var vs let vs const

• Think about what happens inside a for loop with setTimeout.

Model answer.

• Scope: var is function-scoped; let and const are block-scoped (they live inside the nearest { }).
• Hoisting: all three are hoisted, but a var starts as undefined, while let/const sit in the temporal dead zone: touching them before the declaration line throws a ReferenceError.
• Redeclaration: var x twice in one scope is allowed; let/const twice is a SyntaxError.
• Reassignment: const cannot be reassigned, but the object it points to can still change.

const user = { name: 'Asha' };
user.name = 'Ravi';   // fine: the object changed, the binding did not
// user = {};         // TypeError: Assignment to constant variable

A classic bug: for (var i = 0; i < 3; i++) setTimeout(() => console.log(i)) prints 3 3 3, because there is one shared i. With let each iteration gets its own i and it prints 0 1 2.

Default: use const, switch to let only when the value must change, and avoid var.