Asynchronous programming is an essential concept in JavaScript that allows non-blocking execution of code. This means JavaScript can perform multiple operations concurrently without getting blocked. Asynchronous JS relies primarily on callbacks, promises and async/await. In this article, we'll understand how to use these tools to write asynchronous code.

Callbacks

Callbacks are functions that are passed as arguments to other functions to be executed at a later time. For example:

function doTask(callback) {
  // do something
  callback(); // execute callback
}

doTask(function() {
  // this is the callback  
});

Callbacks are used in many asynchronous JavaScript operations like fetching data, event handling, timeouts etc. A drawback is callback hell or pyramid of doom caused by nesting several callbacks.

Promises

Promises represent the future value of an asynchronous operation. They can have three states - pending, fulfilled or rejected. We handle promises with .then() and .catch():

function asyncFunc() {
  return new Promise((resolve, reject) => {
    if (success) {
      resolve(value); // fulfilled
    } else { 
      reject(error); // rejected
    }
  });
}

asyncFunc()
.then((result) => {
  // handle fulfillment 
})
.catch((error) => {
  // handle rejection
});

Promises avoid callback hell and make asynchronous code linear using chaining. Multiple promises can be executed concurrently with Promise.all().

Async/Await

Async/await makes asynchronous code look like synchronous code. The async keyword before a function designates it as asynchronous and await pauses execution until a promise resolves or rejects.

async function asyncFunc() {
  const result = await promiseBasedFunc(); 
  // other sync operations
  const value = await someOtherFunc();
  return result;
}

Try/catch blocks can be used for error handling. Async/await makes asynchronous code very readable compared to callbacks and chaining promises.

So in summary, callbacks offer basic asynchronous capabilities, promises improve code structure, and async/await make it sync-like. Mastering these techniques will help you write efficient asynchronous JavaScript code.