Selenium: How to Inject/execute a Javascript in to a Page before loading/executing any other scripts of the page?

If you want to inject something into the html of a page before it gets parsed and executed by the browser I would suggest that you use a proxy such as Mitmproxy.


Since version 1.0.9, selenium-wire has gained the functionality to modify responses to requests. Below is an example of this functionality to inject a script into a page before it reaches a webbrowser. 

import os
from seleniumwire import webdriver
from gzip import compress, decompress
from urllib.parse import urlparse

from lxml import html
from lxml.etree import ParserError
from lxml.html import builder

script_elem_to_inject = builder.SCRIPT('alert("injected")')

def inject(req, req_body, res, res_body):
    # various checks to make sure we're only injecting the script on appropriate responses
    # we check that the content type is HTML, that the status code is 200, and that the encoding is gzip
    if res.headers.get_content_subtype() != 'html' or res.status != 200 or res.getheader('Content-Encoding') != 'gzip':
        return None
    try:
        parsed_html = html.fromstring(decompress(res_body))
    except ParserError:
        return None
    try:
        parsed_html.head.insert(0, script_elem_to_inject)
    except IndexError: # no head element
        return None
    return compress(html.tostring(parsed_html))

drv = webdriver.Firefox(seleniumwire_options={'custom_response_handler': inject})
drv.header_overrides = {'Accept-Encoding': 'gzip'} # ensure we only get gzip encoded responses

Another way in general to control a browser remotely and be able to inject a script before the pages content loads would be to use a library based on a separate protocol entirely, eg: DevTools Protocol. A Python implementation is available here: https://github.com/pyppeteer/pyppeteer2 (Disclaimer: I'm one of the main authors)


If you cannot modify the page content, you may use a proxy, or use a content script in an extension installed in your browser. Doing it within selenium you would write some code that injects the script as one of the children of an existing element, but you won't be able to have it run before the page is loaded (when your driver's get() call returns.)

String name = (String) ((JavascriptExecutor) driver).executeScript(
    "(function () { ... })();" ...

The documentation leaves unspecified the moment at which the code would start executing. You would want it to before the DOM starts loading so that guarantee might only be satisfiable with the proxy or extension content script route.

If you can instrument your page with a minimal harness, you may detect the presence of a special url query parameter and load additional content, but you need to do so using an inline script. Pseudocode:

 <html>
    <head>
       <script type="text/javascript">
       (function () {
       if (location && location.href && location.href.indexOf("SELENIUM_TEST") >= 0) {
          var injectScript = document.createElement("script");
          injectScript.setAttribute("type", "text/javascript");

          //another option is to perform a synchronous XHR and inject via innerText.
          injectScript.setAttribute("src", URL_OF_EXTRA_SCRIPT);
          document.documentElement.appendChild(injectScript);

          //optional. cleaner to remove. it has already been loaded at this point.
          document.documentElement.removeChild(injectScript);
       }
       })();
       </script>
    ...

Tags:

Python

Javascript

Selenium

Javascript Injection

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