Variables: var, let, and const

Declare a binding with the right reassignment rule

Use const when a binding should not be reassigned and let when it must change. Both are block-scoped. var has function scope or a global-scope behavior that differs from block-scoped declarations, and its redeclaration rules can hide mistakes in older code.

const course = 'JavaScript';
let completed = 0;
completed += 1;
console.log(course, completed); // JavaScript 1

if (true) {
  let inside = 'block only';
  var older = 'function or global scope';
}
console.log(older);
// console.log(inside); // ReferenceError

const does not freeze an object

const student = { name: 'Asha', score: 0 };
student.score = 10; // permitted: the object changes
console.log(student.score); // 10
// student = {}; // TypeError: binding reassignment

The binding still points to the same object. Object mutation and binding reassignment are separate operations. Object.freeze can restrict an object's own properties, but it is shallow and not a general deep-immutability mechanism.

Initialization and the temporal dead zone

A let or const binding cannot be used before its initialization point in the scope. Reading it earlier throws a ReferenceError. A var declaration is initialized to undefined before its assignment executes, which can allow a mistake to travel farther before failing. Do not describe this difference merely as “let is not hoisted”; declaration processing and initialization are distinct.

Scope in a loop

A for loop with let can create separate per-iteration bindings, useful when callbacks remember the loop variable. A var loop uses one binding in its containing scope. The closures chapter explores this with runnable examples.

Practice with answers

1. Use const for a fixed course label and let for a changing question index.
2. Attempt to reassign the label and read the error.
3. Predict console.log(x); var x = 3;: undefined, before the assignment.
4. Predict a read before let y = 3; in the same scope: ReferenceError.

हिन्दी संकेत: const variable binding को दोबारा assign नहीं होने देता; उसके object की properties अपने आप immutable नहीं बनतीं।