map

Transforms each value emitted by the source using a mapping function that also receives the emission index.

Marble Diagram

map((x, i) => `${i}: ${x}`)

Example

//  Timeline:
//
//  num$      "A"      "B"      "C"
//  indexed$  "0: A"   "1: B"   "2: C"
//
//  Explanation:
//  - mapWithIndex transforms each value along with its index
//  - Index starts at 0 and increments with each emission

const num$ = source<string>();

const indexed$ = num$.pipe(map((x, i) => `${i}: ${x}`));

const valueHistory: string[] = [];

indexed$.subscribe((s) => {
    valueHistory.push(s);
});

num$.next('A'); // 0: A

num$.next('B'); // 1: B

num$.next('C'); // 2: C

assert.deepStrictEqual(valueHistory, ['0: A', '1: B', '2: C']);

View source on GitHub