Class FindCursor<TSchema>

Type Parameters

  • TSchema = any

Hierarchy

Properties

CLOSE: "close" = ...

Event

captureRejectionSymbol: typeof captureRejectionSymbol

Value: Symbol.for('nodejs.rejection')

See how to write a custom rejection handler.

Since

v13.4.0, v12.16.0

captureRejections: boolean

Value: boolean

Change the default captureRejections option on all new EventEmitter objects.

Since

v13.4.0, v12.16.0

defaultMaxListeners: 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.defaultMaxListenersproperty can be used. If this value is not a positive number, a RangeErroris thrown.

Take caution when setting the events.defaultMaxListeners because the change affects allEventEmitter 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 singleEventEmitter, 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

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.

Since

v13.6.0, v12.17.0

Accessors

Methods

  • Add a cursor flag to the cursor

    Parameters

    • flag: "tailable" | "oplogReplay" | "noCursorTimeout" | "awaitData" | "exhaust" | "partial"

      The flag to set, must be one of following ['tailable', 'oplogReplay', 'noCursorTimeout', 'awaitData', 'partial' -.

    • value: boolean

      The flag boolean value.

    Returns FindCursor<TSchema>

  • Add a query modifier to the cursor query

    Parameters

    • name: string

      The query modifier (must start with $, such as $orderby etc)

    • value: string | number | boolean | Document

      The modifier value.

    Returns FindCursor<TSchema>

  • Add a comment to the cursor query allowing for tracking the comment in the log.

    Parameters

    • value: string

      The comment attached to this query.

    Returns FindCursor<TSchema>

  • Get the count of documents for this cursor

    Parameters

    Returns Promise<number>

    Deprecated

    Use collection.estimatedDocumentCount or collection.countDocuments instead

  • Iterates over all the documents for this cursor using the iterator, callback pattern.

    If the iterator returns false, iteration will stop.

    Parameters

    • iterator: ((doc) => boolean | void)

      The iteration callback.

        • (doc): boolean | void
        • Parameters

          • doc: TSchema

          Returns boolean | void

    Returns Promise<void>

    Deprecated

    • Will be removed in a future release. Use for await...of instead.
  • Map all documents using the provided function If there is a transform set on the cursor, that will be called first and the result passed to this function's transform.

    Type Parameters

    • T

    Parameters

    • transform: ((doc) => T)

      The mapping transformation method.

        • (doc): T
        • Parameters

          • doc: TSchema

          Returns T

    Returns FindCursor<T>

    Remarks

    Note Cursors use null internally to indicate that there are no more documents in the cursor. Providing a mapping function that maps values to null will result in the cursor closing itself before it has finished iterating all documents. This will not result in a memory leak, just surprising behavior. For example:

    const cursor = collection.find({});
    cursor.map(() => null);

    const documents = await cursor.toArray();
    // documents is always [], regardless of how many documents are in the collection.

    Other falsey values are allowed:

    const cursor = collection.find({});
    cursor.map(() => '');

    const documents = await cursor.toArray();
    // documents is now an array of empty strings

    Note for Typescript Users: adding a transform changes the return type of the iteration of this cursor, it does not return a new instance of a cursor. This means when calling map, you should always assign the result to a new variable in order to get a correctly typed cursor variable. Take note of the following example:

    Example

    const cursor: FindCursor<Document> = coll.find();
    const mappedCursor: FindCursor<number> = cursor.map(doc => Object.keys(doc).length);
    const keyCounts: number[] = await mappedCursor.toArray(); // cursor.toArray() still returns Document[]
  • Set the cursor max

    Parameters

    • max: Document

      Specify a $max value to specify the exclusive upper bound for a specific index in order to constrain the results of find(). The $max specifies the upper bound for all keys of a specific index in order.

