JavaScriptLesson 15 of 16

Promises and async/await

A server, a file or a timer never answers instantly, and a program should not freeze while it waits. A promise stands for a value that arrives a bit later, and await lets you wait for it as if the code ran in order.

Key points

new Promise((resolve, reject) => …): resolve hands over the result, reject an error
An async function always returns a promise, and await inside it waits for the result
Errors from await are caught with an ordinary try...catch
The order: all plain code first, then promises, then timers, even setTimeout with 0
Promise.all([…]) waits for several promises at once

Example

console.log("1: code runs top to bottom");setTimeout(() => console.log("4: a timer, even with 0 ms"), 0);Promise.resolve().then(() => console.log("3: a resolved promise"));console.log("2: still the same pass"); const wait = (ms) => new Promise((resolve) => setTimeout(resolve, ms)); async function later() {  await wait(0);  console.log("5: after await");  return "result";} later().then((value) => console.log("6:", value));

The numbers show the real order: the plain lines, then the promise, then the timer. later() waited for a timer with await, and its return value went to then.

Task

How to use the editor⌘ Enter or Ctrl Enter runs the code, ⇧ ⌘ Enter checks the task.Tab and Shift Tab indent the line or the selection; Esc, then Tab leaves the editor.The check compares what the program printed with the task; spaces at line ends do not matter.After two failed checks you may open the solution. Your code is kept in this browser.

Finish track(id): wait for getOrder(id) with await and print Order 7: Margherita in 20 min, and if the order does not exist, catch the error with try...catch and print its message. Output: Checking orders..., Order 7: Margherita in 20 min, Order 9 not found and All done.

// A pretend server: it answers a moment laterfunction getOrder(id) {  return new Promise((resolve, reject) => {    setTimeout(() => {      if (id === 7) resolve({ id, dish: "Margherita", minutes: 20 });      else reject(new Error(`Order ${id} not found`));    }, 0);  });} async function track(id) {  // TODO: await getOrder(id) and print "Order 7: Margherita in 20 min"  // TODO: wrap it in try...catch and print error.message if the order is missing} async function main() {  console.log("Checking orders...");  await track(7);  await track(9);  console.log("All done");} main();

// A pretend server: it answers a moment later
function getOrder(id) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (id === 7) resolve({ id, dish: "Margherita", minutes: 20 });
      else reject(new Error(`Order ${id} not found`));
    }, 0);
  });
}

async function track(id) {
  // TODO: await getOrder(id) and print "Order 7: Margherita in 20 min"
  // TODO: wrap it in try...catch and print error.message if the order is missing
}

async function main() {
  console.log("Checking orders...");
  await track(7);
  await track(9);
  console.log("All done");
}

main();

Output

What your program prints appears here.