src/custom_js.py

import gradio

# Adapted from https://github.com/gradio-app/gradio/discussions/2932

class JavaScriptLoader:
    def __init__(self, target):
        #Copy the template response
        self.original_template = gradio.routes.templates.TemplateResponse
        #Prep the js files
        self.load_js(target)
        #reassign the template response to your method, so gradio calls your method instead
        gradio.routes.templates.TemplateResponse = self.template_response

    def load_js(self, target):
        with open(target, 'r', encoding="utf-8") as file:
                self.loaded_script = f"<script>\n{file.read()}\n</script>"

    def template_response(self, *args, **kwargs):
        """Once gradio calls your method, you call the original, you modify it to include
        your scripts and you return the modified version
        """
        response = self.original_template(*args, **kwargs)
        response.body = response.body.replace(
            '</head>'.encode('utf-8'), self.loaded_script + "\n</head>".encode("utf-8")
        )
        response.init_headers()
        return response