[BUGFIX] Regression [Ember 5.12+] Non helpful error when the function for on modifier was forgot as param by johanrd · Pull Request #21108 · emberjs/ember.js

johanrd · 2026-02-21T08:48:16Z

Fixes #20783 / #21045

Since Ember 5.12, passing a non-function to the {{on}} modifier produces an unhelpful TypeError: Cannot read properties of undefined (reading 'bind') instead of the previous helpful message: "You must pass a function as the second argument to the on modifier; you passed undefined."

Root cause

The glimmer-vm refactor in glimmerjs/glimmer-vm@2d9fc01 replaced expect() (a runtime check that always executes) with localAssert() (guarded by LOCAL_DEBUG, which is stripped in published builds). This meant the callback validation became a no-op in published ember-source, while the .bind() call further down — guarded by DEBUG — still runs, producing the unhelpful TypeError.

The key distinction: LOCAL_DEBUG is only true when developing the ember.js repo itself. DEBUG (from @glimmer/env) is true in user app development mode. The existing test suite always runs with LOCAL_DEBUG=true, so this gap was never caught.

Fix

Replace localAssert() with an explicit if (DEBUG) check, matching the pattern already used in the same function for positional argument validation
Add a smoke test that runs against built ember-source (LOCAL_DEBUG=false, DEBUG=true) — the exact environment users have — to prevent this class of regression

Cowritten by Claude.

Not sure whether it should be backported to LTS, considering the repo-merge.

github-actions · 2026-02-21T08:49:35Z

Estimated Asset Sizes

Diff

--- main/out.txt	2026-02-17 19:37:18.000000000 +0000
+++ pr/./pr-22260873684/out.txt	2026-02-21 17:14:28.000000000 +0000
@@ -1,62 +1,62 @@
-╔═══════╤═══════════╤═══════════╗
-║       │ Min       │ Gzip      ║
-╟───────┼───────────┼───────────╢
-║ Total │ 351.99 KB │ 203.84 KB ║
-╚═══════╧═══════════╧═══════════╝
+╔═══════╤═══════════╤══════════╗
+║       │ Min       │ Gzip     ║
+╟───────┼───────────┼──────────╢
+║ Total │ 351.99 KB │ 203.7 KB ║
+╚═══════╧═══════════╧══════════╝
 
-╔══════════════════════╤═══════════╤═══════════╗
-║ @ember/*             │ Min       │ Gzip      ║
-╟──────────────────────┼───────────┼───────────╢
-║ Total                │ 313.39 KB │ 181.91 KB ║
-╟──────────────────────┼───────────┼───────────╢
-║ -internals           │ 36.65 KB  │ 26.22 KB  ║
-║ application          │ 13.23 KB  │ 8.05 KB   ║
-║ array                │ 13.01 KB  │ 7.46 KB   ║
-║ canary-features      │ 304 B     │ 389 B     ║
-║ component            │ 2.05 KB   │ 1.64 KB   ║
-║ controller           │ 1.96 KB   │ 1.41 KB   ║
-║ debug                │ 11.69 KB  │ 8.12 KB   ║
-║ deprecated-features  │ 31 B      │ 77 B      ║
-║ destroyable          │ 561 B     │ 383 B     ║
-║ enumerable           │ 259 B     │ 387 B     ║
-║ helper               │ 1.08 KB   │ 811 B     ║
-║ instrumentation      │ 2.43 KB   │ 1.79 KB   ║
-║ modifier             │ 1.22 KB   │ 965 B     ║
-║ object               │ 35.94 KB  │ 22.16 KB  ║
-║ owner                │ 159 B     │ 178 B     ║
-║ renderer             │ 630 B     │ 487 B     ║
-║ routing              │ 59.3 KB   │ 34.12 KB  ║
-║ runloop              │ 2.36 KB   │ 1.5 KB    ║
-║ service              │ 1 KB      │ 845 B     ║
-║ template             │ 654 B     │ 541 B     ║
-║ template-compilation │ 429 B     │ 366 B     ║
-║ template-compiler    │ 123.08 KB │ 59.45 KB  ║
-║ template-factory     │ 370 B     │ 374 B     ║
-║ test                 │ 923 B     │ 627 B     ║
-║ utils                │ 4.11 KB   │ 3.6 KB    ║
-║ version              │ 55 B      │ 131 B     ║
-╚══════════════════════╧═══════════╧═══════════╝
+╔══════════════════════╤═══════════╤══════════╗
+║ @ember/*             │ Min       │ Gzip     ║
+╟──────────────────────┼───────────┼──────────╢
+║ Total                │ 313.39 KB │ 181.9 KB ║
+╟──────────────────────┼───────────┼──────────╢
+║ -internals           │ 36.65 KB  │ 26.23 KB ║
+║ application          │ 13.23 KB  │ 8 KB     ║
+║ array                │ 13.01 KB  │ 7.46 KB  ║
+║ canary-features      │ 304 B     │ 389 B    ║
+║ component            │ 2.05 KB   │ 1.61 KB  ║
+║ controller           │ 1.96 KB   │ 1.41 KB  ║
+║ debug                │ 11.69 KB  │ 8.12 KB  ║
+║ deprecated-features  │ 31 B      │ 77 B     ║
+║ destroyable          │ 561 B     │ 383 B    ║
+║ enumerable           │ 259 B     │ 387 B    ║
+║ helper               │ 1.08 KB   │ 832 B    ║
+║ instrumentation      │ 2.43 KB   │ 1.79 KB  ║
+║ modifier             │ 1.22 KB   │ 963 B    ║
+║ object               │ 35.94 KB  │ 22.16 KB ║
+║ owner                │ 159 B     │ 178 B    ║
+║ renderer             │ 630 B     │ 515 B    ║
+║ routing              │ 59.3 KB   │ 34.14 KB ║
+║ runloop              │ 2.36 KB   │ 1.5 KB   ║
+║ service              │ 1 KB      │ 845 B    ║
+║ template             │ 654 B     │ 530 B    ║
+║ template-compilation │ 429 B     │ 366 B    ║
+║ template-compiler    │ 123.08 KB │ 59.45 KB ║
+║ template-factory     │ 370 B     │ 374 B    ║
+║ test                 │ 923 B     │ 627 B    ║
+║ utils                │ 4.11 KB   │ 3.6 KB   ║
+║ version              │ 55 B      │ 131 B    ║
+╚══════════════════════╧═══════════╧══════════╝
 
