The computed error handler now sends the computed previous value as a…

…n argument

src/lwc/signals/__tests__/computed.test.ts

@@ -128,4 +128,31 @@ describe("computed values", () => {
 
     expect(computed.value).toBe("fallback");
   });
+
+  test("allows for custom error handlers to return the previous value", () => {
+    const signal = $signal(0);
+    function customErrorHandlerFn(_error: unknown, previousValue: number | undefined) {
+      return previousValue;
+    }
+
+    const computed = $computed(() => {
+      if (signal.value === 2) {
+        throw new Error("test");
+      }
+
+      return signal.value;
+    }, {
+      errorHandler: customErrorHandlerFn
+    });
+
+    expect(computed.value).toBe(0);
+
+    signal.value = 1;
+
+    expect(computed.value).toBe(1)
+
+    signal.value = 2;
+
+    expect(computed.value).toBe(1);
+  });
 });

src/lwc/signals/core.ts

@@ -12,6 +12,7 @@ export type Signal<T> = {
   set value(newValue: T);
   readOnly: ReadOnlySignal<T>;
   brand: symbol;
+  peek(): T;
 };
 
 const context: VoidFunction[] = [];
@@ -105,7 +106,7 @@ function handleEffectError(error: unknown, props: EffectProps) {
 type ComputedFunction<T> = () => T;
 type ComputedProps<T> = {
   identifier: string | null;
-  errorHandler?: (error: unknown) => T | undefined;
+  errorHandler?: (error: unknown, previousValue: T | undefined) => T | undefined;
 };
 
 /**
@@ -139,7 +140,8 @@ function $computed<T>(
         try {
           computedSignal.value = fn();
         } catch (error) {
-          computedSignal.value = props.errorHandler(error);
+          const previousValue = computedSignal.peek();
+          computedSignal.value = props.errorHandler(error, previousValue);
         }
       } else {
         // Otherwise, the error handling is done in the $effect
@@ -306,6 +308,9 @@ function $signal<T>(
         get value() {
           return getter();
         }
+      },
+      peek() {
+        return _storageOption.get();
       }
     };