Which keyword is necessary for defining an asynchronous function in JavaScript?

The keyword necessary for defining an asynchronous function in JavaScript is "async." When you prefix a function declaration with the async keyword, it signals that the function will execute asynchronously. This allows the function to use the await keyword within its body, which lets the program pause execution until a promise is settled, either fulfilled or rejected.

Using async functions improves code readability and maintainability by allowing developers to write asynchronous code that appears synchronous. This prevents the need for deeply nested callbacks, often referred to as "callback hell," making the code easier to follow and debug.

The other options provided serve different purposes in JavaScript but do not define an asynchronous function. The await keyword is used within an async function to wait for promises, while the promise itself is an object that represents the eventual completion or failure of an asynchronous operation. The function keyword is used to declare regular functions but does not inherently indicate that a function operates asynchronously.