attachIndex (withIndex)

Attaches a sequential index to each emitted value, producing [index, value] tuples. Index starts at 0 and increments with each emission.

Marble Diagram

attachIndex()

Example

//  Timeline:
//
//  letter$    "A"      "B"      "C"
//  indexed$   [0,"A"]  [1,"B"]  [2,"C"]
//
//  Explanation:
//  - attachIndex attaches a sequential index to each emitted value
//  - Produces [index, value] tuples
//  - Index starts at 0 and increments with each emission

const letter$ = source<string>();

const indexed$ = letter$.pipe(attachIndex());

const valueHistory: (readonly [number, string])[] = [];

indexed$.subscribe(([i, letter]) => {
    valueHistory.push([i, letter]);
});

letter$.next('A');

assert.deepStrictEqual(valueHistory, [[0, 'A']]);

letter$.next('B');

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

letter$.next('C');

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

View source on GitHub