Build the Converter Update Cycle

Open app.js and add the update() function. This function is the center of the converter: it reads the current state and writes the current state back into the UI.

Start with the selected currencies:

function update() {
  $("fromFlag").textContent = currencies[from][0];
  $("fromCode").textContent = from;
  $("toFlag").textContent = currencies[to][0];
  $("toCode").textContent = to;

Then calculate the result:

  const r = rate(from, to);
  const amount = Number($("amount").value) || 0;
  $("result").value = fmt(amount * r);
  $("rateText").textContent = `1 ${from} = ${fmt(r)} ${to}`;
}

Finish the function by keeping the chart pair label and Popular Currencies in sync with the same state.

Test it

Type a different amount in the Send field. The receive field should update immediately.

Checkpoint

Change both currencies and the amount. The receive value and 1 FROM = RATE TO line should always match the selected pair.