1. Blocks & Scope Boundaries

A block is defined by a pair of curly braces {} (such as inside if statements, for loops, or standalone blocks).

2. Why It Matters

let and const are block-scoped. Unlike legacy var (which leaks outside blocks), let and const variables die when their enclosing block finishes executing.

3. What is the Temporal Dead Zone (TDZ)?

The TDZ is the time window between when a let or const variable enters scope (Phase 1 Memory Allocation) and when it is declared and assigned a value (Phase 2 Execution). Accessing a variable while it is in its TDZ throws an uncaught ReferenceError!

if (true) {
  // === TDZ for 'score' starts here! ===
  // console.log(score); // ReferenceError: Cannot access 'score' before initialization
  
  let score = 95; // === TDZ ends here! ===
  console.log(score); // 95 (Safe to access)
}

4. Block Scope vs. Function Scope

function testBlock() {
  if (true) {
    var varVariable = "I leak out!";
    let letVariable = "I stay inside!";
  }
  
  console.log(varVariable); // "I leak out!" (var ignores block boundary!)
  // console.log(letVariable); // ReferenceError! (let is block-scoped)
}

5. Common Mistakes

• Shadowing Outer Variables Unintentionally: Declaring a let variable inside a block with the same name as an outer variable shadows (overrides) the outer variable inside that block.
• Using var in For Loops: Using var i = 0 in loops creates a single global/function loop variable shared across async iterations. Use let i = 0 so each loop iteration gets its own fresh block variable!

6. Try It Yourself

Write it yourself. The tests below run against your code.

function calculateDiscount(amount) {
  if (amount > 50) {
    const total = amount - (amount * rate);
    const rate = 0.2;
    return total;
  }
  return amount;
}

7. Next Steps

Always use let and const to benefit from block scoping and TDZ safety. Next, let's learn JavaScript's superpower: Closures!