Step-by-Step Guide: How to Use Promises and Async/Await for Asynchronous JavaScript

Introduction

If you have ever worked with JavaScript, you probably know that it is a language that supports asynchronous programming. This means that you can write code that does not block the execution of other code, and that can handle events that happen in the future, such as user interactions, network requests, timers, etc.

However, writing asynchronous code in JavaScript can be tricky, especially if you have to deal with multiple asynchronous operations that depend on each other. For example, imagine that you want to fetch some data from an API, process it, and then display it on the web page. How do you make sure that the data is available before you try to process it? How do you handle errors that might occur during the network request or the processing? How do you avoid creating a mess of nested callbacks that make your code hard to read and maintain?

Fortunately, JavaScript has some features that can help you write asynchronous code in a more elegant and manageable way. These features are promises and async/await. In this article, I will explain what they are, how they work, and how to use them to handle asynchronous code in JavaScript.

What are promises?

A promise is an object that represents the result of an asynchronous operation. A promise can be in one of three states: pending, fulfilled, or rejected. When a promise is created, it is initially in the pending state, meaning that the result is not available yet. When the asynchronous operation completes successfully, the promise is fulfilled, meaning that the result is available and can be accessed. When the asynchronous operation fails, the promise is rejected, meaning that an error occurred and can be handled.

A promise has two methods: then and catch. The then method takes two arguments: a callback function that is executed when the promise is fulfilled, and another callback function that is executed when the promise is rejected. The catch method takes one argument: a callback function that is executed when the promise is rejected. You can use these methods to chain multiple promises together, and to handle the results or errors of each promise.

What is a callback?

A callback is a function that is passed as an argument to another function and is executed later, usually after some event or action. For example, you can pass a function that displays a message to another function that performs a calculation, and the message function will be called after the calculation is done. This way, you can control the order and timing of your code execution. A callback is also useful when you want to do something with the result of another function, such as displaying it on the screen or sending it to a server. A callback is sometimes called a "higher-order function" because it is a function that takes another function as a parameter.

Here is an example of a callback in JavaScript:

// This is the callback function
function displayMessage(message) {
  console.log(message);
}

// This is the function that takes a callback as an argument
function addNumbers(num1, num2, callback) {
  let sum = num1 + num2;
  // This is where the callback is called
  callback("The sum is " + sum);
}

// This is how you use the callback
addNumbers(5, 5, displayMessage); // The sum is 10

Let's say that we have a function called fetchData that takes a URL as an argument and returns a promise that resolves with the data from the API. We can use then and catch to handle the promise like this:

fetchData("https://example.com/api/data")
  .then((data) => {
    // this function is executed when the promise is fulfilled
    console.log("Data received:", data);
    // we can return another promise from this function
    return processData(data);
  })
  .then((processedData) => {
    // this function is executed when the previous promise is fulfilled
    console.log("Data processed:", processedData);
    // we can display the processed data on the web page
    displayData(processedData);
  })
  .catch((error) => {
    // this function is executed when any of the promises is rejected
    console.error("Something went wrong:", error);
    // we can display an error message on the web page
    displayError(error);
  });

As you can see, using promises allows us to write asynchronous code in a more linear and readable way, without creating a pyramid of doom with nested callbacks. However, there is still some room for improvement. For example, we have to use anonymous functions as arguments for then and catch, which can make our code less expressive and reusable. Also, we have to use return statements to pass values from one promise to another, which can be easy to forget or miss.

What is async/await?

async/await is a syntactic sugar that makes working with promises even easier and cleaner. It allows us to write asynchronous code as if it was synchronous, using the keywords async and await. The async keyword indicates that a function is asynchronous, meaning that it returns a promise. The await keyword pauses the execution of an async function until a promise is fulfilled or rejected, and then resumes it with the result or error of the promise.

For example, we can rewrite our previous example using async/await like this:

// we declare an async function
async function getData() {
  try {
    // we use await to pause until the promise is fulfilled
    let data = await fetchData("https://example.com/api/data");
    console.log("Data received:", data);
    // we use await again to pause until the next promise is fulfilled
    let processedData = await processData(data);
    console.log("Data processed:", processedData);
    // we display the processed data on the web page
    displayData(processedData);
  } catch (error) {
    // we use catch to handle any errors that might occur
    console.error("Something went wrong:", error);
    // we display an error message on the web page
    displayError(error);
  }
}

// we call our async function
getData();

As you can see, using async/await makes our code look more synchronous and concise, without using then and catch or anonymous functions. We can also use regular try/catch blocks to handle errors, which is more familiar and consistent with synchronous code.

Conclusion

In this article, I have explained what promises and async/await are, how they work, and how to use them to handle asynchronous code in JavaScript. Promises and async/await are powerful features that can help you write asynchronous code in a more elegant and manageable way, avoiding callback hell and improving readability and maintainability.

I hope you have learned something new and useful from this article, and that you will apply it to your own projects.