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Class ClientSession

A class representing a client session on the server

NOTE: not meant to be instantiated directly.




clientOptions?: MongoOptions
clusterTime?: ClusterTime
defaultTransactionOptions: TransactionOptions
explicit: boolean
hasEnded: boolean
operationTime?: Timestamp
supports: { causalConsistency: boolean }

Type declaration

  • causalConsistency: boolean
transaction: Transaction
captureRejectionSymbol: typeof captureRejectionSymbol
captureRejections: boolean

Sets or gets the default captureRejection value for all emitters.

defaultMaxListeners: number
errorMonitor: 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.


  • get isPinned(): boolean
  • get loadBalanced(): boolean
  • get snapshotEnabled(): boolean
  • Whether or not this session is configured for snapshot reads

    Returns boolean


  • Advances the clusterTime for a ClientSession to the provided clusterTime of another ClientSession


    • clusterTime: ClusterTime

      the $clusterTime returned by the server from another session in the form of a document containing the BSON.Timestamp clusterTime and signature

    Returns void

  • advanceOperationTime(operationTime: Timestamp): void
  • Advances the operationTime for a ClientSession.


    • operationTime: Timestamp

      the BSON.Timestamp of the operation type it is desired to advance to

    Returns void

  • emit<EventKey>(event: symbol | EventKey, ...args: Parameters<ClientSessionEvents[EventKey]>): boolean
  • eventNames(): string[]
  • getMaxListeners(): number
  • inTransaction(): boolean
  • Returns boolean

    whether this session is currently in a transaction or not

  • incrementTransactionNumber(): void
  • Increment the transaction number on the internal ServerSession


    This helper increments a value stored on the client session that will be added to the serverSession's txnNumber upon applying it to a command. This is because the serverSession is lazily acquired after a connection is obtained

    Returns void

  • listenerCount<EventKey>(type: string | symbol | EventKey): number
  • removeAllListeners<EventKey>(event?: string | symbol | EventKey): ClientSession
  • toBSON(): never
  • This is here to ensure that ClientSession is never serialized to BSON.

    Returns never

  • Runs a provided callback within a transaction, retrying either the commitTransaction operation or entire transaction as needed (and when the error permits) to better ensure that the transaction can complete successfully.

    IMPORTANT: This method requires the user to return a Promise, and await all operations. Any callbacks that do not return a Promise will result in undefined behavior.


    This function:

    • Will return the command response from the final commitTransaction if every operation is successful (can be used as a truthy object)
    • Will return undefined if the transaction is explicitly aborted with await session.abortTransaction()
    • Will throw if one of the operations throws or throw statement is used inside the withTransaction callback

    Checkout a descriptive example here:



    Type parameters

    • T = void


    Returns Promise<undefined | Document>

    A raw command response or undefined

  • getEventListeners(emitter: EventEmitter | DOMEventTarget, 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.

    const { getEventListeners, EventEmitter } = require('events');

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

    v15.2.0, v14.17.0


    • emitter: EventEmitter | DOMEventTarget
    • name: string | symbol

    Returns Function[]

  • listenerCount(emitter: EventEmitter, eventName: string | symbol): number
  • A class method that returns the number of listeners for the given eventNameregistered on the given emitter.

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



    Since v3.2.0 - Use listenerCount instead.


    • emitter: EventEmitter

      The emitter to query

    • eventName: string | symbol

      The event name

    Returns number

  • on(emitter: EventEmitter, eventName: string, options?: StaticEventEmitterOptions): AsyncIterableIterator<any>
  • ```js const { on, EventEmitter } = require('events');

    (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')) { // 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:

    const { on, EventEmitter } = require('events');
    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());

    v13.6.0, v12.16.0


    • emitter: EventEmitter
    • eventName: string

      The name of the event being listened for

    • Optional options: StaticEventEmitterOptions

    Returns AsyncIterableIterator<any>

    that iterates eventName events emitted by the emitter

  • once(emitter: NodeEventTarget, eventName: string | symbol, options?: StaticEventEmitterOptions): Promise<any[]>
  • once(emitter: DOMEventTarget, eventName: string, 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.

    const { once, EventEmitter } = require('events');

    async function run() {
    const ee = new EventEmitter();

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

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

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

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

    const { EventEmitter, once } = require('events');

    const ee = new EventEmitter();

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

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

    // Prints: ok boom

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

    const { EventEmitter, once } = require('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!

    v11.13.0, v10.16.0


    • emitter: NodeEventTarget
    • eventName: string | symbol
    • Optional options: StaticEventEmitterOptions

    Returns Promise<any[]>

  • Parameters

    • emitter: DOMEventTarget
    • eventName: string
    • Optional options: StaticEventEmitterOptions

    Returns Promise<any[]>

  • setMaxListeners(n?: number, ...eventTargets: (EventEmitter | DOMEventTarget)[]): void
  • 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 EventEmitter.setMaxListeners() method allows the default limit to be modified (if eventTargets is empty) or modify the limit specified in every EventTarget | EventEmitter passed as arguments. The value can be set toInfinity (or 0) to indicate an unlimited number of listeners.

    // Equivalent to
    EventEmitter.defaultMaxListeners = 20;

    const eventTarget = new EventTarget();
    // Only way to increase limit for `EventTarget` instances
    // as these doesn't expose its own `setMaxListeners` method
    EventEmitter.setMaxListeners(20, eventTarget);

    v15.3.0, v14.17.0


    • Optional n: number
    • Rest ...eventTargets: (EventEmitter | DOMEventTarget)[]

    Returns void

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