Fix launch JavaScript function execution

[dawoodkhan82]

Description

Restore the Gradio 5 behavior for function strings passed to Blocks.launch(js=...). The Gradio 6 frontend inserted the function expression into a script element but never called it. This adds a core executor that invokes sync and async function expressions while preserving raw JavaScript support, and runs it after the Blocks component tree is ready so Gradio's own page-title update cannot overwrite user code.

Before / after Spaces

• Before fix
• After fix

Closes: #13636

AI Disclosure

• I used AI (Codex) to reproduce the regression, draft the implementation and tests, and prepare this PR. I reviewed every changed line.
• I did not use AI

🎯 PRs Should Target Issues

This PR targets #13636. No overlapping open PR was found before submission.

Testing and Formatting Your Code

• TEST_MODE=happy-dom pnpm exec vitest run js/core/src/custom_js.test.ts --config .config/vitest.config.ts --browser.enabled=false
• pnpm --filter @self/app build --emptyOutDir
• pnpm --filter @self/spa build --emptyOutDir
• One-off headless browser verification against the local reproduction: final document.title is LAUNCH-JS-RAN with no page errors (no Playwright test was added)
• bash scripts/format_frontend.sh (formatting completed; the repository-wide type check reports 15 unrelated existing errors in js/utils, js/markdown-code, and js/preview)

Minimal reproduction (the local file is untracked and was not committed):

import gradio as gr

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

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

[gradio-pr-bot]

🪼 branch checks and previews

•	Name	Status	URL
	Spaces	ready!	Spaces preview
	Storybook	ready!	Storybook preview
🦄	Changes	detected!	Details

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Install Gradio from this PR

pip install https://huggingface.co/buckets/gradio/pypi-previews/resolve/4acd2f60d9fd674323a7177e8774a75dfb0ea338/gradio-6.20.0-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@4acd2f60d9fd674323a7177e8774a75dfb0ea338#subdirectory=client/python"

Import Gradio JS Client from this PR via CDN

import { Client } from "https://huggingface.co/buckets/gradio/npm-previews/resolve/4acd2f60d9fd674323a7177e8774a75dfb0ea338/browser.js";

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
@gradio/core	patch
@self/app	patch
@self/spa	patch
gradio	patch

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• Execute function strings passed to Blocks.launch(js=...)

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Something isn't right?

• Maintainers can change the version label to modify the version bump.
• If the bot has failed to detect any changes, or if this pull request needs to update multiple packages to different versions or requires a more comprehensive changelog entry, maintainers can update the changelog file directly.

[Copilot]

Pull request overview

Restore Gradio 5-era behavior where JavaScript passed via Blocks.launch(js=...) is actually executed on the client. The previous Gradio 6 approach injected the string into a <script> element without invoking function expressions, resulting in a silent no-op for common () => { ... } usage.

Changes:

• Move config.js execution responsibility from SPA/App shells into @gradio/core’s Blocks lifecycle (after app_tree.ready).
• Add a shared executor (execute_custom_js) that supports function expressions (sync/async) and statement-style JS.
• Add unit tests and a changeset bump for affected packages.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
js/spa/src/Index.svelte	Removes direct <script> injection so launch(js=...) runs via core Blocks instead.
js/app/src/routes/[...catchall]/+page.svelte	Removes duplicate custom-JS injection in the SvelteKit app route.
js/core/src/custom_js.ts	Introduces execute_custom_js() to evaluate and invoke function-expression JS (with raw-JS fallback).
js/core/src/custom_js.test.ts	Adds Vitest coverage for function-expression, async function-expression, and raw JS execution.
js/core/src/Blocks.svelte	Executes js after app_tree.ready, with error logging, before setting ready and dispatching load events.
.changeset/clean-dogs-launch.md	Publishes patch releases for impacted packages to ship the regression fix.

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