Understanding JavaScript Engines and Runtimes

Aug 26, 2024

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.