Lesson 3

Variable Declaration

Learn how to declare variables to reserve memory before storing values.

Variable declaration is the process of creating a variable and telling the programming language that the variable exists. It reserves memory for the variable.

After declaration, we can optionally assign a value to it.

1let age;2const PI = 3.14;3var name;

Declared Variable

let age;

Memory (empty)

Initialized Variable

let age = 25;

Memory (with value)

Bad Example

age = 25;

Error! age is not declared. Declare a variable first.

Good Example

let age = 25;

Correct! Variable is declared and initialized.

Using undeclared variables.
Redeclaring a variable with const.
Confusing let and var.
Forgetting to initialize when needed.

Declare variables as shown below.

JavaScript

1// Declare a variable for your name
2// Declare a variable for your age
3// Declare a constant for PI
4// Print them

Declaration creates a variable.
Initialization assigns a value.
Prefer let and const.
Avoid var in modern JavaScript.