JavaScript Basics

JavaScript Syntax

JavaScript syntax is the set of rules that define how a JavaScript program is written and interpreted.

  • JavaScript is case-sensitive
  • Statements end with a semicolon ; (optional, but recommended)
  • Statements are executed top to bottom, left to right
let message = "Hello!";
console.log(message);

JavaScript Statements

A statement is a line of code that performs an action.

let x = 10;       // Statement 1
x = x + 5;        // Statement 2
console.log(x);   // Statement 3

Multiple statements can be grouped using curly braces {} (called a code block):

{
    let name = "John";
    console.log(name);
}

JavaScript Comments

Comments are used to explain code and are ignored by the browser.

  • Single-line comment:
// This is a single-line comment
  • Multi-line comment:
/* This is a 
multi-line comment */

Variables in JavaScript

Variables store data values. You can declare them using:

var (Old way)

var name = "Alice";

let (Modern, block-scoped)

let age = 25;

const (Constant - value cannot change)

const country = "India";
Keyword Can Reassign? Scope Type
var Yes Function-level
let Yes Block-level
const No Block-level

JavaScript Data Types

JavaScript is dynamically typed, meaning you don't need to declare a data type.

Primitive Types:

  • String - "Hello"
  • Number - 42, 3.14
  • Boolean - true, false
  • Undefined - variable declared but not assigned
  • Null - intentional absence of value
  • Symbol - unique identifiers (advanced use cases)
  • BigInt - very large numbers (added in ES2020)

Non-Primitive Types:

  • Object
  • Array
  • Function
let name = "Tom";           // String
let age = 30;               // Number
let isOnline = true;        // Boolean
let person = {name: "Tom", age: 30}; // Object

JavaScript Operators

JavaScript supports several kinds of operators:

1. Arithmetic Operators

+  -  *  /  %  **  ++  --

2. Assignment Operators

=  +=  -=  *=  /=  %=  **=

3. Comparison Operators

==   ===   !=   !==   >   <   >=   <=
  • == checks value only
  • === checks value and type (recommended)

4. Logical Operators

&&   ||   !
true && false // false
true || false // true
!false        // true

5. String Concatenation Operator

let fullName = "John" + " " + "Doe"; // "John Doe"

Type Conversion (Casting)

👉 Implicit (Automatic)

let result = "5" + 2;    // "52" (number 2 converted to string)

👉 Explicit (Manual)

Number("10");     // 10
String(20);       // "20"
Boolean(0);       // false

typeof Operator

Used to check the type of a variable.

typeof "hello";    // "string"
typeof 5;          // "number"
typeof true;       // "boolean"
typeof undefined;  // "undefined"
typeof null;       // "object" (a quirk in JavaScript)

🧪 Practice Tip:

Create a small script that:

  1. Declares variables using let and const
  2. Uses arithmetic and logical operators
  3. Outputs results with console.log()

Example:

let a = 10;
let b = 5;
let sum = a + b;
                                    
console.log("Sum:", sum);
console.log("Are both greater than 0?", a > 0 && b > 0);
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