Variables: var, let & const

Understand the concept

var, let and const declare variables, but they have different scope and initialization rules. let and const are block-scoped, making it easier to keep a variable inside the code that needs it. const prevents reassignment of the binding; it does not make an object immutable.

Important ideas

• var: function scope; reading before its declaration gives undefined.

• let: block scope; can be reassigned.

• const: block scope; cannot be reassigned, but object contents can change.

• Use const by default when the binding will not change, and let when reassignment is needed.

• Redeclaring a let or const in the same scope is an error; var allows many redeclarations and can hide mistakes.

Comparison:

Work through the example

const user = { name: 'Asha' };
user.name = 'Neha'; // Allowed: changes the object.
// user = {};      // Not allowed: reassigns the binding.