derive-rx

A tiny RxJS helper library to create reactive derived observables from one or more source streams. Designed to simplify combineLatest, withLatestFrom, and switchMap patterns while keeping everything type-safe, reactive, and readable.

Installation

Not been published yet.

Functions

deriveFromBoth(...observables, computeFn)

Creates a derived observable that reacts to any change in the source observables. Similar to combineLatest, but with a clean compute function.

Supports both spread and array style.

Example (spread):

deriveFromBoth(obs1$, obs2$, (a, b) => a + b);

Example (array):

deriveFromBoth([obs1$, obs2$], ([a, b]) => a * b);

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deriveFromAWithB(a$, b$, computeFn)

Emits a derived value only when a$ emits, using the latest value(s) from b$.

Supports one or multiple b$ observables.
Useful for avoiding unnecessary emissions from B.

Example (single B):

deriveFromAWithB(a$, b$, (a, b) => a + b);

Example (multiple Bs):

deriveFromAWithB(a$, [b1$, b2$], (a, b1, b2) => a + b1 - b2);

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deriveFromDynamicB(a$, getB$, computeFn)

Use when B is dynamic based on A — i.e., a$ determines which observable(s) to listen to.

Internally uses switchMap.

Supports single or multiple dynamic B observables.

Example (single B):

deriveFromDynamicB(a$, a => fetchB$(a), (a, b) => a + b);

Example (multiple Bs):

deriveFromDynamicB(
  a$,
  a => [fetchB1$(a), fetchB2$(a)],
  (a, b1, b2) => a + b1 - b2
);

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Type Safety

All functions are fully typed. Your compute function will always get autocompletion for the correct values, with no need to cast or destructure manually.

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Testing

This library uses vitest for unit testing.

npm run test

Tests cover all functional cases, including edge cases like hot/cold observables, multiple subscriptions, and delayed emissions.

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License

MIT