typescript · level 2 check

a weekend weather planner.

Time to build something useful: a weekend planner. It asks which town you are in, fetches that town's forecast from Gwecode's practice server, and suggests a plan for every day. Sun means a picnic, cloud a bike ride, and rain board games at home.

Each day in the forecast has a name, a sky, a high temperature and sometimes a wind speed. Servers are not always kind, so trust nothing. `readDay` validates every day and gives `null` for broken ones, which the planner skips and counts. Wind of 30 or more earns a warning.

The practice server may not know the town, or may send something that is not a list. Throw a clear error for both, and let one `catch` explain it. If nobody types a town, say so before asking the server. Fill in each `____`, then run it and type a town.

Your program

type Sky = "sun" | "cloud" | ____;

interface Day {
  readonly name: string;
  sky: Sky;
  high: number;
  wind____: number;
}

const ideas: ____<Sky, string> = {
  sun: "picnic in the park",
  cloud: "bike ride by the river",
  rain: "board games at home",
};

function readSky(value: unknown): Sky | null {
  if (value === "sun" || value === "cloud" || value === "rain") return value;
  return null;
}

function readDay(value: unknown): Day | null {
  if (typeof value !== "object" || value === null) return null;
  if (!("name" in value) || typeof value.name !== "string") return null;
  if (!("high" in value) || typeof value.high !== "____") return null;
  if (!("sky" in value)) return null;
  const sky = readSky(value.sky);
  if (sky === null) return null;
  const day: Day = { name: value.name, sky: sky, high: value.high };
  if ("wind" in value && typeof value.wind === "number") day.wind = value.wind;
  return day;
}

async function getForecast(town: string): Promise<unknown[]> {
  const reply = await fetch(`https://api.example.com/forecast/${town}`);
  if (!reply.____) throw new Error(`No forecast for ${town} (${reply.status})`);
  const data: unknown = await reply.json();
  if (!Array.____(data)) throw new Error("The forecast was not a list");
  return data;
}

function describe(day: Day): string {
  let line = `${day.name}: ${day.high}°C, ${ideas[day.sky]}`;
  if ((day.wind ____ 0) >= 30) line = line + ". Hold on to your hat!";
  return line;
}

function best<T>(items: T[], score: (item: T) => number): T {
  let top = items[0];
  for (const item of items) {
    if (score(item) > score(top)) top = item;
  }
  return top;
}

async function main(): Promise<void> {
  const town = (prompt("Which town?") ?? "").trim().toLowerCase();
  if (town === "") {
    console.log("Please type the name of a town.");
    return;
  }
  try {
    const items = ____ getForecast(town);
    const days: Day[] = [];
    for (const item of items) {
      const day = readDay(item);
      if (day !== null) days.____(day);
    }
    console.log(`Weekend plan for ${town}`);
    for (const day of days) {
      console.log(describe(day));
    }
    if (days.length > 0) console.log(`Warmest day: ${best(days, (day) => day.high).name}`);
    if (items.length > days.length) console.log(`Days skipped: ${items.length - days.length}`);
  } catch (err) {
    if (err ____ Error) console.log(`Sorry! ${err.message}`);
  }
}

main();

Fill in every ____ then run the checks.

1 It plans every day in the forecastuses: fetch
2 It tidies the town before asking the serveruses: text tricks
3 It skips days with broken datauses: validating
4 It explains when a town has no forecastuses: errors
5 It refuses a reply that is not a listuses: unknown
6 It asks for a town when none is givenuses: null and ??

Take as long as you like and try as many times as you want.

a weekend weather planner.

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