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variable / @typed/fx/Fx

takeWhile

Takes elements from an Fx while a predicate returns true. Stops at the first element for which the predicate returns false; that element is not included.

Package version
2.0.0-beta.7
Category
Selecting values
Since
1.0.0

Follow one execution

Takes elements from an Fx while a predicate returns true. Stops at the first element for which the predicate returns false; that element is not included.

Fx timelinetakeWhile(isInProgress)takeWhile
OperatortakeWhile(isInProgress)
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.

A false result stops upstream immediately. The pure predicate acquires no resources and adds no failures or services.

Source implementation · Learn the surrounding model

Compare all Fx timelines →

Import

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

Signatures

export declare const takeWhile: {
    <A>(predicate: (a: A) => boolean): <E, R>(fx: Fx<A, E, R>) => Fx<A, E, R>;
    <A, E, R>(fx: Fx<A, E, R>, predicate: (a: A) => boolean): Fx<A, E, R>;
};

Why

takeWhile emits the longest matching prefix, in order, and completes before the first false value. It is the predicate complement of takeUntil.

Ownership and lifetime

A false result stops upstream immediately. The pure predicate acquires no resources and adds no failures or services.

Examples

import { Fx } from "@typed/fx"
const small = Fx.fromIterable([1, 2, 3]).pipe(Fx.takeWhile((n) => n < 3))

Other public imports

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

Source