Knowing What an Arrow Returns

Knowing What an Arrow Returns

In this lesson, you grew a one-line arrow into a body and saw why an arrow with braces needs return.

• The translator’s formatters are one-line arrows. She wants each email amount rounded to the whole euro first, as a named step that reads clearly, so forEmail needs a rounding line. Today’s fees are whole euros already, so the quote lines shouldn’t change.
• In Python, for_email() gains rounded = round(amount), and the return line now formats rounded. The script’s two lines print as before.
• In JavaScript, the rounding line is a statement, and only a body can hold statements, so the arrow gets braces. It uses Math.round() in place of Python’s round(). On an exact half they differ: Python’s round() goes to the even whole number, and Math.round() goes up, to the larger number.
• The format line moves into the body, now formatting rounded, still with no return. The Bauer Legal line then reads Bauer Legal: undefined, 2 working days, and nothing complains, so the email would go out to the client like that. The Nordwind line, from the one-line forSheet, prints as before.
• The format line still runs: it works out the euro text, then the text is thrown away, because nothing hands it back to the caller.
• The braced body ends without a return, so the call gives undefined, where a Python def would give None, and the template literal turns that undefined into text.
• Python splits the two forms the same way: a lambda returns its one expression with no return keyword, like an arrow without braces, and a def’s body always needs its return line, like an arrow with braces. Where the comparison stops: a lambda can’t grow a body, but an arrow can, so nothing reminds you to add a return when it does.
• One fix is return euroFormat.format(rounded);. The other keeps the arrow one expression, with no braces: const forEmail = (amount) => euroFormat.format(Math.round(amount));.
• Watch out when an arrow returns an object, like a fresh empty tally for each day’s quotes, which Python’s lambda: {"total": 0} returns as a dict. Without parentheses, () => { total: 0 } returns undefined, because JavaScript reads the braces as a body. () => ({ total: 0 }) returns the object, because the parentheses make JavaScript read the braces as a value.

Here’s the braced forEmail without its return, the version that returns undefined:

const forEmail = (amount) => {
  const rounded = Math.round(amount);
  euroFormat.format(rounded);
};