Searching Without for...else

Searching Without for…else

In this lesson, you ported the night-bus report’s search for the first canceled trip from Python to JavaScript.

• Python’s for...else (and while...else) runs the else only when the loop ends without break. JavaScript loops have no else: written after a loop’s closing brace, it’s a SyntaxError, and JavaScript rejects the whole file before running a line. In JavaScript, else belongs only to if.
• When a loop only searches, JavaScript developers use .find(). It calls a function on each item in turn and returns the first item the function answers true for. The function is usually an arrow function, like (trip) => trip.canceled, much as you’d pass lambda trip: trip["canceled"] in Python.
• When nothing matches, .find() returns undefined, JavaScript’s empty value. It doesn’t throw an error, so you check for it with an ordinary if...else. Python’s closest match is next((trip for trip in trips if trip["canceled"]), None), which hands back the default you give it.
• When the code after a loop needs to know whether the loop found something, keep a found flag, as you would in Python. !found is JavaScript’s not found.
• break leaves only the loop it’s in, as in Python. To leave nested loops, the everyday answer in both languages is a function that returns from inside both loops. Many JavaScript codebases prefer it. When nothing matches, the function ends without reaching return, and a JavaScript function with no return gives back undefined, as Python’s gives back None.
• JavaScript can also put a label, a name and a colon, in front of the outer loop, and break with that name from the inner loop. Labels are rare, and a label can’t be reached from inside a callback function, only from the loops themselves.

The report’s search, with .find():

const trips = [
  { departs: "23:10", canceled: false },
  { departs: "23:40", canceled: false },
  { departs: "00:10", canceled: true },
  { departs: "00:40", canceled: false },
];
const first = trips.find((trip) => trip.canceled);
if (first !== undefined) {
  console.log(`Canceled: ${first.departs}`);
} else {
  console.log("No canceled trips");
}