TLDR;
This video provides a comprehensive overview of asynchronous JavaScript, covering synchronous vs. asynchronous execution, callbacks, promises, and async/await. It explains how to handle asynchronous operations to prevent blocking code execution, and introduces key concepts like callback hell and how promises and async/await can mitigate it. Additionally, the video touches on string methods in JavaScript.
- Synchronous execution is line by line, while asynchronous is non-blocking.
- Callbacks are functions passed as arguments to other functions and executed later.
- Promises have three states: pending, resolved, and rejected, offering a structured way to handle asynchronous operations.
- Async/await simplifies asynchronous code, making it more readable and maintainable.
Introduction to Asynchronous JavaScript [39:22]
The session begins with a welcome and a brief recap of the previous class, which covered function creation, function types (function expression, regular function, arrow function), and callbacks. The focus shifts to promises and async/await in JavaScript for executing asynchronous code.
Synchronous vs. Asynchronous Execution [42:47]
Synchronous execution involves executing code line by line. If a line takes time, the entire execution pauses. In contrast, asynchronous execution is non-blocking. The presenter demonstrates this with setTimeout, showing how asynchronous code allows the program to continue executing other lines while waiting for a timer to complete. JavaScript is single-threaded, but asynchronous operations prevent code from blocking.
Understanding Callbacks [52:17]
A callback is a function passed as an argument to another function and called later, often after a task is complete. An example is given of ordering a pizza, where the pizza place calls back when the order is ready. The presenter provides a code example with greet and sayGoodbye functions to illustrate how callbacks work.
Callback Hell [58:01]
The presenter explains the concept of "callback hell," which occurs when multiple nested callbacks make code difficult to read and maintain. An example is provided using setTimeout to simulate asynchronous steps. Without callbacks, steps execute out of order due to the asynchronous nature of setTimeout. Callbacks are then used to ensure steps execute sequentially, but this leads to deeply nested code, or "callback hell".
Promises in JavaScript [1:14:20]
Promises are used for asynchronous programming and have three states: pending, resolved, and rejected. The presenter uses the analogy of ordering food to explain these states. Initially, a promise is pending. If the order is accepted and delivered, the promise is resolved. If the order is cancelled, the promise is rejected. Promises offer a more structured way to handle asynchronous operations compared to callbacks, making code more readable and debuggable.
Promises Example [1:23:52]
The presenter converts the previous callback example into using promises. Each step is wrapped in a new promise, and the resolve function is called after the setTimeout. The .then() method is used to chain the promises, ensuring each step executes in order. The presenter also demonstrates the reject state and how to use .catch() to handle errors.
Async and Await [1:49:24]
Async/await is introduced as a way to simplify asynchronous code further. The presenter creates an async function called runSteps and uses the await keyword to ensure each step executes in order. A try/catch block is used to handle errors. The presenter emphasises that the await keyword can only be used inside an async function.
String Methods in JavaScript [2:03:16]
The presenter covers several string methods in JavaScript, including toLowerCase(), toUpperCase(), length, charAt(), concat(), includes(), slice(), split(), and trim(). Examples are provided for each method to demonstrate their usage.
Q&A and Closing Remarks [2:18:28]
The presenter enables live chat for questions and shares a compressed file with code examples with the faculty group. The presenter answers a question about the difference between synchronous and asynchronous execution, reiterating the food ordering analogy. The session concludes with thanks and a farewell.