How do I check if the key pressed on a form field is a digit (0 - 9)?

See these:

Use event.key to get the actual value. To check if integer, just use isFinite

input.addEventListener("keydown", function(event) {
    const isNumber = isFinite(event.key);
});

Other option:

const isNumber = /^[0-9]$/i.test(event.key)

An easier HTML solution would be to use the number type input. It restricts to only numbers (kind of).

<input type="number">

Either way, you should clean all user input with:

string.replace(/[^0-9]/g,'');

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