JavaScript Promises are a way to handle asynchronous operations in JavaScript.

What are asynchronous operations you might ask?

Asynchronous operations are operations that do not block or halt the execution of other code while they are running. This means that while an asynchronous operation is running, the program can continue to execute other code.

Examples of asynchronous operations in JavaScript include:

• Making an HTTP request to a server using fetch() or XMLHttpRequest()

• Reading a file using fs.readFile() in Node.js

• Setting a timeout using setTimeout()

• Listening for an event, such as a button click or a keypress

Asynchronous operations are useful because they allow the program to continue running other code while the operation is being performed, rather than being blocked or halted. This can greatly improve the performance and responsiveness of a program, especially when working with external resources such as servers or files.

To simplify it, Asynchronous operations are the operations that keep the program running and don't halt it for the completion of that operation, like fetching data from a server, reading a file and listening to events.

Now that you have understood Asynchronous operations, come back to Promises.

Promises provide a way to handle success and failure scenarios when working with asynchronous code, such as fetching data from a server or reading a file. Promises are a more modern and cleaner way to handle async operations in comparison to callbacks.

A Promise is a JavaScript object that represents the eventual completion (or failure) of an asynchronous operation, and its resulting value. A Promise is in one of three states:

• pending: initial state, neither fulfilled nor rejected.

• fulfilled: meaning that the operation was completed successfully.

• rejected: meaning that the operation failed.

Promises are typically used with the .then() and .catch() methods. The .then() method is called when the promise is fulfilled, and it takes a success callback function as its argument. The .catch() method is called when the promise is rejected, and it takes a failure callback function as its argument.

let promise = new Promise((resolve, reject) => {
  // do some async operation
  if (success) {
    resolve(value);
  } else {
    reject(error);
  }
});

promise.then(value => {
  // success callback
}).catch(error => {
  // failure callback
});

Async Await

Asynchronous and Await are new ways of handling promises. They make the code look more like synchronous code, making it easier to read and understand. The async the keyword is used to define an asynchronous function and the await keyword is used to wait for a promise to resolve before moving on to the next line of code.

async function fetchData() {
  try {
    let response = await fetch('https://example.com/data');
    let data = await response.json();
    console.log(data);
  } catch (error) {
    console.log(error);
  }
}
fetchData();

In this example, the fetchData() function is asynchronous and it uses the await keyword to wait for the fetch() promise to resolve before moving on to the next line of code where it parses the response to JSON. If there is any error it would catch it in the catch block.

Conclusion

In summary, JavaScript Promises are a way to handle asynchronous operations in JavaScript, and they provide a way to handle success and failure scenarios when working with asynchronous code. Async/await is a more modern and clean way to handle promises, which makes the code look more like synchronous code and easier to read and understand.