter();
// First listener
myEmitter.on('event', function firstListener() {
console.log('Helloooo! first listener');
});
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
});
// Third listener
myEmitter.on('event', function thirdListener(...args) {
const parameters = args.join(', ');
console.log(`event with parameters ${parameters} in third listener`);
});
console.log(myEmitter.listeners('event'));
myEmitter.emit('event', 1, 2, 3, 4, 5);
// Prints:
// [
// [Function: firstListener],
// [Function: secondListener],
// [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listener
```emit('event');
}
}
const const myEmitter: MyEmittermyEmitter = new constructor MyEmitter(): MyEmitterMyEmitter();
const myEmitter: MyEmittermyEmitter.NodeJS.EventEmitter<DefaultEventMap>.on<any>(eventName: string | symbol, listener: (...args: any[]) => void): MyEmitterAdds the `listener` function to the end of the listeners array for the event
named `eventName`. No checks are made to see if the `listener` has already
been added. Multiple calls passing the same combination of `eventName` and
`listener` will result in the `listener` being added, and called, multiple times.
```js
server.on('connection', (stream) => {
console.log('someone connected!');
});
```
Returns a reference to the `EventEmitter`, so that calls can be chained.
By default, event listeners are invoked in the order they are added. The `emitter.prependListener()` method can be used as an alternative to add the
event listener to the beginning of the listeners array.
```js
import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
// b
// a
```on('event', () => {
var console: ConsoleThe `console` module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:
* A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream.
* A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and
[`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module.
_**Warning**_: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for
more information.
Example using the global `console`:
```js
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
// Error: Whoops, something bad happened
// at [eval]:5:15
// at Script.runInThisContext (node:vm:132:18)
// at Object.runInThisContext (node:vm:309:38)
// at node:internal/process/execution:77:19
// at [eval]-wrapper:6:22
// at evalScript (node:internal/process/execution:76:60)
// at node:internal/main/eval_string:23:3
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
```
Example using the `Console` class:
```js
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
```console.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to `stdout` with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html)
(the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)).
```js
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
```
See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.log('an event occurred!');
});
You can't emit an event from the constructor immediately
because the script will not have processed to the point where the user
assigns a callback to that event. So, within the constructor itself,
you can use process.nextTick() to set a callback to emit the event
after the constructor has finished, which provides the expected results:
const class EventEmitter<T extends EventMap<T> = DefaultEventMap>The `EventEmitter` class is defined and exposed by the `node:events` module:
```js
import { EventEmitter } from 'node:events';
```
All `EventEmitter`s emit the event `'newListener'` when new listeners are
added and `'removeListener'` when existing listeners are removed.
It supports the following option:EventEmitter = var require: NodeJS.Require
(id: string) => any
class MyEmitterMyEmitter extends class EventEmitter<T extends EventMap<T> = DefaultEventMap>The `EventEmitter` class is defined and exposed by the `node:events` module:
```js
import { EventEmitter } from 'node:events';
```
All `EventEmitter`s emit the event `'newListener'` when new listeners are
added and `'removeListener'` when existing listeners are removed.
It supports the following option:EventEmitter {
constructor() {
super();
// use nextTick to emit the event once a handler is assigned
var process: NodeJS.Processprocess.NodeJS.Process.nextTick(callback: Function, ...args: any[]): void`process.nextTick()` adds `callback` to the "next tick queue". This queue is
fully drained after the current operation on the JavaScript stack runs to
completion and before the event loop is allowed to continue. It's possible to
create an infinite loop if one were to recursively call `process.nextTick()`.
See the [Event Loop](https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick/#process-nexttick) guide for more background.
```js
import { nextTick } from 'node:process';
console.log('start');
nextTick(() => {
console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback
```
This is important when developing APIs in order to give users the opportunity
to assign event handlers _after_ an object has been constructed but before any
I/O has occurred:
```js
import { nextTick } from 'node:process';
function MyThing(options) {
this.setupOptions(options);
nextTick(() => {
this.startDoingStuff();
});
}
const thing = new MyThing();
thing.getReadyForStuff();
// thing.startDoingStuff() gets called now, not before.
```
It is very important for APIs to be either 100% synchronous or 100%
asynchronous. Consider this example:
```js
// WARNING! DO NOT USE! BAD UNSAFE HAZARD!
function maybeSync(arg, cb) {
if (arg) {
cb();
return;
}
fs.stat('file', cb);
}
```
This API is hazardous because in the following case:
```js
const maybeTrue = Math.random() > 0.5;
maybeSync(maybeTrue, () => {
foo();
});
bar();
```
It is not clear whether `foo()` or `bar()` will be called first.
The following approach is much better:
```js
import { nextTick } from 'node:process';
function definitelyAsync(arg, cb) {
if (arg) {
nextTick(cb);
return;
}
fs.stat('file', cb);
}
```nextTick(() => {
this.NodeJS.EventEmitter<DefaultEventMap>.emit<any>(eventName: string | symbol, ...args: AnyRest): booleanSynchronously calls each of the listeners registered for the event named `eventName`, in the order they were registered, passing the supplied arguments
to each.
Returns `true` if the event had listeners, `false` otherwise.
```js
import { EventEmitter } from 'node:events';
const myEmitter = new EventEmitter();
// First listener
myEmitter.on('event', function firstListener() {
console.log('Helloooo! first listener');
});
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
});
// Third listener
myEmitter.on('event', function thirdListener(...args) {
const parameters = args.join(', ');
console.log(`event with parameters ${parameters} in third listener`);
});
console.log(myEmitter.listeners('event'));
myEmitter.emit('event', 1, 2, 3, 4, 5);
// Prints:
// [
// [Function: firstListener],
// [Function: secondListener],
// [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listener
```emit('event');
});
}
}
const const myEmitter: MyEmittermyEmitter = new constructor MyEmitter(): MyEmitterMyEmitter();
const myEmitter: MyEmittermyEmitter.NodeJS.EventEmitter<DefaultEventMap>.on<any>(eventName: string | symbol, listener: (...args: any[]) => void): MyEmitterAdds the `listener` function to the end of the listeners array for the event
named `eventName`. No checks are made to see if the `listener` has already
been added. Multiple calls passing the same combination of `eventName` and
`listener` will result in the `listener` being added, and called, multiple times.
```js
server.on('connection', (stream) => {
console.log('someone connected!');
});
```
Returns a reference to the `EventEmitter`, so that calls can be chained.
By default, event listeners are invoked in the order they are added. The `emitter.prependListener()` method can be used as an alternative to add the
event listener to the beginning of the listeners array.
```js
import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
// b
// a
```on('event', () => {
var console: ConsoleThe `console` module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:
* A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream.
* A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and
[`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module.
_**Warning**_: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for
more information.
Example using the global `console`:
```js
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
// Error: Whoops, something bad happened
// at [eval]:5:15
// at Script.runInThisContext (node:vm:132:18)
// at Object.runInThisContext (node:vm:309:38)
// at node:internal/process/execution:77:19
// at [eval]-wrapper:6:22
// at evalScript (node:internal/process/execution:76:60)
// at node:internal/main/eval_string:23:3
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
```
Example using the `Console` class:
```js
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
```console.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)Prints to `stdout` with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html)
(the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)).
```js
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
```
See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.log('an event occurred!');
});