loop throws helpful error message if value is missing

src/component.ts

@@ -265,8 +265,14 @@ class LoopComponent<O> extends Component<O, {}> {
       parent,
       destroyed
     );
-    const returned: (keyof O)[] = <any>Object.keys(output);
-    for (const name of returned) {
+    const needed = Object.keys(placeholderObject);
+    for (const name of needed) {
+      if (name === "destroyed") {
+        continue;
+      }
+      if (output[name] === undefined) {
+        throw new Error(`The property ${name} is missing.`);
+      }
       placeholderObject[name].replaceWith(output[name]);
     }
     return { available: output, output: {} };

test/component.spec.ts

@@ -272,6 +272,16 @@ describe("component specs", () => {
       expect(dom).to.have.length(0);
       expect(toplevel).to.equal(true);
     });
+    it("throws helpful error is a reactive is missing", () => {
+      const c = loop((props: { foo: H.Behavior<string> }) => {
+        // Access something that isn't there
+        (props as any).bar;
+        return div([dynamic(props.foo)]).output((_) => ({
+          foo: H.Behavior.of("foo")
+        }));
+      });
+      assert.throws(() => testComponent(c), /bar/);
+    });
   });
 });