fileservice.py

from fastapi import FastAPI, Request, Response

filenames = ["js/poseMaker.js"]
contents = '\n'.join([open(x).read() for x in filenames])

app = FastAPI()

@app.middleware("http")
async def insert_js(request: Request, call_next):
    path = request.scope['path']  # get the request route
    response = await call_next(request)
    
    if path == "/":
        response_body = ""
        async for chunk in response.body_iterator:
            response_body += chunk.decode()

        some_javascript = f"""
    <script type="text/javascript" defer>
{contents}
    </script>
        """

        response_body = response_body.replace("</body>", some_javascript + "</body>")

        del response.headers["content-length"]

        return Response(
            content=response_body,
            status_code=response.status_code, 
            headers=dict(response.headers),
            media_type=response.media_type
        )

    return response