map

Transforms each value emitted by the source with a mapping function, and emits the result unless it equals the one already held.

The comparison is Object.is unless equals says otherwise, so a result that did not change wakes nothing below. To pass on every result — an event turned into a value, say — give equals: () => false, or use mapTo. The mapping function receives the value alone; withIndex attaches an index.

Marble Diagram

map((x) => x % 2)

Example

//  Timeline:
//
//  num$      1   3   4   6   7
//  parity$   1       0       1
//
//  Explanation:
//  - map transforms each value
//  - A result equal to the one held is not emitted

const num$ = source<number>();

const parity$ = num$.pipe(map((x) => x % 2));

const valueHistory: number[] = [];

parity$.subscribe((p) => {
    valueHistory.push(p);
});

num$.next(1); // 1

num$.next(3); // still 1: not emitted

num$.next(4); // 0

num$.next(6); // still 0: not emitted

num$.next(7); // 1

assert.deepStrictEqual(valueHistory, [1, 0, 1]);

View source on GitHub