Blocks.launch(js=...) is carried in the config but never executed in Gradio 6 (silent no-op; demo.load(js=...) works)

[timebeing92]

Describe the bug

Blocks.launch(js=...) accepts a JavaScript function string and serializes it into the app config (GET /config returns it under the js key), but the 6.x frontend never evaluates it — the function silently does not run.

The same function string passed to demo.load(js=...) executes normally, so the string itself is valid; only the launch(js=...) channel is dead. Since the parameter is still accepted and documented, apps migrating from Gradio 4/5 lose their page-setup scripts silently — no error, no warning.

Expected: either the config js is executed on page load (as in Gradio 4/5), or launch(js=...)/Blocks(js=...) raises a deprecation error pointing at a working channel.

Have you searched existing issues? 🔎

• I have searched and found no existing issues (Custom js functionality is broken #11644 was a non-function string, closed as user error; example of adding custom js from Gradio docs is not working  #10344 was a docs-example issue — both pre-6.x)

Reproduction

import gradio as gr

with gr.Blocks() as demo:
    gr.Markdown("launch(js=) repro")

demo.launch(js="() => { document.title = 'LAUNCH-JS-RAN'; }")

1. curl http://127.0.0.1:7860/config | jq .js → "() => { document.title = 'LAUNCH-JS-RAN'; }" — the config carries the function.

2. Open the app; wait for full load. document.title remains "Gradio". The function never runs (verified with Playwright; no console errors).

3. Control: change the last line to

   demo.load(None, None, None, js="() => { document.title = 'LAUNCH-JS-RAN'; }")
   demo.launch()

   and the title changes — the identical string executes via the load event.

Screenshot

Not applicable.

Logs

No console errors — the config value is simply never evaluated.

System Info

Gradio version: 6.20.0
gradio_client version: 2.5.0
Python: 3.12.7
OS: macOS 26.3.1 (arm64)
Browser: Chromium (Playwright) and Chrome

Severity

I can work around it (moved the script to demo.load(js=...)), but the silent failure cost real debugging time and will bite anyone migrating 4/5-era apps.
\n\n## Before / after Spaces\n\n- Before fix\n- After fix — draft PR #13642

Before / after Spaces

• Before fix
• After fix — draft PR #13642