Module 43 // Core JavaScript

ECMAScript Internals

Module Objective

Execution contexts, realms, jobs, abstract operations

Mental Model Realtime Simulation

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Practical Code Examples

// Example 1
// 1. Internal Slots
const obj = {};
// We can't access [[Prototype]] directly, 
// but we use getPrototypeOf()
console.log(Object.getPrototypeOf(obj));
💡 Internal slots are bracketed in the spec (like `[[Value]]`). They are hidden data fields within an object that only the engine can manipulate directly.
// Example 2
// 2. Abstract Operations
// When you run obj.prop, the engine triggers [[Get]]
const val = obj.prop;

// When you call a function, the engine triggers [[Call]]
myFunc();
💡 The spec defines algorithms (Abstract Operations) for every action. For example, `[[Get]]` defines how the engine walks the prototype chain.
// Example 3
// 3. Different Realms (Iframes)
const iframe = document.createElement('iframe');
document.body.appendChild(iframe);

const IframeArray = iframe.contentWindow.Array;
console.log([] instanceof IframeArray); // false!
💡 Realms are isolated execution environments with their own global objects and built-ins. An array from one realm is not an `instanceof` the constructor from another realm.

Engine & Memory Architecture

Inside the Specification

1. Internal Slots:

  • Stored in a reserved area of the object's memory in the Heap.
  • Not accessible via standard property iteration.

2. Agents & Jobs:

  • An Agent is a thread plus the Event Loop.
  • The engine uses Job Queues (like the Microtask queue) to manage pending operations defined by the spec.

3. Execution Context Stack:

  • The spec defines exactly how contexts are pushed/popped on the RAM.
  • It tracks the LexicalEnvironment and VariableEnvironment as physical pointers in the stack frame.
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