@efesto-cloud/compute

A Computed<T> observable derived from one or more Observable dependencies. Re-evaluates (and notifies) only when the computed value actually changes.

The package is named @efesto-cloud/compute but lives in packages/computed.

Installation

pnpm add @efesto-cloud/compute @efesto-cloud/observable

Quick Start

import { Observable } from "@efesto-cloud/observable";
import { Computed } from "@efesto-cloud/compute";

const firstName = new Observable("Ada");
const lastName  = new Observable("Lovelace");

const fullName = new Computed(
    [firstName, lastName],
    ([first, last]) => `${first} ${last}`,
);

const off = fullName.subscribe((name) => console.log(name));

firstName.set("Grace"); // logs: "Grace Lovelace"
lastName.set("Lovelace"); // no log — identical result, no notify

off();
fullName.dispose(); // unsubscribes from every dependency

API

Computed<T, DEPS> implements IObservable<T>:

new Computed<T, DEPS>(
    dependencies: { [K in keyof DEPS]: IObservable<DEPS[K]> },
    computeFn: (values: DEPS) => T,
);

get(): T;
set(value: T): void;  // no-op — Computed is read-only
subscribe(cb: (value: T) => void): Unsubscribe;
dispose(): void;      // unsubscribe from deps and drop listeners

Rules

• Computed is read-only: set() is a no-op.
• Subscribers fire only when compute() produces a value not === to the previous one. This is reference equality — return the same array/object reference if you want dedup to work.
• Always call dispose() when the owning object is torn down; otherwise the Computed keeps subscriptions to its dependencies alive.

Example: derived from many sources

const a = new Observable(1);
const b = new Observable(2);
const c = new Observable(3);

const sum = new Computed([a, b, c], ([x, y, z]) => x + y + z);

sum.subscribe((v) => console.log("sum:", v));
b.set(20); // logs: sum: 24

Related

• @efesto-cloud/observable — source of truth for values.
• @efesto-cloud/publisher — events without stored state.