Function: takeWhile()

takeWhile<T>(predicate): Operator<T, T>

Defined in: operators/takeWhile.ts:21

Creates a stream operator that emits values from the source stream as long as a predicate returns true.

This operator is a conditional limiter. It consumes values from the source stream and applies the predicate function to each. As long as the predicate returns true, the value is passed through to the output stream. The first time the predicate returns a falsy value, the operator stops emitting and immediately completes the output stream. The value that caused the predicate to fail is not emitted.

This is useful for taking a contiguous block of data from a stream that meets a certain condition, such as processing user input until an invalid entry is made.

Type Parameters

T

T = any

The type of the values in the source and output streams.

Parameters

predicate

(value) => CallbackReturnType<boolean>

The function to test each value. true means to continue emitting, and false means to stop and complete. It can be synchronous or asynchronous.

Returns

Operator<T, T>

An Operator instance that can be used in a stream's pipe method.

Examples

From takeWhile.spec.ts:4

const testStream = from([1, 2, 3, 4, 5]);
const predicate = (value: number) => value < 4;
const takenWhileStream = testStream.pipe(takeWhile(predicate));
let results: any[] = [];
takenWhileStream.subscribe({
  next: (value) => results.push(value),
  complete: () => {
    expect(results).toEqual([1, 2, 3]); // Should emit values until predicate returns false
    done();
  },
});