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withLatestFrom

Emits [source, latest] whenever the source emits, using the latest value from that. Source values are dropped until that has emitted at least once.

Unlike {@link zipLatest}, this does not emit when that updates.

Completion: Completes when the source completes. Errors: The first failure from either stream fails the result.

Package version
2.0.0-beta.13
Category
Combining sources
Since
1.0.0

Follow one execution

Only source pushes trigger pairs; source-1 is dropped because state is not initialized.

Fx timelinewithLatestFrom(source, state)withLatestFrom
OperatorwithLatestFrom(source, state)
Read this diagram

Follow each lane from left to right. Events stacked vertically share a tick; the green cursor marks the current time across every lane.

  • A value
    The text inside the pill is the emitted value.
  • Work starts
    The raised chevron starts an inner run (^ in the source).
  • The run returns
    The vertical bar ends this lane’s run.
  • A cause is delivered
    The exclamation mark belongs to this lane.
  • Work is interrupted
    The cross marks cancellation of this run.
  • Current time
    The line and diamond move together across all lanes.
  • Happening now
    A highlighted event is at the current tick.
  • Still ahead
    Muted, dashed values have not happened yet.
  • Time continues
    The lane’s arrow is not a return marker. An empty stretch can be quiet work that is still running.

Illustrated ticks start at 0. At 1×, one illustrated tick takes one second; captions specify real durations when timing matters. A cause or interruption belongs to its lane, and other work may continue. Scroll horizontally to inspect the rest of a long timeline.

The right source runs in a child fiber and one latest value is retained. Left completion or consumer interruption interrupts that fiber; failure from either side is forwarded.

Source implementation · Learn the surrounding model

Compare all Fx timelines →

Import

import { withLatestFrom } from "@typed/fx/Fx";

Signatures

export declare const withLatestFrom: {
    <B, E2, R2>(that: Fx<B, E2, R2>): <A, E, R>(self: Fx<A, E, R>) => Fx<readonly [
        A,
        B
    ], E | E2, R | R2>;
    <A, E, R, B, E2, R2>(self: Fx<A, E, R>, that: Fx<B, E2, R2>): Fx<readonly [
        A,
        B
    ], E | E2, R | R2>;
};

Why

withLatestFrom makes the left source the sole emission trigger while enriching it with right-side state. Left values before the right’s first value are dropped; right updates never emit alone.

Ownership and lifetime

The right source runs in a child fiber and one latest value is retained. Left completion or consumer interruption interrupts that fiber; failure from either side is forwarded.

Examples

import { Fx } from "@typed/fx"
const labeled = Fx.withLatestFrom(Fx.fromIterable([1, 2]), Fx.succeed("ready"))

Other public imports

These import paths expose the same declaration. Each page retains its own public name and signature.

Source