Class: Solver

Example

In the following example, we run a solver asynchronously. We subscribe to the solution event to print the objective value of the solution and the value of interval variable x. After finding the first solution, we request the solver to stop. We also subscribe to the summary event to print statistics about the solve.

import * as CP from '@scheduleopt/optalcp';

...
let model = new CP.Model();
let x = model.intervalVar({ ... });
...

// Create a new solver:
let solver = new CP.Solver;

// Subscribe to "solution" events:
solver.on("solution", (msg: CP.SolutionEvent) => {
  // Get Solution from SolutionEvent:
  let solution = msg.solution;
  console.log("At time " + msg.solveTime + ", solution found with objective " + solution.getObjective());
  // Print value of interval variable x:
  if (solution.isAbsent(x))
    console.log("  Interval variable x is absent");
  else
    console.log("  Interval variable x: [" + solution.getStart(x) + " -- " + solution.getEnd(x) + "]");
  // Request the solver to stop as soon as possible
  // (the message is only informative):
  solver.stop("We are happy with the first solution found.");
});

// Subscribe to "summary" events:
solver.on("summary", (msg: CP.SolveSummary) => {
  // Print the statistics. The statistics doesn't exist if an error occurred.
  console.log("Total duration of solve: " + msg.duration);
  console.log("Number of branches: " + msg.nbBranches);
});

try {
  await solver.solve(model, { timeLimit: 60 });
  console.log("All done");
} catch (e) {
  // We did not subscribe to "error" events. So an exception is thrown in
  // case of an error.
  console.error("Exception caught: ", (e as Error).message);
}

Extends

• EventEmitter

Constructors

new Solver()

new Solver(options?: EventEmitterOptions): Solver

Parameters

Parameter	Type
options?	EventEmitterOptions

Returns

Solver

Inherited from

EventEmitter.constructor

Properties

Property	Modifier	Type	Description	Inherited from
captureRejectionSymbol	readonly	typeof captureRejectionSymbol	Value: Symbol.for('nodejs.rejection') See how to write a custom rejection handler. Since v13.4.0, v12.16.0	EventEmitter.captureRejectionSymbol
captureRejections	static	boolean	Value: boolean Change the default captureRejections option on all new EventEmitter objects. Since v13.4.0, v12.16.0	EventEmitter.captureRejections
defaultMaxListeners	static	number	By default, a maximum of 10 listeners can be registered for any single event. This limit can be changed for individual EventEmitter instances using the emitter.setMaxListeners(n) method. To change the default for allEventEmitter instances, the events.defaultMaxListeners property can be used. If this value is not a positive number, a RangeError is thrown. Take caution when setting the events.defaultMaxListeners because the change affects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over events.defaultMaxListeners. This is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single EventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning: import { EventEmitter } from 'node:events'; const emitter = new EventEmitter(); emitter.setMaxListeners(emitter.getMaxListeners() + 1); emitter.once('event', () => { // do stuff emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0)); }); The --trace-warnings command-line flag can be used to display the stack trace for such warnings. The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type, and count properties, referring to the event emitter instance, the event's name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'. Since v0.11.2	EventEmitter.defaultMaxListeners
errorMonitor	readonly	typeof errorMonitor	This symbol shall be used to install a listener for only monitoring 'error' events. Listeners installed using this symbol are called before the regular 'error' listeners are called. Installing a listener using this symbol does not change the behavior once an 'error' event is emitted. Therefore, the process will still crash if no regular 'error' listener is installed. Since v13.6.0, v12.17.0	EventEmitter.errorMonitor

Methods

[captureRejectionSymbol]()?

optional [captureRejectionSymbol]<K>(error: Error, event: string | symbol, ...args: AnyRest): void

Type Parameters

Type Parameter
K

Parameters

Parameter	Type
error	Error
event	string | symbol
...args	AnyRest

Returns

void

Inherited from

EventEmitter.[captureRejectionSymbol]

---

addListener()

addListener<K>(eventName: string | symbol, listener: (...args: any[]) => void): this

Alias for emitter.on(eventName, listener).

