[BUGFIX beta] Fix 'on' modifier error message regression

[crazylogic03]

This PR fixes a regression where the helpful error message for the 'on' modifier was lost when the callback is undefined or not a function.

It adds an explicit runtime check in DEBUG mode to ensure the callback is a function, and includes a reproduction test case to verify the fix.

[crazylogic03]

I have implemented a fix to restore the helpful error message when the

on
modifier is used without a function callback.

[github-actions]

Estimated Asset Sizes

Diff

--- main/out.txt	2026-02-13 16:38:38.000000000 +0000
+++ pr/./pr-22039263969/out.txt	2026-02-15 16:43:00.000000000 +0000
@@ -1,16 +1,16 @@
 ╔═══════╤═══════════╤═══════════╗
 ║       │ Min       │ Gzip      ║
 ╟───────┼───────────┼───────────╢
-║ Total │ 352.02 KB │ 203.77 KB ║
+║ Total │ 352.02 KB │ 203.83 KB ║
 ╚═══════╧═══════════╧═══════════╝
 
 ╔══════════════════════╤═══════════╤═══════════╗
 ║ @ember/*             │ Min       │ Gzip      ║
 ╟──────────────────────┼───────────┼───────────╢
-║ Total                │ 313.42 KB │ 181.95 KB ║
+║ Total                │ 313.42 KB │ 181.89 KB ║
 ╟──────────────────────┼───────────┼───────────╢
-║ -internals           │ 36.65 KB  │ 26.23 KB  ║
-║ application          │ 13.23 KB  │ 8.09 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   ║
@@ -19,16 +19,16 @@
 ║ deprecated-features  │ 31 B      │ 77 B      ║
 ║ destroyable          │ 561 B     │ 383 B     ║
 ║ enumerable           │ 259 B     │ 387 B     ║
-║ helper               │ 1.08 KB   │ 803 B     ║
+║ helper               │ 1.08 KB   │ 811 B     ║
 ║ instrumentation      │ 2.43 KB   │ 1.79 KB   ║
-║ modifier             │ 1.22 KB   │ 966 B     ║
+║ modifier             │ 1.22 KB   │ 965 B     ║
 ║ object               │ 35.94 KB  │ 22.16 KB  ║
 ║ owner                │ 159 B     │ 178 B     ║
-║ renderer             │ 630 B     │ 492 B     ║
-║ routing              │ 59.33 KB  │ 34.13 KB  ║
+║ renderer             │ 630 B     │ 487 B     ║
+║ routing              │ 59.33 KB  │ 34.1 KB   ║
 ║ runloop              │ 2.36 KB   │ 1.5 KB    ║
 ║ service              │ 1 KB      │ 845 B     ║
-║ template             │ 654 B     │ 519 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     ║
@@ -40,19 +40,19 @@
 ╔═════════════════╤══════════╤══════════╗
 ║ @glimmer/*      │ Min      │ Gzip     ║
 ╟─────────────────┼──────────┼──────────╢
-║ Total           │ 38.6 KB  │ 21.82 KB ║
+║ 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    │ 245 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 │ 5.22 KB  ║
+║ 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  ║

Details

This PR

main

╔═══════╤═══════════╤═══════════╗ ║ │ Min │ Gzip ║ ╟───────┼───────────┼───────────╢ ║ Total │ 352.02 KB │ 203.83 KB ║ ╚═══════╧═══════════╧═══════════╝ ╔══════════════════════╤═══════════╤═══════════╗ ║ @ember/* │ Min │ Gzip ║ ╟──────────────────────┼───────────┼───────────╢ ║ Total │ 313.42 KB │ 181.89 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 │ 59.33 KB │ 34.1 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 ║ ╚═════════════════╧══════════╧══════════╝

╔═══════╤═══════════╤═══════════╗ ║ │ Min │ Gzip ║ ╟───────┼───────────┼───────────╢ ║ Total │ 352.02 KB │ 203.77 KB ║ ╚═══════╧═══════════╧═══════════╝ ╔══════════════════════╤═══════════╤═══════════╗ ║ @ember/* │ Min │ Gzip ║ ╟──────────────────────┼───────────┼───────────╢ ║ Total │ 313.42 KB │ 181.95 KB ║ ╟──────────────────────┼───────────┼───────────╢ ║ -internals │ 36.65 KB │ 26.23 KB ║ ║ application │ 13.23 KB │ 8.09 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 │ 803 B ║ ║ instrumentation │ 2.43 KB │ 1.79 KB ║ ║ modifier │ 1.22 KB │ 966 B ║ ║ object │ 35.94 KB │ 22.16 KB ║ ║ owner │ 159 B │ 178 B ║ ║ renderer │ 630 B │ 492 B ║ ║ routing │ 59.33 KB │ 34.13 KB ║ ║ runloop │ 2.36 KB │ 1.5 KB ║ ║ service │ 1 KB │ 845 B ║ ║ template │ 654 B │ 519 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 │ 21.82 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 │ 245 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 │ 5.22 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]

@crazylogic03 looks like tests don't pass

[NullVoxPopuli]

@crazylogic03 can you show a before and after of what you see in the browser after making this change? (for the changelog, it would be helpful to know what the specific problem being solved is) tyty!

[crazylogic03]

@NullVoxPopuli
Specific Problem:
In recent versions (since 5.12), passing a non-function (like undefined or null) to the on modifier resulted in unhelpful or generic errors, making it hard to track down where the invalid value came from.

Before:
The validation was either missing or generic assertion failures that didn't help pinpoint the issue.

After:
The modifier now explicitly checks the callback type in DEBUG mode and throws a descriptive error including the type received and the debug label of the argument.
Example Error:
You must pass a function as the second argument to the 'on' modifier; you passed undefined. While rendering: ...

[NullVoxPopuli]

@crazylogic03 legit thanks for looking in to this! new error phrasing looks good. what was the old error?

[NullVoxPopuli]

Looks like on 6.10, we get "Cannot access bind of undefined" (not good lol)

[crazylogic03]

@NullVoxPopuli Yep, exactly!
The old error Cannot read properties of undefined (reading 'bind') happened because we were trying to .bind() the callback for debug context before actually verifying it was a function.
This PR adds an explicit check before that point, so instead of crashing with a native JS error, we now get a helpful Ember assertion telling you exactly what argument was passed (e.g. undefined or null) and where.

[crazylogic03]

@NullVoxPopuli Can You Review merge this PR and All the Checks have passed

[crazylogic03]

Thanks for the review @NullVoxPopuli! I'm glad we could get this fixed.

[crazylogic03]

@kategengler Can You Review merge this PR and All the Checks have passed 🙂