You can add Javascript to a document via
new_script_element = document.createElement("script")
new_script_element.setAttribute("src","https://domain.com/path/to/hello.js")
document.body.append(new_script_element)and to view it, using the developer tools:
scripts = Array.from(document.querySelectorAll("script"))
script[3] // appropriate index
// you can then view it in the console