-╔═════════════════╤══════════╤══════════╗
-║ @glimmer/*      │ Min      │ Gzip     ║
-╟─────────────────┼──────────┼──────────╢
-║ Total           │ 38.6 KB  │ 21.94 KB ║
-╟─────────────────┼──────────┼──────────╢
-║ destroyable     │ 2.77 KB  │ 1.39 KB  ║
-║ encoder         │ 81 B     │ 171 B    ║
-║ env             │ 38 B     │ 87 B     ║
-║ global-context  │ 886 B    │ 545 B    ║
-║ manager         │ 977 B    │ 608 B    ║
-║ node            │ 175 B    │ 260 B    ║
-║ opcode-compiler │ 1.11 KB  │ 894 B    ║
-║ owner           │ 159 B    │ 202 B    ║
-║ program         │ 252 B    │ 301 B    ║
-║ reference       │ 548 B    │ 531 B    ║
-║ runtime         │ 10.32 KB │ 5.32 KB  ║
-║ tracking        │ 1.34 KB  │ 1.16 KB  ║
-║ util            │ 1.94 KB  │ 1.68 KB  ║
-║ validator       │ 15.75 KB │ 6.96 KB  ║
-║ vm              │ 495 B    │ 569 B    ║
-║ wire-format     │ 1.84 KB  │ 1.35 KB  ║
-╚═════════════════╧══════════╧══════════╝
+╔═════════════════╤══════════╤═════════╗
+║ @glimmer/*      │ Min      │ Gzip    ║
+╟─────────────────┼──────────┼─────────╢
+║ Total           │ 38.6 KB  │ 21.8 KB ║
+╟─────────────────┼──────────┼─────────╢
+║ destroyable     │ 2.77 KB  │ 1.39 KB ║
+║ encoder         │ 81 B     │ 171 B   ║
+║ env             │ 38 B     │ 87 B    ║
+║ global-context  │ 886 B    │ 545 B   ║
+║ manager         │ 977 B    │ 608 B   ║
+║ node            │ 175 B    │ 246 B   ║
+║ opcode-compiler │ 1.11 KB  │ 894 B   ║
+║ owner           │ 159 B    │ 202 B   ║
+║ program         │ 252 B    │ 301 B   ║
+║ reference       │ 548 B    │ 531 B   ║
+║ runtime         │ 10.32 KB │ 5.2 KB  ║
+║ tracking        │ 1.34 KB  │ 1.16 KB ║
+║ util            │ 1.94 KB  │ 1.68 KB ║
+║ validator       │ 15.75 KB │ 6.96 KB ║
+║ vm              │ 495 B    │ 569 B   ║
+║ wire-format     │ 1.84 KB  │ 1.35 KB ║
+╚═════════════════╧══════════╧═════════╝

