When I first encountered asynchronous JavaScript, I struggled with callbacks and had no idea how Promises worked under the hood. Over time, learning about Promises and async/await transformed my approach to coding, making it much more manageable. In this blog, we’ll explore these async patterns step-by-step, revealing how they can streamline your development process and make your code cleaner and more efficient. Let’s dive in and uncover these concepts together!

Why You Need to Learn Asynchronous JavaScript?

Learning asynchronous JavaScript is essential for modern web development. It allows you to handle tasks like API requests efficiently, keeping your applications responsive and fast. Mastering async techniques, such as Promises and async/await, is crucial not only for building scalable applications but also for succeeding in JavaScript job interviews, where understanding these concepts is often a key focus. By mastering asynchronous JavaScript, you'll enhance your coding skills and better prepare yourself for real-world challenges.

What are Async Patterns?

Asynchronous patterns in JavaScript are techniques used to handle tasks that take time, such as fetching data from a server, without freezing the application. Initially, developers used callbacks to manage these tasks, but this approach often led to complex and hard-to-read code, known as "callback hell." To simplify this, Promises were introduced, providing a cleaner way to handle asynchronous operations by chaining actions and handling errors more gracefully. The evolution continued with async/await, which allows you to write asynchronous code that looks and behaves more like synchronous code, making it easier to read and maintain. These patterns are crucial for building efficient, responsive applications and are fundamental in modern JavaScript development. We will explore these concepts in more detail throughout this blog.

what is callback?

Callbacks are functions that you pass as arguments to other functions, with the intention that the receiving function will execute the callback at some point. This is useful for scenarios where you want to ensure some code runs after a specific task is complete, like after fetching data from a server or finishing a computation.

How Callbacks Work:

You define a function (callback).
You pass this function as an argument to another function.
The receiving function executes the callback at the appropriate time.

example 1

function fetchData(callback) {
  // Simulate fetching data with a delay
  setTimeout(() => {
    const data = "Data fetched";
    callback(data); // Call the callback function with the fetched data
  }, 1000);
}

function processData(data) {
  console.log("Processing:", data);
}

fetchData(processData); // fetchData will call processData with the data

example 2

// Function that adds two numbers and uses a callback to return the result
function addNumbers(a, b, callback) {
  const result = a + b;
  callback(result); // Call the callback function with the result
}

// Callback function to handle the result
function displayResult(result) {
  console.log("The result is:", result);
}

// Call addNumbers with the displayResult callback
addNumbers(5, 3, displayResult);

Note: I think callbacks are effective for handling asynchronous operations, but be cautious: as your code complexity increases, especially with nested callbacks, you may encounter a problem known as callback hell. This issue arises when callbacks are deeply nested within each other, leading to readability problems and making the code harder to maintain.

Callback Hell

Callback Hell (also known as Pyramid of Doom) refers to the situation where you have multiple nested callbacks. This happens when you need to perform several asynchronous operations in sequence, and each operation relies on the previous one.

ex. This creates a "pyramid" structure which can be hard to read and maintain.

fetchData(function(data1) {
  processData1(data1, function(result1) {
    processData2(result1, function(result2) {
      processData3(result2, function(result3) {
        console.log("Final result:", result3);
      });
    });
  });
});

Issues with Callback Hell:

Readability: The code becomes difficult to read and understand.
Maintainability: Making changes or debugging becomes challenging.
Error Handling: Managing errors can get complicated.

Handling Errors with Callbacks

When working with callbacks, it's common to use a pattern known as error-first callbacks. In this pattern, the callback function takes an error as its first argument. If there is no error, the first argument is usually null or undefined, and the actual result is provided as the second argument.

function fetchData(callback) {
  setTimeout(() => {
    const error = null; // Or `new Error("Some error occurred")` if there's an error
    const data = "Data fetched";
    callback(error, data); // Pass error and data to the callback
  }, 1000);
}

function processData(error, data) {
  if (error) {
    console.error("Error:", error);
    return;
  }
  console.log("Processing:", data);
}

fetchData(processData); // `processData` will handle both error and data

Note: After callbacks, Promises were introduced to handle asynchronous processes in JavaScript. We will now dive deeper into Promises and explore how they work under the hood.

Introduction to Promises

Promises are objects that represent the eventual completion (or failure) of an asynchronous operation and its resulting value. They provide a cleaner way to handle asynchronous code compared to callbacks.

Purpose of Promises:

Avoid Callback Hell: Promises help manage multiple asynchronous operations without deep nesting.
Improve Readability: Promises provide a more readable way to handle sequences of asynchronous tasks.

Promise States

A Promise can be in one of three states:

Pending: The initial state, before the promise has been resolved or rejected.
Fulfilled: The state when the operation completes successfully, and resolve has been called.
Rejected: The state when the operation fails, and reject has been called.

