Practise timers, promises, async/await, error handling and running work in parallel. Every solution has been run with Node.js and shows its real output.
Review first: Async Code and the Event Loop, Promises, async and await, Fetching Data from APIs, Error Handling
Most problems use this helper, which resolves with value after ms milliseconds:
const wait = (ms, value) => new Promise((resolve) => setTimeout(() => resolve(value), ms));Predict the output of this code, then run it:
console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => console.log("3"));
console.log("4");Synchronous code first, then microtasks (promises), then timers.
console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => console.log("3"));
console.log("4");1
4
3
2Using wait, write an async function that prints "Boiling water", waits 30 ms, prints "Adding tea", waits 20 ms, then prints "Tea is ready".
await wait(30) pauses the async function.
const wait = (ms, value) => new Promise((resolve) => setTimeout(() => resolve(value), ms));
async function makeTea() {
console.log("Boiling water");
await wait(30);
console.log("Adding tea");
await wait(20);
console.log("Tea is ready");
}
await makeTea();Boiling water
Adding tea
Tea is readyLoad three values in parallel with wait(40, "profile"), wait(10, "orders") and wait(20, "messages"), and print the results as an array in that order.
await Promise.all([...]) keeps the original order.
const wait = (ms, value) => new Promise((resolve) => setTimeout(() => resolve(value), ms));
const results = await Promise.all([wait(40, "profile"), wait(10, "orders"), wait(20, "messages")]);
console.log(results);[ 'profile', 'orders', 'messages' ]Write getPrice(item) that resolves with 20 for "tea" after 10 ms and rejects with new Error("Unknown item: ...") for anything else. Call it for "tea" and "cake" with await inside try...catch, printing either the price or the error message.
Return new Promise((resolve, reject) => ...) and catch errors where you await.
function getPrice(item) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (item === "tea") resolve(20);
else reject(new Error(`Unknown item: ${item}`));
}, 10);
});
}
for (const item of ["tea", "cake"]) {
try {
console.log(item, await getPrice(item));
} catch (error) {
console.log(error.message);
}
}tea 20
Unknown item: cakeRun wait(10, "ok") and a promise that rejects with new Error("timeout") together, and print each result's status and its value or reason message.
Promise.allSettled never rejects; each result has a status.
const wait = (ms, value) => new Promise((resolve) => setTimeout(() => resolve(value), ms));
const results = await Promise.allSettled([wait(10, "ok"), Promise.reject(new Error("timeout"))]);
for (const r of results) {
console.log(r.status, r.status === "fulfilled" ? r.value : r.reason.message);
}fulfilled ok
rejected timeoutWrite retry(fn, attempts) that calls the async function fn and, if it throws, tries again, up to attempts times in total, then rethrows the last error. Test it with a function that fails twice and succeeds on the third call, printing each attempt.
Loop from 1 to attempts, return await fn() inside try, and remember the error in catch.
async function retry(fn, attempts) {
let lastError;
for (let i = 1; i <= attempts; i++) {
try {
return await fn(i);
} catch (error) {
console.log(`attempt ${i} failed: ${error.message}`);
lastError = error;
}
}
throw lastError;
}
const flaky = async (attempt) => {
if (attempt < 3) throw new Error("network error");
return "data loaded";
};
console.log(await retry(flaky, 5));attempt 1 failed: network error
attempt 2 failed: network error
data loaded