Normalize & Zero.

Theory. Field data is raw: " 40 " has spaces, "" is empty, "0" is a real zero. Three different cases — and the most common bug is mixing them up.

Normalize text: const t = el.value.trim(). Then: if t === "" — the value is missing (show "empty"). Otherwise const n = Number(t) — a real number, and 0 is perfectly valid.

Traps: - Number("") is 0 — an empty field looks like zero! - if (!t) works for "", but if (!n) also rejects a real 0. - Number("abc") is NaN — check with Number.isNaN(n).

Small example:

function describe(el) { const t = el.value.trim(); if (t === "") return "empty"; return String(Number(t)); }

Common mistake: empty = 0; 0 = empty; forgotten trim.