Details

This PR main

╔═══════╤═══════════╤══════════╗
║       │ Min       │ Gzip     ║
╟───────┼───────────┼──────────╢
║ Total │ 351.99 KB │ 203.7 KB ║
╚═══════╧═══════════╧══════════╝

╔══════════════════════╤═══════════╤══════════╗
║ @ember/*             │ Min       │ Gzip     ║
╟──────────────────────┼───────────┼──────────╢
║ Total                │ 313.39 KB │ 181.9 KB ║
╟──────────────────────┼───────────┼──────────╢
║ -internals           │ 36.65 KB  │ 26.23 KB ║
║ application          │ 13.23 KB  │ 8 KB     ║
║ array                │ 13.01 KB  │ 7.46 KB  ║
║ canary-features      │ 304 B     │ 389 B    ║
║ component            │ 2.05 KB   │ 1.61 KB  ║
║ controller           │ 1.96 KB   │ 1.41 KB  ║
║ debug                │ 11.69 KB  │ 8.12 KB  ║
║ deprecated-features  │ 31 B      │ 77 B     ║
║ destroyable          │ 561 B     │ 383 B    ║
║ enumerable           │ 259 B     │ 387 B    ║
║ helper               │ 1.08 KB   │ 832 B    ║
║ instrumentation      │ 2.43 KB   │ 1.79 KB  ║
║ modifier             │ 1.22 KB   │ 963 B    ║
║ object               │ 35.94 KB  │ 22.16 KB ║
║ owner                │ 159 B     │ 178 B    ║
║ renderer             │ 630 B     │ 515 B    ║
║ routing              │ 59.3 KB   │ 34.14 KB ║
║ runloop              │ 2.36 KB   │ 1.5 KB   ║
║ service              │ 1 KB      │ 845 B    ║
║ template             │ 654 B     │ 530 B    ║
║ template-compilation │ 429 B     │ 366 B    ║
║ template-compiler    │ 123.08 KB │ 59.45 KB ║
║ template-factory     │ 370 B     │ 374 B    ║
║ test                 │ 923 B     │ 627 B    ║
║ utils                │ 4.11 KB   │ 3.6 KB   ║
║ version              │ 55 B      │ 131 B    ║
╚══════════════════════╧═══════════╧══════════╝

╔═════════════════╤══════════╤═════════╗
║ @glimmer/*      │ Min      │ Gzip    ║
╟─────────────────┼──────────┼─────────╢
║ Total           │ 38.6 KB  │ 21.8 KB ║
╟─────────────────┼──────────┼─────────╢
║ destroyable     │ 2.77 KB  │ 1.39 KB ║
║ encoder         │ 81 B     │ 171 B   ║
║ env             │ 38 B     │ 87 B    ║
║ global-context  │ 886 B    │ 545 B   ║
║ manager         │ 977 B    │ 608 B   ║
║ node            │ 175 B    │ 246 B   ║
║ opcode-compiler │ 1.11 KB  │ 894 B   ║
║ owner           │ 159 B    │ 202 B   ║
║ program         │ 252 B    │ 301 B   ║
║ reference       │ 548 B    │ 531 B   ║
║ runtime         │ 10.32 KB │ 5.2 KB  ║
║ tracking        │ 1.34 KB  │ 1.16 KB ║
║ util            │ 1.94 KB  │ 1.68 KB ║
║ validator       │ 15.75 KB │ 6.96 KB ║
║ vm              │ 495 B    │ 569 B   ║
║ wire-format     │ 1.84 KB  │ 1.35 KB ║
╚═════════════════╧══════════╧═════════╝

╔═══════╤═══════════╤═══════════╗
║       │ Min       │ Gzip      ║
╟───────┼───────────┼───────────╢
║ Total │ 351.99 KB │ 203.84 KB ║
╚═══════╧═══════════╧═══════════╝

