[BUGFIX]: throws helpful error when event name is missing in {{on}} modifier.

 + Cleanup redundant test from 5b6f80b

Author: johanrd

packages/@glimmer/runtime/lib/modifiers/on.ts

@@ -14,7 +14,7 @@ import {
   CheckString,
   CheckUndefined,
 } from '@glimmer/debug';
-import { buildUntouchableThis, localAssert } from '@glimmer/debug-util';
+import { buildUntouchableThis } from '@glimmer/debug-util';
 import { registerDestructor } from '@glimmer/destroyable';
 import { setInternalModifierManager } from '@glimmer/manager';
 import { valueForRef } from '@glimmer/reference';
@@ -57,13 +57,16 @@ export class OnModifierState {
   updateListener(): void {
     let { element, args, listener } = this;
 
-    localAssert(
-      args.positional[0],
-      'You must pass a valid DOM event name as the first argument to the `on` modifier'
-    );
+    let arg0 = args.positional[0];
+
+    if (DEBUG && !arg0) {
+      throw new Error(
+        'You must pass a valid DOM event name as the first argument to the `on` modifier'
+      );
+    }
 
     let eventName = check(
-      valueForRef(args.positional[0]),
+      arg0 ? valueForRef(arg0) : undefined,
       CheckString,
       () => 'You must pass a valid DOM event name as the first argument to the `on` modifier'
     );

smoke-tests/scenarios/basic-test.ts

@@ -285,7 +285,7 @@ function basicTest(scenarios: Scenarios, appName: string) {
                   resetOnerror();
                 });
 
-                test('throws helpful error when callback is undefined', async function (assert) {
+                test('throws helpful error when callback is missing', async function (assert) {
                   assert.expect(1);
                   const noop = undefined;
                   setupOnerror((error) => {
@@ -297,16 +297,16 @@ function basicTest(scenarios: Scenarios, appName: string) {
                   await render(<template><div {{on "click" noop}}>Click</div></template>);
                 });
 
-                test('throws helpful error when callback is null', async function (assert) {
+                test('throws helpful error when event name is missing', async function (assert) {
                   assert.expect(1);
-                  const noop = null;
+                  const noop = () => {};
                   setupOnerror((error) => {
                     assert.true(
-                      /You must pass a function as the second argument to the \`on\` modifier/.test(error.message),
+                      /You must pass a valid DOM event name as the first argument to the \`on\` modifier/.test(error.message),
                       'Expected helpful error message, got: ' + error.message
                     );
                   });
-                  await render(<template><div {{on "click" noop}}>Click</div></template>);
+                  await render(<template><div {{on}}>Click</div></template>);
                 });
               });
             `,