2 1 1 Declaring Variables

Key Concepts

To understand declaring variables in JavaScript, it is essential to grasp the following key concepts:

• Variable Declaration
• Data Types
• Variable Naming Conventions
• Scope of Variables

Variable Declaration

In JavaScript, variables are declared using the keywords var, let, and const. Each of these keywords has different behaviors and use cases:

• var: Declares a variable with function scope or global scope if declared outside any function.
• let: Declares a block-scoped variable, which is limited to the block, statement, or expression where it is defined.
• const: Declares a block-scoped, read-only named constant.

Data Types

JavaScript is a dynamically typed language, meaning variables can hold values of any data type. Common data types include:

• Number: For numeric values (e.g., 10, 3.14).
• String: For textual data (e.g., "Hello", 'World').
• Boolean: For logical values (true or false).
• Object: For complex data structures (e.g., {name: "John", age: 30}).
• Array: For ordered collections of data (e.g., [1, 2, 3]).
• Undefined: Represents a variable that has been declared but not assigned a value.
• Null: Represents the intentional absence of any object value.

Variable Naming Conventions

When naming variables in JavaScript, it is important to follow these conventions:

• Use descriptive names that reflect the purpose of the variable.
• Start variable names with a letter, underscore (_), or dollar sign ($).
• Subsequent characters can be letters, digits, underscores, or dollar signs.
• Use camelCase for variable names (e.g., firstName).
• Avoid using JavaScript reserved words (e.g., if, else, function).

Scope of Variables

The scope of a variable determines where in the code the variable can be accessed:

• var: Function-scoped or globally scoped.
• let: Block-scoped, meaning it is limited to the block, statement, or expression where it is defined.
• const: Block-scoped and read-only.

Examples

Here are some examples to illustrate the concepts:

Example 1: Variable Declaration with var

var age = 30;
function printAge() {
    var age = 25;
    console.log(age); // Output: 25
}
printAge();
console.log(age); // Output: 30
    

Example 2: Variable Declaration with let

let count = 10;
if (true) {
    let count = 20;
    console.log(count); // Output: 20
}
console.log(count); // Output: 10
    

Example 3: Variable Declaration with const

const PI = 3.14;
// PI = 3.14159; // This will throw an error
console.log(PI); // Output: 3.14
    

Example 4: Variable Naming Conventions

let firstName = "John";
let lastName = "Doe";
let age = 30;
let isStudent = false;
    

By understanding these concepts and examples, you can effectively declare and use variables in JavaScript to build dynamic and interactive web applications.