JavaScript - Variables

Overview

Estimated time: 15–25 minutes

Variables store values for later use. In modern JavaScript, prefer let and const over var.

Learning Objectives

• Declare variables and initialize values.
• Choose between let and const.
• Follow clear naming and scoping conventions.

Prerequisites

• JavaScript - Strict Mode

Declaring variables

let count = 0;       // changeable
const pi = 3.14159;  // constant binding
// Avoid var in modern code (function-scoped, hoisted)

Naming

• Use letters, digits, _, $; cannot start with a digit.
• Use camelCase for variables and functions; PascalCase for classes.
• Be descriptive: userCount, totalPrice.

Initialization and reassignment

let total = 10;
total = total + 5;  // ok

const max = 100;
// max = 200; // TypeError: Assignment to constant variable.

Common Pitfalls

• Using var leads to function-scoped variables and hoisting surprises.
• Reassigning const bindings throws; for objects, the binding is constant but contents can mutate.

Checks for Understanding

1. When should you choose const?
2. Is let block-scoped or function-scoped?

Exercises

1. Refactor a snippet using var to use let/const appropriately; explain your choices.
2. Create a constant object and mutate one of its properties; explain why this is allowed.