Input Value.

For <input>, the value is .value (a string!). Number(value) makes a number for math. Empty string → Number("") is 0 — careful with validation later.

Typical pattern: read value inside a click listener, compute, write the result to another element.

Mini example:

const amount = document.querySelector("#amount"); const out = document.querySelector("#out"); document.querySelector("#run").addEventListener("click", () => { out.textContent = String(Number(amount.value) * 2); });

Common mistake: adding strings ("1"+"2"="12") instead of Number; or reading textContent on an input.