curriculum/challenges/english/blocks/lecture-understanding-asynchronous-programming/673407d56c3dce67fa97969b.md
In the previous lessons, you learned about asynchronous programming which allows other code to run while we wait for some time-consuming tasks to complete, like fetching data from a server, reading data from a file, and so on.
async/await, built on top of promises, makes writing and reading asynchronous code easier. When you put the async keyword before a function, it means that function will always return a Promise. Only inside an async function, you can use the await keyword, which allows you to wait for a Promise to resolve before moving on to the next line of code. Here's an example to illustrate how async/await works:
:::interactive_editor
async function delayedGreeting(name) {
console.log("A Messenger entered the chat...");
await new Promise(resolve => setTimeout(resolve, 2000));
console.log(`Hello, ${name}!`);
}
delayedGreeting("Alice");
console.log("First Printed Message!");
:::
In this code, we define an async function called delayedGreeting. Inside this function, we use await to pause the execution for 2 seconds. After the delay, it prints a greeting.
When we call this function, you'll see First Printed Message! appear before the greeting. This is because the function is asynchronous - it doesn't block the rest of the code from running.
One of the biggest advantages of async/await is error handling. With promises, we often had to use .catch() method to handle errors. With async/await, we can use try/catch blocks. Here's an example:
async function fetchUserData() {
try {
let response = await fetch(`https://api.example.com/users`);
let userData = await response.json();
console.log(userData);
} catch (error) {
console.log("Error fetching user data:", error);
}
}
fetchUserData();
In this example, we're using async/await to fetch user data from an API. The await keyword is used twice: once to wait for the fetch operation to complete, and again to wait for the JSON parsing to finish. If any error occurs during this process, it will be caught in the catch block.
What is the primary purpose of the async keyword in JavaScript?
To make a function run faster.
Think about what async does to the function's return value.
To indicate that a function will always return a Promise.
To create a new thread for the function to run on.
Think about what async does to the function's return value.
To prevent a function from being called asynchronously.
Think about what async does to the function's return value.
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Where can the await keyword be used in JavaScript?
Anywhere in the code.
Consider the restrictions on where await can be placed.
At very beginning of your code.
Consider the restrictions on where await can be placed.
Inside async functions.
Only in try...catch blocks.
Consider the restrictions on where await can be placed.
3
What happens when an error occurs in an awaited Promise?
The program crashes.
Think about how error handling works with async/await.
The error is automatically logged to the console.
Think about how error handling works with async/await.
Nothing, errors in Promises are ignored.
Think about how error handling works with async/await.
The error can be caught using a try...catch block.
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