╔══════════════════════╤═══════════╤═══════════╗
║ @ember/*             │ Min       │ Gzip      ║
╟──────────────────────┼───────────┼───────────╢
║ Total                │ 313.39 KB │ 181.91 KB ║
╟──────────────────────┼───────────┼───────────╢
║ -internals           │ 36.65 KB  │ 26.22 KB  ║
║ application          │ 13.23 KB  │ 8.05 KB   ║
║ array                │ 13.01 KB  │ 7.46 KB   ║
║ canary-features      │ 304 B     │ 389 B     ║
║ component            │ 2.05 KB   │ 1.64 KB   ║
║ controller           │ 1.96 KB   │ 1.41 KB   ║
║ debug                │ 11.69 KB  │ 8.12 KB   ║
║ deprecated-features  │ 31 B      │ 77 B      ║
║ destroyable          │ 561 B     │ 383 B     ║
║ enumerable           │ 259 B     │ 387 B     ║
║ helper               │ 1.08 KB   │ 811 B     ║
║ instrumentation      │ 2.43 KB   │ 1.79 KB   ║
║ modifier             │ 1.22 KB   │ 965 B     ║
║ object               │ 35.94 KB  │ 22.16 KB  ║
║ owner                │ 159 B     │ 178 B     ║
║ renderer             │ 630 B     │ 487 B     ║
║ routing              │ 59.3 KB   │ 34.12 KB  ║
║ runloop              │ 2.36 KB   │ 1.5 KB    ║
║ service              │ 1 KB      │ 845 B     ║
║ template             │ 654 B     │ 541 B     ║
║ template-compilation │ 429 B     │ 366 B     ║
║ template-compiler    │ 123.08 KB │ 59.45 KB  ║
║ template-factory     │ 370 B     │ 374 B     ║
║ test                 │ 923 B     │ 627 B     ║
║ utils                │ 4.11 KB   │ 3.6 KB    ║
║ version              │ 55 B      │ 131 B     ║
╚══════════════════════╧═══════════╧═══════════╝

╔═════════════════╤══════════╤══════════╗
║ @glimmer/*      │ Min      │ Gzip     ║
╟─────────────────┼──────────┼──────────╢
║ Total           │ 38.6 KB  │ 21.94 KB ║
╟─────────────────┼──────────┼──────────╢
║ destroyable     │ 2.77 KB  │ 1.39 KB  ║
║ encoder         │ 81 B     │ 171 B    ║
║ env             │ 38 B     │ 87 B     ║
║ global-context  │ 886 B    │ 545 B    ║
║ manager         │ 977 B    │ 608 B    ║
║ node            │ 175 B    │ 260 B    ║
║ opcode-compiler │ 1.11 KB  │ 894 B    ║
║ owner           │ 159 B    │ 202 B    ║
║ program         │ 252 B    │ 301 B    ║
║ reference       │ 548 B    │ 531 B    ║
║ runtime         │ 10.32 KB │ 5.32 KB  ║
║ tracking        │ 1.34 KB  │ 1.16 KB  ║
║ util            │ 1.94 KB  │ 1.68 KB  ║
║ validator       │ 15.75 KB │ 6.96 KB  ║
║ vm              │ 495 B    │ 569 B    ║
║ wire-format     │ 1.84 KB  │ 1.35 KB  ║
╚═════════════════╧══════════╧══════════╝

NullVoxPopuli · 2026-02-21T13:02:46Z

Is the problem still present an canary?

Because: #21091

johanrd · 2026-02-21T15:21:50Z

@NullVoxPopuli yes, as I understand it, this needs to be tested in smoke tests to fully emulate the user dev mode?

on main(canary), the smoke tests in this PR #21108 (which run with LOCAL_DEBUG=false, DEBUG=true, matching real user dev mode) confirm this. On main/canary:

  not ok 8 - on modifier | error handling: throws helpful error when callback is missing                                                                                                                                                      
      message: Expected helpful error message, got: Cannot read properties of undefined (reading 'bind')                                                                                                                                      
                                                                                                                                                                                                                                              
  not ok 9 - on modifier | error handling: throws helpful error when event name is missing                                                                                                                                                    
      message: Expected helpful error message, got: Cannot destructure property 'tag' of 'ref' as it is undefined.

PR #21091 improved error messages inside check() and localAssert(), but both are gated by LOCAL_DEBUG (not DEBUG), so they're no-ops in published builds. Users still get unhelpful TypeErrors. The fix in fix/20783 correctly replaces localAssert() with if (DEBUG) { throw ... }.

