JavaScript is executed by a JavaScript engine, which is a program that interprets and executes JavaScript code. Most modern web browsers, such as Google Chrome, Mozilla Firefox, and Safari, include a built-in JavaScript engine that can execute JavaScript code in web pages.

When a web page is loaded in a browser, the JavaScript engine first parses the HTML and CSS to construct the Document Object Model (DOM) and the CSS Object Model (CSSOM) respectively. Once the DOM and CSSOM are constructed, the JavaScript engine then begins to execute any JavaScript code that is included in the web page.

JavaScript code can be included in a web page in several ways, such as:

  • Inline scripts: JavaScript code that is included directly in an HTML file using the
  • External scripts: JavaScript code that is included in a separate file and referenced from the HTML file using the
  • Script modules: JavaScript code that is loaded as a module using the import statement.

Regardless of how the JavaScript code is included in the web page, the JavaScript engine will interpret and execute the code in a single-threaded manner. This means that JavaScript code is executed one statement at a time, in the order that it appears in the code.

As the JavaScript engine executes the code, it creates and manages various data structures, such as the call stack, heap, and event queue, to ensure that the code is executed correctly and efficiently. The exact implementation details of these data structures can vary between different JavaScript engines.

Additionally, because JavaScript is executed in a sandboxed environment within the web browser, it is not able to access system resources or perform low-level operations that could potentially compromise the security of the user’s computer. This helps to ensure that JavaScript code running in a web page cannot cause harm to the user’s system.

While JavaScript is not compiled in the traditional sense, modern JavaScript engines employ various techniques to optimize the performance of JavaScript code. For example, many JavaScript engines use Just-in-Time (JIT) compilation to dynamically compile frequently-used code into machine code at runtime, which can significantly improve the performance of the code. Additionally, some JavaScript engines employ Ahead-of-Time (AOT) compilation techniques to pre-compile JavaScript code into machine code before it is executed, which can further improve performance in certain scenarios.

Execution

The JavaScript interpreter in Google Chrome, like most web browsers, uses a multi-process architecture to execute JavaScript code. Here is a simplified overview of how it works:

  • Parsing: When a web page is loaded in Chrome, the HTML and JavaScript code is parsed and a Document Object Model (DOM) is created. The JavaScript code is then passed to the V8 JavaScript engine for execution.

  • V8 JavaScript engine: The V8 engine is a standalone JavaScript engine developed by Google that is used in Chrome and other Chromium-based browsers. It compiles JavaScript code into machine code using just-in-time (JIT) compilation, which optimizes the code for faster execution.

  • Execution: Once the JavaScript code is compiled, it is executed in the browser’s rendering engine, which is responsible for displaying the page to the user. The rendering engine communicates with the JavaScript engine using a messaging system, allowing the two processes to work together.

  • Garbage collection: The JavaScript engine also includes a garbage collector, which automatically frees up memory that is no longer being used by the application.

  • Web APIs: In addition to executing JavaScript code, the browser provides a set of Web APIs that allow JavaScript code to interact with the browser and the underlying operating system. These APIs include the Document Object Model (DOM), XMLHttpRequest, and Web Storage, among others.

Overall, the JavaScript interpreter in Chrome works by parsing and executing JavaScript code using the V8 engine, communicating with the rendering engine, and providing access to Web APIs for interacting with the browser and operating system.

V8

V8 engine works by parsing and executing JavaScript code using a combination of an interpreter and JIT compilation, profiling the code to identify performance bottlenecks, optimizing the code with specialized machine code, and automatically freeing up unused memory with a garbage collector.