portfolio/packages/frptools/spec/computed.spec.js

const { observable, computed } = require('../lib/index.js');

describe('computed', () => {
	const add = (a, b) => a + b;
	const square = a => a * a;

	it('returns the value computed from its dependencies', () => {
		const a = observable(0);
		const b = computed(square, [a]);
		const c = computed(add, [a, b]);

		expect(b()).toEqual(0);
		expect(c()).toEqual(0);

		a(1);
		expect(b()).toEqual(1);
		expect(c()).toEqual(2);

		a(2);
		expect(b()).toEqual(4);
		expect(c()).toEqual(6);

		a(3);
		expect(b()).toEqual(9);
		expect(c()).toEqual(12);
	});

	it('only computes when called', () => {
		let runCount = 0;
		let currentValue = 1;
		const a = observable(0);
		const b = computed(
			val => {
				runCount += 1;
				expect(val).toEqual(currentValue);
				return val * val;
			},
			[a]
		);

		a(1);
		expect(runCount).toEqual(0);
		// b evaluates
		expect(b()).toEqual(1);
		expect(runCount).toEqual(1);
		// b does not evaluate
		expect(b()).toEqual(1);
		expect(runCount).toEqual(1);
		currentValue = 3;
		// b does not evaluate
		a(3);
		expect(runCount).toEqual(1);
		// b evaluates
		expect(b()).toEqual(9);
		expect(runCount).toEqual(2);
	});

	it('computes automatically when subscribed', () => {
		let runCount = 0;
		let subRunCount = 0;
		let currentValue = 1;
		const a = observable(0);
		const b = computed(
			val => {
				runCount += 1;
				expect(val).toEqual(currentValue);
				return val * val;
			},
			[a]
		);

		// b does not evaluate
		a(1);
		expect(runCount).toEqual(0);
		// b evaluates
		expect(b()).toEqual(1);
		expect(runCount).toEqual(1);
		// b does not evaluate
		expect(b()).toEqual(1);
		expect(runCount).toEqual(1);

		const cancelSubscription = b.subscribe(val => {
			subRunCount += 1;
			expect(val).toEqual(currentValue * currentValue);
		});

		currentValue = 3;
		// b evaluates
		a(3);
		expect(runCount).toEqual(2);
		expect(subRunCount).toEqual(1);
		// b does not evaluate
		expect(b()).toEqual(9);
		expect(runCount).toEqual(2);
		expect(subRunCount).toEqual(1);
	});

	it('honors cancelled subscriptions', () => {
		let runCount = 0;
		let subRunCount = 0;
		let currentValue = 1;
		const a = observable(0);
		const b = computed(
			val => {
				runCount += 1;
				expect(val).toEqual(currentValue);
				return val * val;
			},
			[a]
		);
		const cancelSubscription = b.subscribe(val => {
			subRunCount += 1;
			expect(val).toEqual(currentValue * currentValue);
		});

		const cancelSubscription2 = b.subscribe(val => {
			subRunCount += 1;
		});

		// b evaluates
		a(1);
		expect(runCount).toEqual(1);
		expect(subRunCount).toEqual(2);
		// b does not evaluate
		expect(b()).toEqual(1);
		expect(runCount).toEqual(1);
		expect(subRunCount).toEqual(2);

		expect(cancelSubscription()).toEqual(1);

		currentValue = 3;
		// b evaluates
		a(3);
		expect(runCount).toEqual(2);
		expect(subRunCount).toEqual(3);
		// b does not evaluate
		expect(b()).toEqual(9);
		expect(runCount).toEqual(2);
		expect(subRunCount).toEqual(3);

		expect(cancelSubscription2()).toEqual(0);
	});

	it('can be detached', () => {
		const a = observable(2);
		const b = computed(square, [a]);
		const c = computed(add, [a, b]);

		expect(b()).toEqual(4);
		expect(c()).toEqual(6);

		b.detach();

		a(3);
		expect(b()).toEqual(4);
		expect(c()).toEqual(7);
	});

	it('uses a comparator', () => {
		function setEquals(a, b) {
			return (
				a instanceof Set &&
				b instanceof Set &&
				[...a].reduce((acc, d) => acc && b.has(d), true) &&
				[...b].reduce((acc, d) => acc && a.has(d), true)
			);
		}

		let runCount = 0;

		function intersection(a, b) {
			runCount += 1;
			return new Set([...a].filter(x => b.has(x)));
		}

		const a = observable(new Set([1, 2]), setEquals);
		const b = observable(new Set([2, 3]), setEquals);
		const ABintersection = computed(intersection, [a, b], setEquals);

		expect(runCount).toEqual(0);
		expect([...ABintersection()]).toEqual([2]);
		expect(runCount).toEqual(1);
		b(new Set([3, 2]));
		expect([...ABintersection()]).toEqual([2]);
		expect(runCount).toEqual(1);
		b(new Set([3, 2, 1]));
		expect(runCount).toEqual(1);
		expect([...ABintersection()]).toEqual([1, 2]);
		expect(runCount).toEqual(2);
	});
});