Type Parameters

Type Parameter
K

Parameters

Parameter	Type
eventName	string | symbol
listener	(...args: any[]) => void

Returns

this

Since

v0.1.26

Inherited from

EventEmitter.addListener

---

emit()

emit<K>(eventName: string | symbol, ...args: AnyRest): boolean

Synchronously 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.

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

Type Parameters

Type Parameter
K

Parameters

Parameter	Type
eventName	string | symbol
...args	AnyRest

Returns

boolean

Since

v0.1.26

Inherited from

EventEmitter.emit

---

eventNames()

eventNames(): (string | symbol)[]

Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.

import { EventEmitter } from 'node:events';

const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});

const sym = Symbol('symbol');
myEE.on(sym, () => {});

console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]

Returns

(string | symbol)[]

Since

v6.0.0

Inherited from

EventEmitter.eventNames

---

getMaxListeners()

getMaxListeners(): number

Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to defaultMaxListeners.

Returns

number

Since

v1.0.0

Inherited from

EventEmitter.getMaxListeners

---

listenerCount()

listenerCount<K>(eventName: string | symbol, listener?: Function): number

Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.

Type Parameters

Type Parameter
K

Parameters

Parameter	Type	Description
eventName	string | symbol	The name of the event being listened for
listener?	Function	The event handler function

Returns

number

Since

v3.2.0

Inherited from

EventEmitter.listenerCount

---

listeners()

listeners<K>(eventName: string | symbol): Function[]

Returns a copy of the array of listeners for the event named eventName.

server.on('connection', (stream) => {
  console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]

Type Parameters

Type Parameter
K

Parameters

Parameter	Type
eventName	string | symbol

Returns

Function[]

Since

v0.1.26

Inherited from

EventEmitter.listeners

---

off()

off<K>(eventName: string | symbol, listener: (...args: any[]) => void): this

Alias for emitter.removeListener().

Type Parameters

Type Parameter
K

Parameters

Parameter	Type
eventName	string | symbol
listener	(...args: any[]) => void

Returns

this

Since

v10.0.0

Inherited from

EventEmitter.off

---

on()

on(event, listener)

on(event: "error", listener: (err: Error) => void): this

With event="error", register the given listener function to error events. The function should take an Error parameter (standard Node.js class) and return void.

Parameters

Parameter	Type	Description
event	"error"	-
listener	(err: Error) => void	The function to be registered.

Returns

this

The Solver itself for chaining.

Remarks

This function is equivalent to the function EventEmitter.on. As usual with EventEmitter, if there is no listener registered for the 'error' event and an 'error' event is emitted, then the error is thrown.

Example

In the following example, we simply log all errors on the console.

let solver = new CP.Solver;
solver.on('error', (err: Error) => {
  console.error('There was an error: ', err);
});
let result = await solver.solve(myModel, { timeLimit: 60 });

Overrides

EventEmitter.on

on(event, listener)

on(event: "warning", listener: (msg: string) => void): this

With event="warning", register a listener function for warning events.

Parameters

Parameter	Type	Description
event	"warning"	-
listener	(msg: string) => void	The listener function to register. It should take a string parameter (the warning) and return void.

Returns

this

Remarks

This function is equivalent to function EventEmitter.on for event type warning. The registered listener function is called for every warning issued by the solver. The warning message is passed as a parameter to the function.

Alternatively, you can use function redirectOutput to redirect all solver output (including the warnings) to a stream.

The amount of warning messages can be configured using the parameter Parameters.warningLevel.

Example

In the following example, we simply log all warnings on the console using console.warn.

const solver = new CP.Solver;
solver.on('warning', (msg: string) => {
  console.warn(`Warning: ${msg}`);
});
let result = wait solver.solve(myModel, { searchType: "LNS"});

Overrides

EventEmitter.on

on(event, listener)

on(event: "log", listener: (msg: string) => void): this

With event="log", add a listener function for log events.

Parameters

Parameter	Type	Description
event	"log"	-
listener	(msg: string) => void	The listener function to add. It should take a string parameter (the log message) and return void.

Returns

this

Remarks

This function is equivalent to function EventEmitter.on for event type log. The registered listener function is called for every log message issued by the solver. The log message is passed as a parameter to the function.