    Returns FindCursor<TSchema>

  • Set a maxAwaitTimeMS on a tailing cursor query to allow to customize the timeout value for the option awaitData (Only supported on MongoDB 3.2 or higher, ignored otherwise)

    Parameters

    • value: number

      Number of milliseconds to wait before aborting the tailed query.

    Returns FindCursor<TSchema>

  • Set the cursor min

    Parameters

    • min: Document

      Specify a $min value to specify the inclusive lower bound for a specific index in order to constrain the results of find(). The $min specifies the lower bound for all keys of a specific index in order.

    Returns FindCursor<TSchema>

  • Add a project stage to the aggregation pipeline

    Type Parameters

    Parameters

    Returns FindCursor<T>

    Remarks

    In order to strictly type this function you must provide an interface that represents the effect of your projection on the result documents.

    By default chaining a projection to your cursor changes the returned type to the generic Document type. You should specify a parameterized type to have assertions on your final results.

    Example

    // Best way
    const docs: FindCursor<{ a: number }> = cursor.project<{ a: number }>({ _id: 0, a: true });
    // Flexible way
    const docs: FindCursor<Document> = cursor.project({ _id: 0, a: true });

    Example

    const cursor: FindCursor<{ a: number; b: string }> = coll.find();
    const projectCursor = cursor.project<{ a: number }>({ _id: 0, a: true });
    const aPropOnlyArray: {a: number}[] = await projectCursor.toArray();

    // or always use chaining and save the final cursor

    const cursor = coll.find().project<{ a: string }>({
    _id: 0,
    a: { $convert: { input: '$a', to: 'string' }
    }});
  • Set the cursor returnKey. If set to true, modifies the cursor to only return the index field or fields for the results of the query, rather than documents. If set to true and the query does not use an index to perform the read operation, the returned documents will not contain any fields.

    Parameters

    • value: boolean

      the returnKey value.

    Returns FindCursor<TSchema>

  • Rewind this cursor to its uninitialized state. Any options that are present on the cursor will remain in effect. Iterating this cursor will cause new queries to be sent to the server, even if the resultant data has already been retrieved by this cursor.

    Returns void

  • Modifies the output of a query by adding a field $recordId to matching documents. $recordId is the internal key which uniquely identifies a document in a collection.

    Parameters

    • value: boolean

      The $showDiskLoc option has now been deprecated and replaced with the showRecordId field. $showDiskLoc will still be accepted for OP_QUERY stye find.

    Returns FindCursor<TSchema>

  • Returns an array of documents. The caller is responsible for making sure that there is enough memory to store the results. Note that the array only contains partial results when this cursor had been previously accessed. In that case, cursor.rewind() can be used to reset the cursor.

    Returns Promise<TSchema[]>

  • 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

    • signal: AbortSignal
    • resource: ((event) => void)
        • (event): void
        • Parameters

          • event: Event

          Returns void

    Returns Disposable

    that removes the abort listener.

    Since

    v20.5.0

  • 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

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

    Returns Function[]

    Since

    v15.2.0, v14.17.0

  • 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

    • emitter: EventEmitter | _DOMEventTarget

    Returns number

    Since

    v19.9.0

  • A class method that returns the number of listeners for the given eventNameregistered 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

    • emitter: EventEmitter

      The emitter to query

    • eventName: string | symbol

      The event name

    Returns number

    Since

    v0.9.12

    Deprecated

    Since v3.2.0 - Use listenerCount instead.

  • 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());

    Parameters

    • 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

    Since

    v13.6.0, v12.16.0

  • 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

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

    Returns Promise<any[]>

    Since

    v11.13.0, v10.16.0

  • Parameters

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

    Returns Promise<any[]>

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

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

    setMaxListeners(5, target, emitter);

    Parameters

    • Optional n: number

      A non-negative number. The maximum number of listeners per EventTarget event.

    • Rest ...eventTargets: (EventEmitter | _DOMEventTarget)[]

    Returns void

    Since

    v15.4.0

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