Merge pull request #245 from preactjs/fix-effect-barf

Fix effect behavior when first run throws

Author: jviide

.changeset/modern-bulldogs-perform.md

@@ -0,0 +1,5 @@
+---
+"@preact/signals-core": patch
+---
+
+Fix effect behavior when first run throws

packages/core/src/index.ts

@@ -648,7 +648,12 @@ Effect.prototype._dispose = function () {
 
 function effect(compute: () => unknown): () => void {
 	const effect = new Effect(compute);
-	effect._callback();
+	try {
+		effect._callback();
+	} catch (err) {
+		effect._dispose();
+		throw err;
+	}
 	// Return a bound function instead of a wrapper like `() => effect._dispose()`,
 	// because bound functions seem to be just as fast and take up a lot less memory.
 	return effect._dispose.bind(effect);

packages/core/test/signal.test.tsx

@@ -461,6 +461,19 @@ describe("effect()", () => {
 		expect(spy).to.be.calledOnce;
 	});
 
+	it("should not subscribe to anything if first run throws", () => {
+		const s = signal(0);
+		const spy = sinon.spy(() => {
+			s.value;
+			throw new Error("test");
+		});
+		expect(() => effect(spy)).to.throw("test");
+		expect(spy).to.be.calledOnce;
+
+		s.value++;
+		expect(spy).to.be.calledOnce;
+	});
+
 	it("should reset the cleanup if the effect throws", () => {
 		const a = signal(0);
 		const spy = sinon.spy();