Web Page Field Autofill Script

I need a small, self-contained script that automatically populates every input on a specific web page as soon as it loads. The page contains regular text boxes, dropdown menus, checkboxes, numeric inputs and a date picker, and each of those must be filled without any manual clicks.

All data will come straight from the user—either hard-coded variables at the top of the file or a simple prompt/local-storage read is fine, as long as no external API or database is involved. Once the values are set, the script should:

• Detect the page’s load event (plain JavaScript preferred; lightweight libraries like jQuery acceptable if really useful).
• Locate each target field reliably with selectors that will survive minor layout changes.
• Inject the predefined values immediately so the form appears fully completed the moment the page is visible.
• Leave the page otherwise untouched so validation or submission continues working normally.

Deliverable: a single, commented .js file plus a quick read-me explaining how to edit the default values and attach the script (userscript header, browser extension injection, or simple <script> tag—whichever you recommend). No server code or backend integration is required.