An Observable-ish Value

Publish changes to subscriber functions when values change.

const fn = function(current, previous) {}

const obsValue = ov('initial');
obsValue.subscribe(fn);      // subscribe to changes
obsValue();                  // 'initial'
obsValue('initial');         // identical value, no change
obsValue('new');             // fn('new', 'initial')
obsValue.value = 'silent';   // silent update

Modifying arrays and objects will not publish, but replacing them will.

const obsArray = ov([1, 2, 3]);
obsarray().subscribe(fn);
obsArray().push(4);          // silent update
obsArray.publish();          // fn([1, 2, 3, 4]);
obsArray([4, 5]);            // fn([4, 5], [1, 2, 3]);

Passing a function caches the result as the value. Any extra arguments will be passed to the function. Any observables called within the function will be subscribed to, and updates to those observables will recompute the value. Child observables must be called, mere references are ignored. If the function returns a Promise, the value is assigned async after resolution.

const a = ov(1);
const b = ov(2);
const computed = ov(arg => { a() + b() + arg }, 3);
computed.subscribe(fn);
computed();                  // fn(6)
a(2);                        // fn(7, 6)