Promises

1// Creating a typed Promise
2function fetchUser(id: number): Promise<{ id: number; name: string }> {
3  return new Promise((resolve, reject) => {
4    setTimeout(() => {
5      if (id > 0) {
6        resolve({ id, name: `User ${id}` });
7      } else {
8        reject(new Error("Invalid user ID"));
9      }
10    }, 100);
11  });
12}
13 
14// Promise chaining
15fetchUser(1)
16  .then((user) => {
17    console.log(`Found: ${user.name}`);
18    return user.id;
19  })
20  .then((id) => {
21    console.log(`User ID: ${id}`);
22  })
23  .catch((error: Error) => {
24    console.error(`Error: ${error.message}`);
25  });
26 
27// Promise.all - run promises in parallel
28async function fetchAllUsers(ids: number[]): Promise<{ id: number; name: string }[]> {
29  const promises = ids.map((id) => fetchUser(id));
30  return Promise.all(promises);
31}
32 
33// Promise.race - first to complete wins
34function timeout<T>(promise: Promise<T>, ms: number): Promise<T> {
35  const timeoutPromise = new Promise<never>((_, reject) => {
36    setTimeout(() => reject(new Error("Timeout")), ms);
37  });
38  return Promise.race([promise, timeoutPromise]);
39}
40 
41// Promise.allSettled - wait for all, regardless of success/failure
42async function fetchWithStatus(ids: number[]) {
43  const results = await Promise.allSettled(ids.map((id) => fetchUser(id)));
44 
45  results.forEach((result, index) => {
46    if (result.status === "fulfilled") {
47      console.log(`User ${ids[index]}: ${result.value.name}`);
48    } else {
49      console.log(`User ${ids[index]} failed: ${result.reason}`);
50    }
51  });
52}
53 
54fetchWithStatus([1, 2, -1, 3]);