[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 ... }.

NullVoxPopuli · 2026-03-26T15:52:45Z

@johanrd I forgot to merge this :( any chance you'd be willing to rebase, and one more time show the error message(s) before and after?

… 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

johanrd · 2026-03-26T16:39:58Z

@NullVoxPopuli Rebased now. See current CI for passing tests, and here for failing tests without the fix, e.g:

not ok 9 Chrome 145.0 - [36 ms] - on modifier | error handling: throws helpful error when callback is missing
        ---
            actual: >
                false
            expected: >
                true
            stack: >
                    at http://localhost:7357/assets/tests-DSd2Dm9s.js:10690:18
            message: >
                Expected helpful error message, got: Cannot read properties of undefined (reading 'bind')
            negative: >
                false
            browser log: |
        ...
    not ok 10 Chrome 145.0 - [52 ms] - on modifier | error handling: throws helpful error when event name is missing
        ---
            actual: >
                false
            expected: >
                true
            stack: >
                    at http://localhost:7357/assets/tests-DSd2Dm9s.js:10710:18
            message: >
                Expected helpful error message, got: Cannot destructure property 'tag' of 'ref' as it is undefined.
            negative: >
                false
            browser log: |
        ...
    not ok 11 Chrome 145.0 - [35 ms] - on modifier | error handling: error message includes element selector
        ---
            actual: >
                false
            expected: >
                true
            stack: >
                    at http://localhost:7357/assets/tests-DSd2Dm9s.js:10730:18
            message: >
                Expected element selector in error, got: Cannot read properties of undefined (reading 'bind')
            negative: >
                false
            browser log: |
        ...

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

NullVoxPopuli reviewed Feb 21, 2026

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Comment thread packages/@glimmer/runtime/lib/modifiers/on.ts Outdated

NullVoxPopuli reviewed Feb 21, 2026

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Comment thread packages/@glimmer/runtime/lib/modifiers/on.ts Outdated

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 4 times, most recently from dddb309 to 871db22 Compare March 7, 2026 18:32

NullVoxPopuli reviewed Mar 7, 2026

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Comment thread smoke-tests/scenarios/basic-test.ts

NullVoxPopuli previously approved these changes Mar 7, 2026

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johanrd added 6 commits March 26, 2026 17:09

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

b468185

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

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

53ece6a

…odifier. + Cleanup redundant test from 5b6f80b

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

df17c40

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

Include element tag name in callback error message

d477ac6

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

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

5818cc5

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

adding a test for selector being present in errors

8847937

johanrd dismissed NullVoxPopuli’s stale review via 8847937 March 26, 2026 16:09

johanrd force-pushed the fix/20783 branch from 4bfdd86 to 8847937 Compare March 26, 2026 16:09

NullVoxPopuli approved these changes Mar 26, 2026

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NullVoxPopuli merged commit f9c4357 into emberjs:main Mar 26, 2026
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[BUGFIX] Regression [Ember 5.12+] Non helpful error when the function for on modifier was forgot as param#21108

[BUGFIX] Regression [Ember 5.12+] Non helpful error when the function for on modifier was forgot as param#21108
NullVoxPopuli merged 6 commits intoemberjs:mainfrom
johanrd:fix/20783

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