Node.js: How to write input values of a formular into a JSON file when the submit button is pressed?

I'm programming a website with different product pages which all have a formular and a submit button to write comments. When a user fills the input fields and presses the submit utton, the input values should be append to a JSON file and the website should update immediately to see the new entry on the page. Each page has its own JSON file. I'm working with node.js. Currently, the system is able to read from the JSON file, but I have problems with the writing. How can I do that? Should I write this into the app.js or into the specific js of the page?

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