JavaScript has three ways to declare a variable, and choosing the wrong one is one of the most common interview mistakes.

var   → function-scoped, hoisted, re-declarable   (avoid)
let   → block-scoped, not hoisted*, reassignable   (default choice)
const → block-scoped, not hoisted*, not reassignable (prefer for values that don't change)

Block scope vs function scope — the most important distinction:

if (true) {
  var x = 1;   // leaks out of the block!
  let y = 2;   // stays inside
}
console.log(x);  // 1 — var ignores block boundaries
console.log(y);  // ReferenceError — let respects them

JavaScript's seven primitive types

JavaScript has exactly 7 primitive types. Memorise them — interviewers ask this directly:

Everything else — arrays, functions, dates, regular expressions — is an object at runtime.

The const trap

const means the binding can't be reassigned — not that the value is immutable:

const nums = [1, 2, 3];
nums.push(4);       // ✓ fine — mutating the array, not reassigning the binding
nums = [1, 2, 3, 4]; // ✗ TypeError: Assignment to constant variable

const config = { debug: false };
config.debug = true; // ✓ fine — mutating the object
config = {};         // ✗ TypeError

For true immutability, use Object.freeze() — but note it's shallow.

Which to use when

• const — default. Use it whenever you won't reassign.
• let — when you need to reassign (loop counters, accumulator variables).
• var — never in new code. Only in old codebases you're maintaining.

The next section covers the typeof operator — and its famous exceptions.