Conversion.

Sixth class. Forms and notes often give text "50", but math needs the number 50.

Number("50") → 50. Number("99.5") → 99.5. If the string is not a valid number, you get NaN (Not a Number) — an error signal.

After conversion, typeof amount should be "number". Then amount + 10 adds instead of concatenating.

Mini example:

const raw = "12"; console.log(Number(raw));

Common mistake: Number("50 EUR") → NaN because of letters. Start with a clean digit-only string.