Note: If you want to explore more, you should check out Understand How Promises Work Under the Hood where I discuss how promises work under the hood.

example 1

// Creating a new promise
const myPromise = new Promise((resolve, reject) => {
  const success = true; // Simulate success or failure
  if (success) {
    resolve("Operation successful!"); // If successful, call resolve
  } else {
    reject("Operation failed!"); // If failed, call reject
  }
});

// Using the promise
myPromise
  .then((message) => {
    console.log(message); // Handle the successful case
  })
  .catch((error) => {
    console.error(error); // Handle the error case
  });

example 2

const examplePromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    const success = Math.random() > 0.5; // Randomly succeed or fail
    if (success) {
      resolve("Success!");
    } else {
      reject("Failure.");
    }
  }, 1000);
});

console.log("Promise state: Pending...");

// To check the state, you would use `.then()` or `.catch()`
examplePromise
  .then((message) => {
    console.log("Promise state: Fulfilled");
    console.log(message);
  })
  .catch((error) => {
    console.log("Promise state: Rejected");
    console.error(error);
  });

Chaining Promises

Chaining allows you to perform multiple asynchronous operations in sequence, with each step depending on the result of the previous one.

Chaining promises is a powerful feature of JavaScript that allows you to perform a sequence of asynchronous operations where each step depends on the result of the previous one. This approach is much cleaner and more readable compared to deeply nested callbacks.

How Promise Chaining Works

Promise chaining involves connecting multiple promises in a sequence. Each promise in the chain executes only after the previous promise is resolved, and the result of each promise can be passed to the next step in the chain.

function step1() {
  return new Promise((resolve) => {
    setTimeout(() => resolve("Step 1 completed"), 1000);
  });
}

function step2(message) {
  return new Promise((resolve) => {
    setTimeout(() => resolve(message + " -> Step 2 completed"), 1000);
  });
}

function step3(message) {
  return new Promise((resolve) => {
    setTimeout(() => resolve(message + " -> Step 3 completed"), 1000);
  });
}

// Chaining the promises
step1()
  .then(result => step2(result))
  .then(result => step3(result))
  .then(finalResult => console.log(finalResult))
  .catch(error => console.error("Error:", error));

Disadvantages of Chaining:
While chaining promises improves readability compared to nested callbacks, it can still become unwieldy if the chain becomes too long or complex. This can lead to readability issues similar to those seen with callback hell.

Note: To address these challenges, async and await were introduced to provide an even more readable and straightforward way to handle asynchronous operations in JavaScript.

Introduction to Async/Await

async and await are keywords introduced in JavaScript to make handling asynchronous code more readable and easier to work with.

async: Marks a function as asynchronous. An async function always returns a promise, and it allows the use of await within it.
await: Pauses the execution of the async function until the promise resolves, making it easier to work with asynchronous results in a synchronous-like fashion.

async function fetchData() {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve("Data fetched");
    }, 1000);
  });
}

async function getData() {
  const data = await fetchData(); // Wait for fetchData to resolve
  console.log(data); // Logs "Data fetched"
}

getData();

How Async/Await Works

1. Async Functions Always Return a Promise:

No matter what you return from an async function, it will always be wrapped in a promise. For example:

async function example() {
  return "Hello";
}

example().then(console.log); // Logs "Hello"

Even though example() returns a string, it is automatically wrapped in a promise.

2. Await Pauses Execution:

The await keyword pauses the execution of an async function until the promise it is waiting for resolves.

async function example() {
  console.log("Start");
  const result = await new Promise((resolve) => {
    setTimeout(() => {
      resolve("Done");
    }, 1000);
  });
  console.log(result); // Logs "Done" after 1 second
}

example();

In this example:

"Start" is logged immediately.
The await pauses execution until the promise resolves after 1 second.
"Done" is logged after the promise resolves.

Error Handling with Async/Await

Handling errors with async/await is done using try/catch blocks, which makes error handling more intuitive compared to promise chains.

async function fetchData() {
  throw new Error("Something went wrong!");
}

async function getData() {
  try {
    const data = await fetchData();
    console.log(data);
  } catch (error) {
    console.error("Error:", error.message); // Logs "Error: Something went wrong!"
  }
}

getData();

With Promises, you handle errors using .catch():

fetchData()
  .then(data => console.log(data))
  .catch(error => console.error("Error:", error.message));

Using async/await with try/catch often results in cleaner and more readable code.

Combining Async/Await with Promises

You can use async/await with existing promise-based functions seamlessly.

example

function fetchData() {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve("Data fetched");
    }, 1000);
  });
}

async function getData() {
  const data = await fetchData(); // Wait for the promise to resolve
  console.log(data); // Logs "Data fetched"
}

getData();

Best Practices:

Use async/await for readability: When dealing with multiple asynchronous operations, async/await can make the code more linear and easier to understand.
Combine with Promises: Continue using async/await with promise-based functions to handle complex asynchronous flows more naturally.
Error Handling: Always use try/catch blocks in async functions to handle potential errors.

Conclusion

async and await provide a cleaner and more readable way to handle asynchronous operations compared to traditional promise chaining and callbacks. By allowing you to write asynchronous code that looks and behaves like synchronous code, they simplify complex logic and improve error handling with try/catch blocks. Using async/await with promises results in more maintainable and understandable code.

Mastering JavaScript Async Patterns: From Callbacks to Async/Await