27. What is the purpose of the 'async' and 'await' keywords in JavaScript?

Understanding 'async' and 'await' in JavaScript

The async and await keywords in JavaScript are used to work with Promises more easily and to write cleaner asynchronous code. These keywords are part of ES2017 (ES8) and they help to handle asynchronous operations in a more synchronous-like fashion.

How 'async' Works

The async keyword is used to define an asynchronous function. When a function is declared with async, it automatically returns a Promise. The execution of an async function is paused at each await keyword, waiting for the Promise to resolve.

async function fetchData() {
  return 'Data fetched';
}

fetchData().then(result => console.log(result)); // Output: Data fetched

How 'await' Works

The await keyword can be used inside async functions to pause the execution of the function until the Promise is resolved. It makes the code easier to read and write, as it avoids the need for .then() and .catch() methods.

async function fetchData() {
  let dataPromise = new Promise((resolve, reject) => {
    setTimeout(() => resolve('Data fetched'), 1000);
  });

  let result = await dataPromise;
  console.log(result); // Output: Data fetched
}

fetchData();

Benefits

1. Readability: Code with async and await is more readable and looks more like synchronous code.
2. Error Handling: You can use try and catch blocks to handle errors, making error management straightforward.
3. Avoiding Callback Hell: Helps to avoid deeply nested callbacks, making the code cleaner.

Important Notes

• await only works inside async functions.
• await pauses the function execution, but it does not block the execution of other operations.
• If a Promise is rejected, an error is thrown, which you can catch using try-catch blocks.

In conclusion, async and await provide a powerful way to write cleaner and more manageable asynchronous JavaScript code.