Note: I’ve explained the entire topic step by step, but feel free to skip to any section where you have a question or need clarification.

Before we get into asynchronous javascript it’s important to understand what is synchronous javascript and why we shall need asynchronous way to write javascript code, right?

What is Synchronous JavaScript?

In synchronous programming, tasks are executed one after the other, in a sequential manner. The next task can only start after the current task is finished. If one task takes a long time, everything else must wait.

Think of it like waiting in line at a grocery store: if the person ahead of you has a lot of items and takes a long time to check out, you must wait your turn until they are done before you can proceed.

console.log("Start cooking");

for (let i = 0; i < 5; i++) {
  console.log("Cooking dish " + (i + 1));
}

console.log("Finish cooking");

Here’s what happens:

1. It prints "Start cooking".

2. Then it enters a loop and prints each "Cooking dish X" one after another.

3. Only after the loop is done, it prints "Finish cooking".

In this case, the code runs in order, and nothing else can happen until the current task (cooking each dish) is completed. Imagine if one dish took 10 minutes to cook—everything else would have to wait for that dish to finish.

What is Asynchronous JavaScript?

In asynchronous programming, tasks can be started, and while they are still running (e.g., waiting for data from a server), other tasks can continue to run. You don’t have to wait for one task to finish before starting another.

Think of it like placing an order at a restaurant: you order your food, and while it’s being prepared, you can continue talking to your friends or checking your phone. Once the food is ready, the waiter will bring it to you.

Example of Asynchronous Code:

console.log("Start cooking");

setTimeout(() => {
  console.log("Cooking is done!");
}, 3000); // Simulate a task that takes 3 seconds

console.log("Prepare other things while waiting");

Here’s what happens:

1. It prints "Start cooking".

2. The setTimeout function starts a 3-second timer. But instead of waiting, JavaScript immediately moves on to the next line.

3. It prints "Prepare other things while waiting".

4. After 3 seconds, the timer completes and prints "Cooking is done!".

There are three main ways to write asynchronous JavaScript, and they are:

1. Callbacks

2. Promises

3. Async/Await

These are the primary approaches to handling asynchronous code in JavaScript.

Callbacks

A callback function in JavaScript is a function that you pass as an argument to another function. The basic idea is that you pass or define a function as an argument to another function, and that passed function is called a ‘Callback function’. The callback is called (or "invoked") after a certain task is completed, often after an asynchronous task, like fetching data from a server.

This allows your main function to continue doing other things without waiting for the task to finish. When the task is done, the callback is triggered to handle the result.

function mainFunc(callback){
  console.log("this is set before setTimeout")
  callback()
  console.log("this is set after setTimeout")
}

function cb(){
  setTimeout(()=>{
    console.log("This is supposed to be painted after 3 second")
  },3000)
}

mainFunc(cb)

This code demonstrates the callback concept in JavaScript. Here's how it works:

1. mainFunc takes a callback function as an argument.

2. Inside mainFunc, the callback is executed immediately, but since the callback itself (the cb function) contains a setTimeout, it schedules the console.log to run after 3 seconds.

3. Meanwhile, mainFunc continues executing, printing messages before and after calling the callback.

4. After 3 seconds, the setTimeout inside the callback finishes, and the delayed message is printed.

This shows how the main function continues its tasks without waiting for the asynchronous operation (the 3-second delay in the callback) to complete.

What is a Callback hell and when does it happen?

Callback hell refers to a situation in JavaScript where multiple callbacks are nested within each other in an unmanageable way, making the code difficult to read, maintain, and debug. It often looks like a pyramid of nested functions, where each asynchronous operation relies on the completion of the previous one, leading to deeply nested callbacks.

Callback hell typically occurs when you have multiple asynchronous tasks that need to be performed in sequence, and each task depends on the result of the previous one. As more tasks are added, the code gets increasingly indented and harder to follow, resulting in a messy and complex structure.

getData((data) => {
  processData(data, (processedData) => {
    saveData(processedData, (savedData) => {
      notifyUser(savedData, () => {
        console.log("All tasks complete!");
      });
    });
  });
});

Here, each task is dependent on the previous one, leading to multiple levels of indentation and difficult-to-follow logic. If there’s an error at any point, handling it properly becomes even more complex.

To rescue you from callback hell, modern JavaScript provides solutions like:

1. Promises – which flatten the code and make it more readable.

2. Async/Await – which simplifies chaining asynchronous operations and makes the code look synchronous.

Promises

A Promise in JavaScript is an object that represents the eventual completion (or failure) of an asynchronous operation and its result. It’s like a promise you make in real life: something may not happen immediately, but you either fulfill it or fail to fulfill it.

In JavaScript, Promises allow you to write asynchronous code in a cleaner way, avoiding callback hell. A Promise is created using the new Promise syntax, and it takes a function (which is constructor function) with two parameters: resolve (if the task is successful) and reject (if it fails).

const myPromise = new Promise((resolve, reject) => {
  // Simulating an asynchronous task like fetching data
  const success = true; // Change to false to simulate failure

  setTimeout(() => {
    if (success) {
      resolve("Task completed successfully!"); // Success
    } else {
      reject("Task failed!"); // Failure
    }
  }, 2000); // 2-second delay
});

Here:

• The promise simulates a task that takes 2 seconds.