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

NullVoxPopuli · 2026-02-21T19:05:29Z

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

+
+    if (DEBUG && !arg0) {
+      throw new Error(
+        'You must pass a valid DOM event name as the first argument to the `on` modifier'


Should we say what was received instead? Can we provide even more information in the error? maybe the element tag name? (at least since it's kinda hard right now to find what element a modifier is on rn)

yes, good idea.

Sentry reports something like button.font-bold.hover:bg-white.focus-m-1[type="submit"]

Even better would maybe be to use element.cloneNode(false).outerHTML which would print the whole node as in the dom, like <button type="submit" class="font-bold hover:bg-white" id="save">

It may be too long, but that may not be an issue?

how does sentry get that?

I do think it would b e too long to include the HTML

heh, checked now, Sentry's _htmlElementAsString builds a compact CSS-selector-style representation: button#save.btn.primary[type="submit"]. See their implementation: https://github.com/getsentry/sentry-javascript/blob/b3d336c98ee58f124004debf2e6aca60365d15b4/packages/core/src/utils/browser.ts#L68-L122

It is quite complex, though. element.cloneNode(false).outerHTML is simpler and gives the full opening tag from the DOM, but it can get verbose with data attributes, analytics attributes etc. But is that an issue in dev mode? alternatively substring it.

how are they getting access to the element at all tho? 🤔

Including html is questionable -- you don't want to leak any data from an app into an exception

@kategengler ok, yes. something like this would be safe i guess:

You must pass a valid DOM event name as the first argument to the `on` modifier, but you passed undefined. Usage example: {{on "click" this.handleClick}}. (Element: <button>)

Adding the id and/or the class list would sometimes be helpful, but potentially leaking.

pushed an update now with the tag name

I think the classlist is probably fine. Sentry doesn't hide the classes.

okay, added the id and class list now in css selector / sentry style: button#save.btn.primary

… for on modifier was forget to add emberjs#21045 emberjs#20783

…odifier. + Cleanup redundant test from 5b6f80b

…e? so that way we aren't defining two variables that have the same value?

Show which element the {{on}} modifier is attached to, helping locate the issue when modifiers are forwarded via ...attributes.

… classList, like button#my-id.class1.class2

NullVoxPopuli · 2026-03-07T19:19:36Z

smoke-tests/scenarios/basic-test.ts


              });
            `,
+            'on-modifier-error-test.gjs': `


one of these tests should probably include the selector text, ya?

yes, good point,
I added it as its own test now: 4bfdd86

johanrd force-pushed the fix/20783 branch 2 times, most recently from 674a781 to 680a063 Compare February 21, 2026 12:11

johanrd force-pushed the fix/20783 branch from 680a063 to 6d922cd Compare February 21, 2026 15:18

NullVoxPopuli reviewed Feb 21, 2026

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NullVoxPopuli reviewed Feb 21, 2026

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NullVoxPopuli reviewed Feb 21, 2026

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johanrd added 4 commits March 7, 2026 14:19

[BUGFIX] Regression [Ember 5.12+] Non helpful error when the function…

c179871

… for on modifier was forget to add emberjs#21045 emberjs#20783

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

ccaf140

…odifier. + Cleanup redundant test from 5b6f80b

fix review comment: why not move eventName above the if statement her…

ebe7060

…e? so that way we aren't defining two variables that have the same value?

Include element tag name in callback error message

5cb50b2

Show which element the {{on}} modifier is attached to, helping locate the issue when modifiers are forwarded via ...attributes.

johanrd force-pushed the fix/20783 branch from 72429bf to 5cb50b2 Compare March 7, 2026 13:21

NullVoxPopuli previously approved these changes Mar 7, 2026

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johanrd dismissed NullVoxPopuli’s stale review via a29a483 March 7, 2026 18:01

johanrd force-pushed the fix/20783 branch 3 times, most recently from 788bea1 to dddb309 Compare March 7, 2026 18:09

update to output tag name with css selector style context from id and…

871db22

… classList, like button#my-id.class1.class2

johanrd force-pushed the fix/20783 branch from dddb309 to 871db22 Compare March 7, 2026 18:32

NullVoxPopuli reviewed Mar 7, 2026

View reviewed changes

adding a test for selector being present in errors

4bfdd86

NullVoxPopuli approved these changes Mar 7, 2026

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