Validation.

Validation checks input before acting. If the value is bad — show a message in #err; if good — clear the error and continue.

Typical: Number(input.value) < 1 → "too small". Empty input is also a problem.

Mini example:

document.querySelector("#check").addEventListener("click", () => { const n = Number(document.querySelector("#amount").value); const err = document.querySelector("#err"); if (n < 1) err.textContent = "too small"; else err.textContent = ""; });

Common mistake: validating only in your head without writing to the DOM; or comparing a string to a number without Number.