• If the task is successful (success is true), it calls resolve with a message.

• If it fails (success is false), it calls reject with an error message.

How to handle a Promise

To handle the result of a Promise, we use two methods:

• .then() for a successful result (when the Promise is fulfilled).

• .catch() for an error (when the Promise is rejected).
  

myPromise
  .then((message) => {
    console.log(message); // "Task completed successfully!" (if resolve was called)
  })
  .catch((error) => {
    console.log(error); // "Task failed!" (if reject was called)
  });

Chaining in Promises

Promises allow you to chain asynchronous operations using the .then() method, which makes the code more linear and easier to follow. Each .then() represents a step in the asynchronous process. The .catch() method allows you to centralize error handling at the end of the promise chain, making it more organized.

fetchData()
  .then(data => processData1(data))
  .then(result1 => processData2(result1))
  .then(result2 => processData3(result2))
  .catch(error => handleError(error));

Promises promote a more readable and maintainable code structure by avoiding deeply nested callbacks. The chaining and error-handling mechanisms contribute to a clearer and more organized codebase.

// Callback hell
fetchData1(data1 => {
  processData1(data1, result1 => {
    fetchData2(result1, data2 => {
      processData2(data2, result2 => {
        fetchData3(result2, data3 => {
          processData3(data3, finalResult => {
            console.log("Final result:", finalResult);
          });
        });
      });
    });
  });
});

// Using Promises
fetchData1()
  .then(result1 => processData1(result1))
  .then(data2 => fetchData2(data2))
  .then(result2 => processData2(result2))
  .then(data3 => fetchData3(data3))
  .then(finalResult => {
    console.log("Final result:", finalResult);
  })
  .catch(error => handleError(error));

finally Method

The finally method is used to execute code, regardless of whether the Promise is resolved or rejected.

myPromise
  .then((data) => {
    console.log("Data:", data);
  })
  .catch((error) => {
    console.error("Error:", error);
  })
  .finally(() => {
    console.log("Finally block"); // This runs no matter what
  });

If the promise is fulfilled (i.e., it successfully gets data), the .then() method will be executed. On the other hand, if the promise encounters an error, the .catch() method will be called to handle the error. However, the .finally() method has no such condition—it will always be executed, regardless of whether the promise was resolved or rejected. Whether the .then() or .catch() is triggered, the .finally() block will definitely run at the end.

It's useful for cleaning up resources, stopping loading indicators, or doing something that must happen after the promise completes, regardless of its result.

JavaScript Promise Methods

JavaScript provides several Promise methods that make working with asynchronous tasks more flexible and powerful. These methods allow you to handle multiple promises, chain promises, or deal with various outcomes in different ways

Promise.all() - Parallel Execution

You can use Promise.all() to execute multiple asynchronous operations concurrently and handle their results collectively.

This method takes an array of promises and runs them in parallel which returns a new Promise. This new Promise fulfils with an array of resolved values when all the input Promises have fulfilled, or rejects with the reason of the first rejected Promise.

Copy code
const promise1 = Promise.resolve(10);
const promise2 = Promise.resolve(20);
const promise3 = Promise.resolve(30);

Promise.all([promise1, promise2, promise3]).then((values) => {
  console.log(values); // [10, 20, 30]
});

If one of the promises rejects, the whole Promise.all() will reject:

const promise1 = Promise.resolve(10);
const promise2 = Promise.reject("Error!");

Promise.all([promise1, promise2])
  .then((values) => {
    console.log(values);
  })
  .catch((error) => {
    console.log(error); // "Error!"
  });

Promise.allSettled()

This method takes an array of promises and returns a new promise after all of them finish their execution, whether they succeed or fail. Unlike Promise.all(), it doesn't fail if one promise fails. Instead, it waits for all the promises to complete and gives you an array of results that show if each one succeeded or failed.

It’s useful when you want to know the result of every promise, even if some fail.

const promise1 = Promise.resolve("Success!");
const promise2 = Promise.reject("Failure!");

Promise.allSettled([promise1, promise2]).then((results) => {
  console.log(results);
  // [{ status: 'fulfilled', value: 'Success!' }, { status: 'rejected', reason: 'Failure!' }]
});

Promise.any()

Promise.any() takes an array of promises and returns a promise that resolves as soon as any one promise resolves. If all input Promises are rejected, then Promise.any() rejects with an AggregateError containing an array of rejection reasons.

It’s useful when you need just one successful result from multiple promises.

const promise1 = fetchData1();
const promise2 = fetchData2();
const promise3 = fetchData3();

Promise.any([promise1, promise2, promise3])
  .then((firstFulfilledValue) => {
    console.log("First promise fulfilled with:", firstFulfilledValue);
  })
  .catch((allRejectedReasons) => {
    console.error("All promises rejected with:", allRejectedReasons);
  });

Promise.race()

This method takes an array of promises and returns a new promise that resolves or rejects as soon as the first promise settles (either resolves or rejects).