The amount of log messages and their periodicity can be controlled by parameters Parameters.logLevel and Parameters.logPeriod.

Alternatively, you can use function redirectOutput to redirect all solver output (including the logs) to a stream.

Example

In the following example, we simply log all logs on the console using console.log.

const solver = new CP.Solver;
solver.on('log', (msg: string) => {
 console.log(`Log: ${msg}`);
});
let result = await solver.solve(myModel, { logPeriod: 1 });

Overrides

EventEmitter.on

on(event, listener)

on(event: "trace", listener: (msg: string) => void): this

With event="trace", add a listener function for trace events.

Parameters

Parameter	Type	Description
event	"trace"	-
listener	(msg: string) => void	The listener function to add. It should take a string parameter (the trace message) and return void.

Returns

this

Remarks

This function is equivalent to function EventEmitter.on for event type trace. The registered listener function is called for every trace message the solver sends. The trace message is passed as a parameter to the function.

The types of trace messages can be controlled by parameters such as Parameters.searchTraceLevel or Parameters.propagationTraceLevel. Note that traces are available only in the Development version of the solver.

Alternatively, you can use function redirectOutput to redirect all solver output (including the traces) to a stream.

Example

In the following example, we simply log all traces on the console using console.log.

const solver = new CP.Solver;
solver.on('trace', (msg: string) => {
  console.log(`Trace: ${msg}`);
});
let result = await solver.solve(myModel, { nbWorkers: 2 });

Overrides

EventEmitter.on

on(event, listener)

on(event: "solution", listener: (msg: SolutionEvent) => void): this

With event="solution", add a listener function for solution events.

Parameters

Parameter	Type	Description
event	"solution"	-
listener	(msg: SolutionEvent) => void	The listener function to add. It should take a SolutionEvent parameter and return void.

Returns

this

Remarks

This function is equivalent to function EventEmitter.on for event type solution. The registered listener function is called for every solution the solver finds. The solution is passed to the function via SolutionEvent parameter.

Example

In the following example, we log the value of the interval variable x in every solution using console.log.

let model = new CP.Model();
let x = model.intervalVar({ length: 5 });
...
const solver = new CP.Solver;
solver.on('solution', (msg: SolutionEvent) => {
  let solution = msg.solution;
  let start = solution.getStart(x);
  let end = solution.getEnd(x);
  if (start === undefined)
    console.log("Solution found with x=absent");
  else
    console.log("Solution found with x=[" + start + "," + end + "]");
});
let result = await solver.solve(myModel);

Note that in the Evaluation version of the solver, the reported value of interval variable x will be always absent because the real variable values are masked.

Overrides

EventEmitter.on

on(event, listener)

on(event: "lowerBound", listener: (msg: LowerBoundEvent) => void): this

With event="lowerBound", add a listener function for lower bound events.

Parameters

Parameter	Type	Description
event	"lowerBound"	-
listener	(msg: LowerBoundEvent) => void	The listener function to add. It should take a LowerBoundEvent parameter and return void.

Returns

this

Remarks

This function is equivalent to function EventEmitter.on for event type lowerBound. The registered listener function is called for every lower bound update issued by the solver. The lower bound is passed via LowerBoundEvent parameter to the function.

Overrides

EventEmitter.on

on(event, listener)

on(event: "summary", listener: (msg: SolveSummary) => void): this

With event="summary", add a listener function for summary events.

Parameters

Parameter	Type	Description
event	"summary"	-
listener	(msg: SolveSummary) => void	The listener function to add. It should take a SolveSummary parameter and return void.

Returns

this

Remarks

This function is equivalent to function EventEmitter.on for event type summary. The registered listener function is called at the end of the search, and the summary is passed via SolveSummary parameter to the function. The summary contains information about the search such as the number of solutions found, the number of failures, the search time, etc.

