In JavaScript, the async function is used to define a function that returns a promise. The async keyword is placed before the function keyword when declaring a function. An async function can contain an await expression, which pauses the execution of the function until the promise is resolved, and then resumes the execution and returns the resolved value.

Here's a basic example:

async function exampleAsyncFunction() {
  console.log("Before await");
  const result = await somePromise(); // Pauses execution until somePromise is resolved
  console.log("After await, result:", result);
  return result;
}

function somePromise() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve("Promise resolved!");
    }, 1000);
  });
}

// Using the async function
exampleAsyncFunction().then((result) => {
  console.log("Async function result:", result);
});

In this example, exampleAsyncFunction is an asynchronous function that uses the await keyword to wait for the resolution of the somePromise function before moving on. The somePromise function returns a promise that resolves after a timeout of 1000 milliseconds.

Keep in mind that the async keyword can only be used with function declarations, function expressions, and arrow functions. Additionally, an async function always returns a promise, and the value it returns will be wrapped in a resolved promise or the function itself can explicitly return a promise using Promise.resolve() or Promise.reject().

Async/await is especially useful for working with asynchronous code in a more synchronous-like manner, making it easier to read and reason about asynchronous code.