Variable Declarations: var vs let vs const & Temporal Dead Zone (TDZ)

1Concept

var is function-scoped and hoisted with undefined. let and const are block-scoped {} and hoisted into the Temporal Dead Zone (TDZ), throwing ReferenceError if accessed before declaration.

2Architecture Diagram

Scope Entry {
  [ Temporal Dead Zone (TDZ) - accessing let/const throws ReferenceError ]
  let x = 10; // Initialization ends TDZ
  // Accessible here
}

3Code Example

function scopeDemo() {
    if (true) {
        var functionScoped = "I leak out of if-blocks!";
        let blockScoped = "I am contained inside this block!";
        const immutableRef = { score: 100 };
        immutableRef.score = 200; // Mutating object property is ALLOWED
    }
    console.log(functionScoped);
    // console.log(blockScoped); -> ReferenceError!
}
scopeDemo();

4Expected Output

I leak out of if-blocks!

5Key Takeaways

• ✓Default to const; use let only when variable reassignment is required.
• ✓Never use var in modern JavaScript.
• ✓const objects can still have their properties mutated.