fix(v3): resolve 'Unexpected token <' error from missing custom.js

[leaanthony]

Summary

• Adds explicit 404 response for /wails/custom.js in desktop mode asset middleware, preventing the request from reaching Vite/user handlers
• Adds Content-Type validation in loadOptionalScript to only inject actual JavaScript, not HTML fallback pages
• Rebuilds runtime bundles via esbuild

Root Cause

In desktop mode, no handler existed for /wails/custom.js. Vite's SPA fallback returned index.html with status 200 for the unknown route. loadOptionalScript only checked response.ok, so it injected HTML as JavaScript, causing the Unexpected token '<' parse error.

Test plan

• Verify wails3 dev with Vite frontend no longer shows "Unexpected token '<'" in console
• Verify server mode still loads custom.js correctly (WebSocket events work)
• Verify desktop mode gracefully skips the optional script without errors

Fixes #5068

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Only injects scripts when the response is OK and confirmed JavaScript, preventing HTML or other content from being loaded as scripts.
  • Returns 404 for missing custom runtime script paths to avoid SPA HTML being served as JS in desktop dev mode.
  • Corrected mismapped dialog and screen behaviors so dialogs and screens appear with the intended types.

• Documentation

  • Updated unreleased changelog to record these fixes.

[github-actions]

⚠️ Missing Changelog Update

Hi @leaanthony, please update v3/UNRELEASED_CHANGELOG.md with a description of your changes.

This helps us keep track of changes for the next release.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 6e5fe943-aa68-4b3c-b6d2-2840d7e47ae1

📥 Commits

Reviewing files that changed from the base of the PR and between 052168e and 722ee9a.

📒 Files selected for processing (5)

• v3/UNRELEASED_CHANGELOG.md
• v3/internal/assetserver/bundledassets/runtime.debug.js
• v3/internal/assetserver/bundledassets/runtime.js
• v3/internal/runtime/desktop/@wailsio/runtime/src/index.ts
• v3/pkg/application/application.go

✅ Files skipped from review due to trivial changes (1)

• v3/UNRELEASED_CHANGELOG.md

🚧 Files skipped from review as they are similar to previous changes (2)

• v3/pkg/application/application.go
• v3/internal/runtime/desktop/@wailsio/runtime/src/index.ts

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Walkthrough

Verifies HEAD responses' case-insensitive Content-Type contains "javascript" before injecting optional scripts, and adds an explicit 404 response for /wails/custom.js. Also updates minified export symbol wiring in bundled runtime.

Changes

Cohort / File(s)	Summary
Runtime (source) v3/internal/runtime/desktop/@wailsio/runtime/src/index.ts	loadOptionalScript(url) now requires response.ok AND a case-insensitive Content-Type containing "javascript" before inserting a <script>; retains silent error handling.
Runtime (bundled/minified) v3/internal/assetserver/bundledassets/runtime.js	Mirrors the loadOptionalScript Content-Type check. Updated minified export wiring: Screens now maps Ne → je; Dialogs.Info now maps Nn → jn; Dialogs.Warning now maps jn → Nn.
Application router v3/pkg/application/application.go	Added explicit branch to respond with http.NotFound() for requests to "/wails/custom.js" instead of letting normal asset routing handle it.
Changelog v3/UNRELEASED_CHANGELOG.md	Added entry documenting the fix: explicit 404 for /wails/custom.js and case-insensitive Content-Type validation in loadOptionalScript.

Sequence Diagram(s)

sequenceDiagram
    participant Client as Client Runtime
    participant DevServer as Dev/Asset Server
    participant App as App HTTP Handler

    Client->>DevServer: HEAD /wails/custom.js
    alt DevServer 200 and Content-Type contains "javascript"
        DevServer-->>Client: 200, Content-Type: application/javascript
        Client->>Client: insert <script src="/wails/custom.js">
    else DevServer 200 with non-JS or 404
        DevServer->>App: (if routed) GET /wails/custom.js
        App-->>DevServer: 404 Not Found
        DevServer-->>Client: 404 or 200 with non-JS
        Client->>Client: do not inject script
    end

Loading

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Suggested labels

Bug, runtime, v3-alpha, Documentation, size:L

Poem

🐰 I sniff the headers, ears held high,
If "javascript" winks, I give a try.
If not, I hop away, no broken view—
A tidy patch, a safer chew.
Happy hops for code anew 🥕✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 1.02% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely summarizes the main change: fixing the 'Unexpected token <' error caused by missing custom.js.
Description check	✅ Passed	The description provides a clear summary, root cause analysis, and test plan, though it does not formally check all template boxes or reference the changelog update checklist.
Linked Issues check	✅ Passed	The PR addresses all coding requirements from #5068: explicit 404 for /wails/custom.js, Content-Type validation in loadOptionalScript, and preserves server-mode behavior.
Out of Scope Changes check	✅ Passed	All changes (Go middleware, TypeScript runtime logic, runtime bundles, and changelog) are directly related to fixing the custom.js issue and contain no out-of-scope modifications.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch v3-alpha-bugfix/issue-5068-custom-js

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[Copilot]

Pull request overview

This PR addresses the Unexpected token '<' runtime error caused by /wails/custom.js being missing in desktop dev mode (and being served as an HTML SPA fallback), by preventing the HTML fallback from being injected as JavaScript.

Changes:

• Adds an explicit 404 handler for /wails/custom.js in the application asset middleware so the request doesn’t reach Vite/user SPA fallback handlers in desktop mode.
• Hardens loadOptionalScript() to only inject the script when the Content-Type indicates JavaScript.
• Rebuilds the production runtime bundle.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
v3/pkg/application/application.go	Intercepts /wails/custom.js and returns 404 to avoid SPA fallbacks being treated as JS.
v3/internal/runtime/desktop/@wailsio/runtime/src/index.ts	Adds Content-Type validation before injecting an “optional” script.
v3/internal/assetserver/bundledassets/runtime.js	Updates the bundled production runtime output to include the new optional-script behavior.

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