Automatically Generating Extra Form Fields

physicsfag here looking to expand my horizons by building a web app. I've seen on some sites forms that will automatically generate an extra field when you fill them up, so that there's is always one more at the bottom. This probably isn't descriptive enough.

To be more exact, I need to get some input from text input form fields from my users. So I want to start with just one text field. Once a user types something in it, another one is generated underneath it.

If anyone could just tell me what this process is called, some Google search terms, or even what language would do this, that would be very helpful.

Thanks

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