1. Primary data types

→ They are immutable

→ JS have seven primitive data types

2. Non Primitive data types ( objects , stores by refrence)

   Objects are the collection of key- value pairs and value can be of any type , including function and other object.

   We create object using following ways in JS:

   1. Using Object Literals

    let person = {
        name: "John",
        age: 30,
        greet: function() {  // value is of function type here 
            console.log("Hello, " + this.name);
        }
    };
    person.greet(); // Hello, John

2. Using new Object()

    let car = new Object();
    car.brand = "Toyota";
    car.model = "Corolla";
    car.getInfo = function() {
        return `${this.brand} ${this.model}`;
    };
    console.log(car.getInfo()); // Toyota Corolla

3. Built-in Objects in JS

   Built-in objects, or “global objects”, are the objects that are accessible at the global scope.

   Some Imp. built in objects are

   1. Math Object - Provides mathematical functions and constants.

   2. Date Object - Handles date and time.

   3. String Object - Provides string manipulation methods.

   4. Array Object - Handles lists of data.

   5. JSON Object - Used for working with JSON data.

// Maths Object
console.log(Math.PI); // 3.141592653589793
console.log(Math.sqrt(16)); // 4
console.log(Math.random()); // Random number between 0 and 1

// date object 
let now = new Date();
console.log(now.toDateString()); // Example: Mon Feb 06 2025

// String object
let text = "Hello, World!";
console.log(text.length); // 13
console.log(text.toUpperCase()); // HELLO, WORLD!
console.log(text.includes("World")); // true

// Arary object
let numbers = [10, 20, 30];
console.log(numbers.push(40)); // Adds 40 to the array
console.log(numbers.pop()); // Removes last element


//JSON Object
let jsonString = '{"name": "John", "age": 30}';
let obj = JSON.parse(jsonString);
console.log(obj.name); // John
console.log(JSON.stringify(obj)); // Convert back to JSON

4. typeof operator

   The typeof operator is used to determine the type of a variable.

console.log(typeof 42); // "number"
console.log(typeof "Hello"); // "string"
console.log(typeof true); // "boolean"
console.log(typeof {}); // "object"
console.log(typeof []); // "object" (Arrays are objects)
console.log(typeof function(){}); // "function"
console.log(typeof null); // "object" (This is a JavaScript bug)
console.log(typeof undefined); // "undefined"

Interview Questions

1. What is the difference between null and undefined?

   undefined means a variable is declared but not assigned a value.

   null is explicitly assigned to indicate "no value".

2. What will be the output of the following?

    console.log(typeof NaN); 
    console.log(typeof null);
   

   number

   object

3. How do you add and delete properties from an object?

    let person = { name: "Alice" };
    person.age = 25; // Adding property
    delete person.name; // Deleting property
    console.log(person); // { age: 25 }
   

4. What is the difference between JSON.parse() and JSON.stringify()?

   • JSON.parse() converts a JSON string into an object.

   • JSON.stringify() converts an object into a JSON string.

    let jsonStr = '{"name": "John"}';
    let obj = JSON.parse(jsonStr); // Converts to object
    console.log(obj.name); // John

    let str = JSON.stringify(obj); // Converts to JSON string
    console.log(str); // '{"name":"John"}'

5. How can you generate a random number in JavaScript?

    let randomNum = Math.random(); // Between 0 and 1
    let randomInt = Math.floor(Math.random() * 10) + 1; // Between 1 and 10
   

Intermediate Topics

1. Object Prototype

   Click Here

2. Prototypal Inheritence

   Click Here

Next Topic

Type conversion and == vs ===