What are the major steps involved in the execution of JavaScript code in a web browser?

When a web page containing JavaScript is loaded in a web browser, several steps are involved in the execution of the JavaScript code. These steps can be summarized as follows:

1. HTML Parsing: The web browser begins by parsing the HTML content of the web page. It constructs a Document Object Model (DOM) tree, which represents the structure of the HTML elements on the page.
2. JavaScript Parsing: When the browser encounters a script tag or an external JavaScript file, it initiates the JavaScript parsing process. The JavaScript code is fetched and parsed by the browser's JavaScript engine. The parsing stage involves tokenizing the code and creating an abstract syntax tree (AST) to represent the structure and semantics of the code.
3. Variable and Function Hoisting: During the parsing phase, the browser identifies variables and function declarations and hoists them to the top of their respective scopes. This means that they are registered in memory before the actual execution starts.
4. Execution Context Creation: Once the parsing is complete, the browser creates an execution context for the JavaScript code. An execution context consists of the scope chain, the variable object (containing variables, functions, and function parameters), and the value of the 'this' keyword.
5. Variable Initialization: In this step, the browser initializes variables and assigns them a default value (undefined) within the execution context. This process is known as variable instantiation.
6. Code Execution: The JavaScript code is executed line by line, following the order of the statements. As the code executes, it may involve various operations such as variable assignments, function calls, conditional statements, loops, and more.
7. Function Execution: When a function is encountered, a new execution context is created and pushed onto the execution context stack (call stack). The function code is executed within this new context, and the process continues until the function completes.
8. Event Handling: JavaScript in the browser often interacts with user events such as mouse clicks, keyboard input, or network events. When an event occurs, the browser executes the corresponding event handler code associated with the event.
9. DOM Manipulation: JavaScript has direct access to the DOM, allowing it to manipulate HTML elements, modify their styles, add or remove elements dynamically, or change their content. This step involves updating the DOM based on the JavaScript code's instructions.
10. Garbage Collection: As the JavaScript code executes, it creates objects and consumes memory. To manage memory efficiently, the browser's JavaScript engine performs garbage collection. It automatically identifies and removes objects that are no longer reachable, freeing up memory for reuse.
11. Error Handling: During execution, if an error occurs, JavaScript's error-handling mechanism comes into play. The browser will pause execution and look for error-handling code, such as try-catch blocks, to handle and recover from the error gracefully.
12. Completion and Page Rendering: Once the JavaScript code has been executed, the browser completes any remaining tasks, such as rendering the updated DOM and displaying the final content on the web page.

It's important to note that the execution of JavaScript code is often asynchronous due to events, callbacks, and network requests. Asynchronous operations are scheduled and executed separately from the main execution flow, allowing the browser to remain responsive to user interactions.

Understanding the major steps involved in the execution of JavaScript code helps developers optimize their code, handle errors effectively, and build performant web applications.