Example

In the following example, we log part of the summary message on the console using console.log.

const solver = new CP.Solver;
solver.on('summary', (msg: SolveSummary) => {
  console.log(`Solutions: ${msg.nbSolution}`);
  console.log(`Branches: ${msg.nbBranches}`);
  console.log(`Duration: ${msg.duration}`);
});
let result = await solver.solve(myModel);

Overrides

EventEmitter.on

on(event, listener)

on(event: "close", listener: () => void): this

With event="close", add a listener function for close events.

Parameters

Parameter	Type	Description
event	"close"	-
listener	() => void	The listener function to add. It should take no parameter and return void.

Returns

this

Remarks

This function is equivalent to function EventEmitter.on for event type close. The registered listener function is called when the solver is closed. The function takes no parameter.

The event close is always the last event emitted by the Solver, even in the case of an error. It could be used, for example, to wait for solver completion.

See Solver for an example of use.

Overrides

EventEmitter.on

---

once()

once<K>(eventName: string | symbol, listener: (...args: any[]) => void): this

Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

server.once('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

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.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

Type Parameters

Type Parameter
K

Parameters

Parameter	Type	Description
eventName	string | symbol	The name of the event.
listener	(...args: any[]) => void	The callback function

Returns

this

Since

v0.3.0

Inherited from

EventEmitter.once

---

prependListener()

prependListener<K>(eventName: string | symbol, listener: (...args: any[]) => void): this

Adds the listener function to the beginning 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.

server.prependListener('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

Type Parameters

Type Parameter
K

Parameters

Parameter	Type	Description
eventName	string | symbol	The name of the event.
listener	(...args: any[]) => void	The callback function

Returns

this

Since

v6.0.0

Inherited from

EventEmitter.prependListener

---

prependOnceListener()

prependOnceListener<K>(eventName: string | symbol, listener: (...args: any[]) => void): this

Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

server.prependOnceListener('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

Type Parameters

Type Parameter
K

Parameters

Parameter	Type	Description
eventName	string | symbol	The name of the event.
listener	(...args: any[]) => void	The callback function

Returns

this

Since

v6.0.0

Inherited from

EventEmitter.prependOnceListener

---

rawListeners()

rawListeners<K>(eventName: string | symbol): Function[]

Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));

// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];

// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();

// Logs "log once" to the console and removes the listener
logFnWrapper();

emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');

// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');

Type Parameters

Type Parameter
K

Parameters

Parameter	Type
eventName	string | symbol

Returns

Function[]

Since

v9.4.0

Inherited from

EventEmitter.rawListeners

---

redirectOutput()

redirectOutput(stream: null | WritableStream): Solver

Redirects log, trace, and warnings to the given stream. Or suppresses them when stream is null.

Parameters

Parameter	Type	Description
stream	null | WritableStream	The stream to write the output into, or null to suppress the output.

Returns

Solver

The Solver itself for chaining.

Normally, Solver writes log, trace, and warning messages to its standard output. This function allows to redirect those messages to another stream (e.g., a file) or suppress them entirely.

Note that besides writing the messages to the standard output, the solver also emits events for log, trace, and warning messages. Those events can be intercepted using functions on.

If the output stream becomes non-writable (e.g., a broken pipe), then the solver stops as soon as possible (function stop is called).

Example

In the following example, we store all the solver text output in a file named log.txt.

import * as fs from 'fs';
import * as CP from '@scheduleopt/optalcp';

...
let solver = new CP.Solver;
solver.redirectOutput(fs.createWriteStream('log.txt'));

---

removeAllListeners()

removeAllListeners(eventName?: string | symbol): this

Removes all listeners, or those of the specified eventName.

It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters

Parameter	Type
eventName?	string | symbol

Returns

this

Since

v0.1.26

Inherited from

EventEmitter.removeAllListeners

---

removeListener()

removeListener<K>(eventName: string | symbol, listener: (...args: any[]) => void): this

Removes the specified listener from the listener array for the event named eventName.

const callback = (stream) => {
  console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);

removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();

const callbackA = () => {
  console.log('A');
  myEmitter.removeListener('event', callbackB);
};

const callbackB = () => {
  console.log('B');
};

myEmitter.on('event', callbackA);

myEmitter.on('event', callbackB);

// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
//   A
//   B

// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
//   A

Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:

import { EventEmitter } from 'node:events';
const ee = new EventEmitter();

function pong() {
  console.log('pong');
}

ee.on('ping', pong);
ee.once('ping', pong);
ee.removeListener('ping', pong);

ee.emit('ping');
ee.emit('ping');

Returns a reference to the EventEmitter, so that calls can be chained.