It’s useful when you want the result of the fastest promise, ignoring the others.

const promise1 = new Promise((resolve) => setTimeout(resolve, 100, "First"));
const promise2 = new Promise((resolve) => setTimeout(resolve, 200, "Second"));

Promise.race([promise1, promise2]).then((value) => {
  console.log(value); // "First" (because promise1 resolves first)
});

Promise.resolve()

This method immediately resolves a promise with a given value. It’s useful for creating a promise that you already know will be fulfilled.

const resolvedPromise = Promise.resolve("Resolved!");
resolvedPromise.then((value) => {
  console.log(value); // "Resolved!"
});

Promise.reject()

This method immediately rejects a promise with a given error or reason. It’s useful for creating a promise that you know will fail.

const rejectedPromise = Promise.reject("Rejected!");
rejectedPromise.catch((error) => {
  console.log(error); // "Rejected!"
});

I hope you have no confusion regarding promises after carefully reading this explanation of promises. Now, it’s time to move on Async/Await.

Async/Await

async / await is a modern way to handle asynchronous operations in JavaScript, introduced in ES2017 (ES8). It’s built on top of Promises but provides a cleaner and more readable syntax, making asynchronous code look and behave more like synchronous code. This makes it easier to understand and debug.

How async and await Work

When you declare a function with the async keyword, it always returns a Promise. Even if you don't explicitly return a promise, JavaScript wraps the return value inside a resolved promise.

The await keyword can only be used inside an async function. It makes JavaScript wait for the promise to resolve before moving on to the next line of code. It “pauses” the execution of the function, allowing you to handle asynchronous tasks more sequentially, just like synchronous code.

Let’s look at a simple example to see how this works:

async function fetchData() {
  try {
    const response = await fetch("https://api.example.com/data");
    const data = await response.json();
    console.log("Data:", data);
  } catch (error) {
    console.error("Error fetching data:", error);
  } finally {
    console.log("Fetch operation complete");
  }
}

fetchData();

How it works:

1. async function: Since fetchData function is marked as async, that means it returns a promise.

2. await fetch(): The await pauses the execution of the function until the promise returned by fetch() resolves. It then continues with the next line after the promise is resolved.

3. try/catch: We use a try/catch block to handle any potential errors during the async operation.

4. finally: Regardless of success or failure, the finally block will execute.

Why Use async / await?

With async/await, your code becomes more readable and flows more naturally. This makes it easier to follow the logic, especially when dealing with multiple asynchronous operations.

Compare this to handling promises with .then():

fetch("https://api.example.com/data")
  .then(response => response.json())
  .then(data => {
    console.log("Data:", data);
  })
  .catch(error => {
    console.error("Error fetching data:", error);
  })
  .finally(() => {
    console.log("Fetch operation complete");
  });

The async/await version looks cleaner and easier to understand. async/await helps avoid the nesting of callbacks or .then() chains, making your code more linear and easier to follow.

Example with Multiple await

You can also handle multiple asynchronous tasks in a sequence without needing to chain promises.

async function processOrders() {
  const user = await getUserDetails();  // Waits for user details
  const orders = await getOrders(user.id);  // Waits for orders
  console.log("Orders:", orders);
}

processOrders();

In this example, the function waits for each task to finish before moving on to the next, just like how synchronous code would behave.

Parallel Execution with Promise.all() and async/await

If you want to perform multiple asynchronous operations at the same time (in parallel), you can still use Promise.all() with async/await:

async function getAllData() {
  const [user, orders] = await Promise.all([getUserDetails(), getOrders()]);
  console.log("User:", user);
  console.log("Orders:", orders);
}

Here, both getUserDetails() and getOrders() run simultaneously, and the function waits for both to finish before logging the results.

Conclusion

In JavaScript, handling asynchronous operations has evolved over time, offering different tools to make code more manageable and efficient. Callbacks were the first approach, but as the complexity of code grew, they often led to issues like "callback hell." Promises came next, providing a cleaner, more structured way to manage async tasks with .then() and .catch(), improving readability and reducing nesting.

Finally, async/await was introduced as a modern syntax built on promises, making asynchronous code look more like synchronous code. It simplifies the process even further, allowing for easier-to-read, more maintainable code. Each of these techniques plays an important role in JavaScript, and mastering them helps you write more efficient, clear, and robust code.

Understanding when to use each method—callbacks for simple tasks, promises for structured handling, and async/await for readable, scalable async code—will empower you to make the best choices for your projects.

My Last Words

I used to get confused by the concept of promises, especially the different methods of promises. Callbacks were a big challenge for me because the syntax always seemed very confusing. So, I read various sources online, including chatbots, to come up with this description. To be honest, chatbots don’t always provide a straight and understandable answer. So, I didn’t just copy and paste from different places—I simplified everything so that it can serve as a clear note for me and for anyone else who has difficulty grasping these concepts. I hope this note leaves you with zero confusion.