scan

Applies an accumulator function over the source observable and emits each intermediate result. Similar to Array.reduce but emits accumulated value after each source emission.

Marble Diagram

scan((sum, x) => sum + x, 0)

Example

//  Timeline (accumulating sum):
//
//  num$    1     2     3     4     5
//  sum$    1     3     6     10    15
//          |     |     |     |     |
//          0+1   1+2   3+3   6+4   10+5
//
//  Explanation:
//  - scan accumulates values over time using a reducer function
//  - Starting with seed value 0, each emission adds to the accumulator
//  - Similar to Array.reduce, but for streams

const num$ = source<number>();

const sum$ = num$.pipe(scan((acc, curr) => acc + curr, 0));

const valueHistory: number[] = [];

sum$.subscribe((x) => {
    valueHistory.push(x);
});

assert.deepStrictEqual(valueHistory, [0]);

num$.next(1); // logs: 1

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

num$.next(2); // logs: 3

assert.deepStrictEqual(valueHistory, [0, 1, 3]);

num$.next(3); // logs: 6

assert.deepStrictEqual(valueHistory, [0, 1, 3, 6]);

View source on GitHub