Type Parameters

Type Parameter
K

Parameters

Parameter	Type
eventName	string | symbol
listener	(...args: any[]) => void

Returns

this

Since

v0.1.26

Inherited from

EventEmitter.removeListener

---

sendSolution()

sendSolution(solution: Solution): Promise<void>

Send an external solution to the solver.

Parameters

Parameter	Type	Description
solution	Solution	The solution to send. It must be compatible with the model; otherwise, an error is raised.

Returns

Promise<void>

Remarks

This function can be used to send an external solution to the solver, e.g. found by another solver, a heuristic, or a user. The solver will take advantage of the solution to speed up the search: it will search only for better solutions (if it is a minimization or maximization problem). The solver may try to improve the provided solution by Large Neighborhood Search.

The solution does not have to be better than the current best solution found by the solver. It is up to the solver whether or not it will use the solution in this case.

Sending a solution to a solver that has already stopped has no effect.

The solution is sent to the solver asynchronously. Unless parameter Parameters.logLevel is set to 0, the solver will log a message when it receives the solution.

---

setMaxListeners()

setMaxListeners(n: number): this

By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters

Parameter	Type
n	number

Returns

this

Since

v0.3.5

Inherited from

EventEmitter.setMaxListeners

---

solve()

solve(model: Model, params?: Parameters, warmStart?: Solution, log?: null | WritableStream): Promise<SolveResult>

Solves a given model with the given parameters.

Parameters

Parameter	Type	Description
model	Model	The model to solve
params?	Parameters	The parameters for the solver
warmStart?	Solution	An initial solution to start the solver with
log?	null | WritableStream	A stream to redirect the solver output to. If null, the output is suppressed. If undefined, the output stream is not changed (the default is standard output).

Returns

Promise<SolveResult>

A promise that resolves to a SolveResult object when the solve is finished.

Remarks

The solving process starts asynchronously. Use await to wait for the solver to finish. During the solve, the solver emits events that can be intercepted (see on) to execute a code when the event occurs.

Note that JavaScript is single-threaded. Therefore, it cannot communicate with the solver subprocess while the user code runs. The user code must be idle (using await or waiting for an event) for the solver to function correctly.

Note that function solve cannot be called only once. If you need to solve multiple models or run multiple solves in parallel, then create multiple Solver objects.

Warm start and external solutions

If the warmStart parameter is specified, the solver will start with the given solution. The solution must be compatible with the model; otherwise an error is raised. The solver will take advantage of the solution to speed up the search: it will search only for better solutions (if it is a minimization or maximization problem). The solver may try to improve the provided solution by Large Neighborhood Search.

There are two ways to pass a solution to the solver: using warmStart parameter and using function sendSolution. The difference is that warmStart is guaranteed to be used by the solver before the solve starts. On the other hand, sendSolution can be called at any time during the solve.

---

stop()

stop(reason: string): Promise<void>

Instruct the solver to stop ASAP.

Parameters

Parameter	Type	Description
reason	string	The reason why to stop. The reason will appear in the log.

Returns

Promise<void>

Remarks

A stop message is sent to the server asynchronously. The server will stop as soon as possible and will send a summary event and close event. However, due to the asynchronous nature of the communication, other events may be sent before the summary event (e.g., another solution found or a log message).

Requesting a stop on a solver that has already stopped has no effect.

Example

In the following example, we issue a stop command 1 minute after the first solution is found.

let solver = new CP.Solver;
timerStarted = false;
solver.on('solution', (_: SolutionEvent) => {
  // We just found a solution. Set a timeout if there isn't any.
 if (!timerStarted) {
   timerStarted = true;
    // Register a function to be called after 60 seconds:
   setTimeout(() => {
     console.log("Requesting solver to stop");
     solver.stop("Stop because I said so!");
   }, 60); // The timeout is 60 seconds
 }
});
let result = await solver.solve(model, { timeLimit: 300 });

---

addAbortListener()

static addAbortListener(signal: AbortSignal, resource: (event: Event) => void): Disposable

Experimental

Listens once to the abort event on the provided signal.

