JS Functions, closures, and var/let/const scoping #96

Implement JavaScript function objects, lexical closures, and the full variable scoping model.

Scope#

Build function objects with closure support and implement var/let/const scoping rules per ECMAScript specification.

Function Objects#

• Function as a first-class value (callable object)
• Function.prototype with call, apply, bind
• Constructor calls (new operator)
• arguments object (non-strict mode)
• Rest parameters (...args)
• Default parameter values
• Arrow functions (lexical this, no arguments, no new)

this Binding#

• Default binding (global or undefined in strict mode)
• Implicit binding (method call: obj.method())
• Explicit binding (call, apply, bind)
• new binding (newly created object)
• Arrow function: inherits this from enclosing scope

Closures#

• Capture variables from enclosing scope
• Environment chain: each function has reference to outer environment
• Captured variables are live references (not copies)
• Multiple closures can share the same environment

Scoping#

• var: function-scoped, hoisted to top of function
• let/const: block-scoped, temporal dead zone (TDZ)
• const: immutable binding (assignment after init is TypeError)
• Hoisting: function declarations hoisted entirely, var declarations hoisted (init to undefined)
• Scope chain: local → enclosing → ... → global

Strict Mode#

• "use strict" directive (function and script level)
• No implicit global variable creation
• this is undefined (not global) in default binding
• No arguments.callee

Acceptance Criteria#

• Function objects are callable and constructable
• Arrow functions with lexical this
• Closures correctly capture and mutate outer variables
• var is function-scoped with hoisting
• let/const are block-scoped with TDZ enforcement
• const assignment throws TypeError
• this binding follows all four rules
• call/apply/bind work correctly
• Rest parameters and default values work
• arguments object works in non-arrow functions
• Unit tests for scoping, closures, this binding

Phase 10 — JavaScript Engine (issue 7 of 15). Depends on: JS Object model.