The JavaScript environment

All JavaScript code runs in an environment such as a web browser (e.g., Chrome, Firefox) or a server-side runtime (e.g., Node.js, Deno). These environments provide essential components without which JavaScript wouldn’t be very useful. Here are the main components:

• Runtime engine (e.g., V8, Chakra, SpiderMonkey): parses and executes JavaScript source code. Manages memory allocation and the call stack.
• APIs: this will be dependent on the environment. For example, browsers provide document, XMLHttpRequest, setTimeout, etc. Node.js provides fs, process, buffer, etc. Electron provides clipboard, systemPreferences, etc.
• Event loop: simulates concurrency when combined with other APIs (e.g., setTimeout, XMLHttpRequest) and the callback queue.
• Callback queue: list of callbacks waiting to be added to the call stack.