Listening to the abort event on abort signals is unsafe and may lead to resource leaks since another third party with the signal can call e.stopImmediatePropagation(). Unfortunately Node.js cannot change this since it would violate the web standard. Additionally, the original API makes it easy to forget to remove listeners.

This API allows safely using AbortSignals in Node.js APIs by solving these two issues by listening to the event such that stopImmediatePropagation does not prevent the listener from running.

Returns a disposable so that it may be unsubscribed from more easily.

import { addAbortListener } from 'node:events';

function example(signal) {
  let disposable;
  try {
    signal.addEventListener('abort', (e) => e.stopImmediatePropagation());
    disposable = addAbortListener(signal, (e) => {
      // Do something when signal is aborted.
    });
  } finally {
    disposable?.[Symbol.dispose]();
  }
}

Parameters

Parameter	Type
signal	AbortSignal
resource	(event: Event) => void

Returns

Disposable

Disposable that removes the abort listener.

Since

v20.5.0

Inherited from

EventEmitter.addAbortListener

---

getEventListeners()

static getEventListeners(emitter: EventTarget | EventEmitter<DefaultEventMap>, name: string | symbol): Function[]

Returns a copy of the array of listeners for the event named eventName.

For EventEmitters this behaves exactly the same as calling .listeners on the emitter.

For EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.

import { getEventListeners, EventEmitter } from 'node:events';

{
  const ee = new EventEmitter();
  const listener = () => console.log('Events are fun');
  ee.on('foo', listener);
  console.log(getEventListeners(ee, 'foo')); // [ [Function: listener] ]
}
{
  const et = new EventTarget();
  const listener = () => console.log('Events are fun');
  et.addEventListener('foo', listener);
  console.log(getEventListeners(et, 'foo')); // [ [Function: listener] ]
}

Parameters

Parameter	Type
emitter	EventTarget | EventEmitter<DefaultEventMap>
name	string | symbol

Returns

Function[]

Since

v15.2.0, v14.17.0

Inherited from

EventEmitter.getEventListeners

---

getMaxListeners()

static getMaxListeners(emitter: EventTarget | EventEmitter<DefaultEventMap>): number

Returns the currently set max amount of listeners.

For EventEmitters this behaves exactly the same as calling .getMaxListeners on the emitter.

For EventTargets this is the only way to get the max event listeners for the event target. If the number of event handlers on a single EventTarget exceeds the max set, the EventTarget will print a warning.

import { getMaxListeners, setMaxListeners, EventEmitter } from 'node:events';

{
  const ee = new EventEmitter();
  console.log(getMaxListeners(ee)); // 10
  setMaxListeners(11, ee);
  console.log(getMaxListeners(ee)); // 11
}
{
  const et = new EventTarget();
  console.log(getMaxListeners(et)); // 10
  setMaxListeners(11, et);
  console.log(getMaxListeners(et)); // 11
}

Parameters

Parameter	Type
emitter	EventTarget | EventEmitter<DefaultEventMap>

Returns

number

Since

v19.9.0

Inherited from

EventEmitter.getMaxListeners

---

listenerCount()

static listenerCount(emitter: EventEmitter<DefaultEventMap>, eventName: string | symbol): number

A class method that returns the number of listeners for the given eventName registered on the given emitter.

import { EventEmitter, listenerCount } from 'node:events';

const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2

Parameters

Parameter	Type	Description
emitter	EventEmitter<DefaultEventMap>	The emitter to query
eventName	string | symbol	The event name

Returns

number

Since

v0.9.12

Deprecated

Since v3.2.0 - Use listenerCount instead.

