Intro to JavaScript Variables

Learning objective: By the end of this lesson, students will understand the basics of JavaScript variables, including naming conventions and the difference between let and const.

Concepts

The purpose of a variable in programming is to hold or reference data.

📚 A variable is a named container used to store and retrieve data used within a program.

We use the let or const (short for constant) keyword to declare a variable in JavaScript. These are special keywords known as declarations. The variable name or identifier follows the declaration.

📚 A declaration is used to indicate a variable is being created. An identifier is then used to identify a variable.

let favSnack;
// let is the declaration
// favSnack is the identifier

We can also assign a value to a variable at the time we declare it by using the = (assignment) operator:

let name = 'Tony Stark';

and change its value later:

name = 'Pepper Potts';
// note that we only declare a variable once

Naming rules

In JavaScript, when naming variables, the convention is to name the identifiers using lowerCamelCase. Good examples are numActivePlayers, tacoFlavor, or name.

Identifiers in JS:

• Are case-sensitive
• Cannot begin with a number
• Can contain letters, digits, underscores, and dollar signs
• Cannot contain spaces
• Cannot be reserved words or keywords

❓ Is car-3 a valid variable identifier?

let and const

The difference between let and const is that a const variable cannot be re-assigned to - you can assign data to it once, and that’s it:

let x = 25;
x = 100; // no problem
const z = 25;
z = 100; // Uncaught TypeError: Assignment to constant variable