Inject JS script into ASGI application

main.py

from __future__ import annotations

from starlette.datastructures import MutableHeaders

BROWSER_SYNC_SCRIPT =b"<script>console.log('Hi there!');</script>"

class InjectScriptMiddleware:
    def __init__(self, app, script: bytes = BROWSER_SYNC_SCRIPT):
        self.app = app
        self.script = script

    async def __call__(self, scope, receive, send):
        if scope["type"] != "http":
            return await self.app(scope, receive, send)

        inject_script = False

        async def send_wrapper(message):
            nonlocal inject_script
            if message["type"] == "http.response.start":
                headers = MutableHeaders(raw=message.get("headers", []))
                if headers.get("content-type", "").startswith("text/html"):
                    inject_script = True
                    headers["content-length"] = str(int(headers["content-length"]) + len(self.script))
            elif message["type"] == "http.response.body":
                if not message.get("more_body", False) and inject_script:
                    message["body"] += self.script
            await send(message)

        return await self.app(scope, receive, send_wrapper)