JavaScript Engines and Runtimes

Overview of JavaScript Engines

• Definition: A JavaScript engine is a computer program that executes JavaScript code.
• Examples:
  • Google's V8 engine (powers Chrome and Node.js)
  • Different browsers have their own engines (look up online for more info)

Components of a JavaScript Engine

• Call Stack: Executes code using execution contexts.
• Heap: Unstructured memory pool for storing application objects.

Code Translation: Compilation vs. Interpretation

• Compilation:
  • Entire source code is converted to machine code at once.
  • Resulting machine code is saved in a portable file for future execution.
• Interpretation:
  • Code is executed line by line by an interpreter.
  • Machine code conversion happens just before execution.

JavaScript's Historical Context

• Previously considered a purely interpreted language.
• Modern practices have evolved due to performance needs.
• Just-in-Time (JIT) Compilation:
  • Combines aspects of both compilation and interpretation.
  • Code is compiled to machine code immediately before execution without portable files.

Execution Process in JavaScript Engines

1. Parsing:
   • JavaScript code is read and converted into an Abstract Syntax Tree (AST).
   • Syntax errors are checked during this process.
2. Compilation:
   • AST is compiled into machine code.
   • Code is executed immediately after compilation.
3. Optimization:
   • Initial unoptimized code is executed while background optimizations occur.
   • Optimized code replaces unoptimized code without stopping execution.

JavaScript Runtime

• Definition: A JavaScript runtime is a container that includes everything needed to execute JavaScript, primarily in a browser context.
• Components:
  • JavaScript Engine: Core component that executes code.
  • Web APIs: Provide additional functionalities (e.g., DOM manipulation, console logging).
  • Callback Queue: Stores callback functions ready for execution.

Event Loop

• Manages the Call Stack and Callback Queue.
• Executes callbacks when the Call Stack is empty, facilitating non-blocking programming.

Node.js and Other Runtimes

• Node.js: A JavaScript runtime outside of the browser.
• Lacks web APIs; instead includes C++ bindings and a thread pool.

Conclusion

• Understanding JavaScript engines and runtimes is crucial for effective JavaScript development.
• Next lecture will cover JavaScript execution in the call stack.