Inherited from

EventEmitter.listenerCount

---

on()

on(emitter, eventName, options)

static on(emitter: EventEmitter<DefaultEventMap>, eventName: string | symbol, options?: StaticEventEmitterIteratorOptions): AsyncIterableIterator<any[]>

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
});

for await (const event of on(ee, 'foo')) {
  // The execution of this inner block is synchronous and it
  // processes one event at a time (even with await). Do not use
  // if concurrent execution is required.
  console.log(event); // prints ['bar'] [42]
}
// Unreachable here

Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.

An AbortSignal can be used to cancel waiting on events:

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ac = new AbortController();

(async () => {
  const ee = new EventEmitter();

  // Emit later on
  process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
  });

  for await (const event of on(ee, 'foo', { signal: ac.signal })) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
  }
  // Unreachable here
})();

process.nextTick(() => ac.abort());

Use the close option to specify an array of event names that will end the iteration:

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
  ee.emit('close');
});

for await (const event of on(ee, 'foo', { close: ['close'] })) {
  console.log(event); // prints ['bar'] [42]
}
// the loop will exit after 'close' is emitted
console.log('done'); // prints 'done'

Parameters

Parameter	Type
emitter	EventEmitter<DefaultEventMap>
eventName	string | symbol
options?	StaticEventEmitterIteratorOptions

Returns

AsyncIterableIterator<any[]>

An AsyncIterator that iterates eventName events emitted by the emitter

Since

v13.6.0, v12.16.0

Inherited from

EventEmitter.on

on(emitter, eventName, options)

static on(emitter: EventTarget, eventName: string, options?: StaticEventEmitterIteratorOptions): AsyncIterableIterator<any[]>

Parameters

Parameter	Type
emitter	EventTarget
eventName	string
options?	StaticEventEmitterIteratorOptions

Returns

AsyncIterableIterator<any[]>

Inherited from

EventEmitter.on

---

once()

once(emitter, eventName, options)

static once(emitter: EventEmitter<DefaultEventMap>, eventName: string | symbol, options?: StaticEventEmitterOptions): Promise<any[]>

Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.

This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.

import { once, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

process.nextTick(() => {
  ee.emit('myevent', 42);
});

const [value] = await once(ee, 'myevent');
console.log(value);

const err = new Error('kaboom');
process.nextTick(() => {
  ee.emit('error', err);
});

try {
  await once(ee, 'myevent');
} catch (err) {
  console.error('error happened', err);
}

The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:

import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();

once(ee, 'error')
  .then(([err]) => console.log('ok', err.message))
  .catch((err) => console.error('error', err.message));

ee.emit('error', new Error('boom'));

// Prints: ok boom

An AbortSignal can be used to cancel waiting for the event:

import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();
const ac = new AbortController();

async function foo(emitter, event, signal) {
  try {
    await once(emitter, event, { signal });
    console.log('event emitted!');
  } catch (error) {
    if (error.name === 'AbortError') {
      console.error('Waiting for the event was canceled!');
    } else {
      console.error('There was an error', error.message);
    }
  }
}

foo(ee, 'foo', ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit('foo'); // Prints: Waiting for the event was canceled!

Parameters

Parameter	Type
emitter	EventEmitter<DefaultEventMap>
eventName	string | symbol
options?	StaticEventEmitterOptions

Returns

Promise<any[]>

Since

v11.13.0, v10.16.0

Inherited from

EventEmitter.once

once(emitter, eventName, options)

static once(emitter: EventTarget, eventName: string, options?: StaticEventEmitterOptions): Promise<any[]>

Parameters

Parameter	Type
emitter	EventTarget
eventName	string
options?	StaticEventEmitterOptions

Returns

Promise<any[]>

Inherited from

EventEmitter.once

---

setMaxListeners()

static setMaxListeners(n?: number, ...eventTargets?: (EventTarget | EventEmitter<DefaultEventMap>)[]): void

import { setMaxListeners, EventEmitter } from 'node:events';

const target = new EventTarget();
const emitter = new EventEmitter();

setMaxListeners(5, target, emitter);

Parameters

Parameter	Type	Description
n?	number	A non-negative number. The maximum number of listeners per EventTarget event.
...eventTargets?	(EventTarget | EventEmitter<DefaultEventMap>)[]	-

Returns

void

Since

v15.4.0

Inherited from

EventEmitter.setMaxListeners