JavaScript Runtime Environment

JavaScript Runtime Environment

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3 min read

A JavaScript (JS) runtime is a complete environment designed to execute JavaScript code. It comprises several components that work together to support the functioning of JavaScript applications. When we talk about a JS runtime, we generally mean the entire ecosystem, which goes beyond merely running the code.

The features of the runtime can vary depending on where JavaScript is executed—whether in a web browser or server-side using Node.js. For example, in a browser, the runtime includes tools for handling browser events, interacting with the DOM, and using browser-specific functionalities.

In this discussion, we will focus solely on understanding the JavaScript runtime as it operates within a web browser.

  1. The JavaScript engine is a core component of the JavaScript runtime, but it doesn’t operate in isolation. To function effectively, it relies on additional tools and resources, such as Web APIs.

    In the context of web browsers, JS runtimes include Web APIs that extend JavaScript's capabilities beyond its core language features. These APIs enable functionalities like interacting with the Document Object Model (DOM), making HTTP requests using XMLHttpRequest or fetch, managing timers, and much more. These features bridge the gap between JavaScript and the browser environment, allowing developers to build dynamic and interactive web applications.

    JS Engine contain two things

    1. Heap Memory: it is Store JS objects (object, array and function)

    2. Call Stack:

    • The JavaScript engine keeps track of the execution of synchronous code, such as functions, objects, and variables.

    • When the engine encounters a function call, it creates an execution context for that function and pushes it onto the call stack.

    • Once the function completes its execution, the engine removes (or "pops") the function's execution context from the call stack.

  1. Web APIs enhance JavaScript's functionality, allowing it to interact seamlessly with the browser environment. They enable tasks like modifying webpage structures, responding to user events, and making network requests.

    In essence, Web APIs are additional features provided to the JavaScript engine but are not inherently part of the JavaScript language itself. JavaScript accesses these APIs through the window object, which serves as a bridge between the language and the browser-specific functionalities.

  2. Asynchronous operations in JavaScript, like responding to user input or performing network requests, rely on callback functions. These callbacks are added to a queue called the callback queue, where they wait their turn for execution. The callback queue plays a crucial role in managing asynchronous tasks in a systematic and orderly way.

The event loop ensures that asynchronous operations do not block the execution thread, keeping the application responsive.

It continuously monitors the call stack and the callback queue for tasks and if when the call stack becomes empty, the event loop transfers a callback from the callback queue to the call stack, allowing it to execute.

When exploring the JavaScript runtime in a browser, it comprises several key components: the JavaScript Engine, Web APIs, callback queue, and the event loop. Among these, the event loop is vital as it transfers callback functions from the callback queue to the call stack, ensuring smooth and efficient execution of JavaScript applications.

If you want a deep dive into the JavaScript engine, be sure to check out my below detailed article on the topic!

What is a JavaScript Engine and How Does It Work?


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