In the previous lessons, we discussed Promise.all() and how it works when executing multiple asynchronous operations in parallel. You also saw an important limitation: If any one promise rejects, Promise.all() immediately rejects and stops waiting for the remaining promises. In this tutorial, we’ll explore a safer alternative — Promise.allSettled() This method allows you to run…

In many real-world applications, we often need to execute multiple asynchronous operations at the same time and wait for all of them to finish before proceeding. A very common example of this is when an application needs to make multiple HTTP requests in parallel — such as fetching data from different services — and only…

Understanding Promise.resolve() and Promise.reject() In this tutorial, we are going to understand two very important JavaScript methods that help us convert any value into a Promise: These methods are extremely useful when working with existing code that expects Promises, but sometimes you only have normal (non-promise) values. Why Do We Need This? In real-world JavaScript…

Using .then() with One or Two Arguments The .then() method can accept two arguments: So far, we mostly used .then() with only one argument, which handles successful cases. Example: Handling Both Success and Failure Let’s use our familiar calculateSquare function. This works correctly: However, this approach has a limitation. The Hidden Problem Let’s introduce an…

The Key Idea: .then() Returns a New Promise This is extremely important: Every .then() call returns a new promise. Because of this: This allows us to create a chain of asynchronous operations. Step 1: Basic Promise Chain Let’s start with a simple example. Output: So far, nothing new. Step 2: Adding Another .then() Now let’s…

Why We Need .then() Let’s start with a simple question: Once a promise is resolved, how do we access its value? You might think we can simply do something like: But that does not work. Why? Because: So how do we get the result? By using .then() Creating a Simple Promise Let’s start with a…

In this section, we are going to explore Promises in JavaScript — one of the most important concepts for handling asynchronous operations in a clean and readable way. Promises were introduced to solve many of the problems caused by callbacks, especially callback hell. Before we see how promises work in practice, we must clearly understand…

In this section, we are going to understand how errors are handled in asynchronous JavaScript code. So far, we have seen how callbacks work and how asynchronous execution is handled using the event loop. Now we’ll focus on what happens when something goes wrong inside an asynchronous operation and why traditional try…catch does not work…

What Is a Callback? A callback is simply a function that is passed as an argument to another function, and then invoked (called) inside that function. This concept is possible because functions in JavaScript are first-class citizens. This means: Because of this flexibility, JavaScript heavily relies on callbacks to control execution flow. Why Callbacks Exist…

When we write JavaScript code, it may look simple and linear on the surface, but a lot is happening behind the scenes. Every browser and runtime environment uses a JavaScript engine to execute code, and understanding how this engine works gives you a much deeper insight into asynchronous behavior, callbacks, and performance issues. Almost every…