Scope & Hoisting

Scope determines where variables and functions are accessible in the code.

Hoisting is a mechanism where declarations are registered in memory before program execution.

Scope

• Function Scope — variables declared with var are visible only inside the function.
• Block Scope — variables declared with let and const are accessible only within the { ... } block.

When creating a scope, the engine forms a Lexical Environment — an internal structure where links to identifiers and their values are stored.

Each environment is linked to an external one, forming a Scope Chain.

Hoisting

• During the compilation phase, variable and function declarations are registered in memory before code execution.
• For var, the declaration is hoisted and initialized as undefined.
• For let and const, declarations are hoisted but remain in the Temporal Dead Zone (TDZ) until initialization.
• Function declarations are hoisted completely (both name and function body), so they can be called before declaration.

Key Idea

• Scope defines boundaries of variable accessibility.
• Hoisting explains why access is possible before declaration.
• Lexical Environment forms the basis for the scope chain and closures.
• TDZ prevents the use of let and const before their initialization.

The Lexical Environment lies at the core of the Closures mechanism — functions in JavaScript remember their environment and can access variables from the outer scope even after it has completed.