11. Built-in Functions in JS

1. String Built in Function

2. Array Built in Functions

3. Maths Built-in functions

Date built-in functions

Interview Questions

1.   let str = "Hello JavaScript!";
     console.log(str.charAt(6));
     console.log(str.indexOf("Java"));
     console.log(str.includes("Script"));
   

   Output:

   J

   6

   true

2. What does Math.random() return, and how can you generate a random number between 1 and 10?

   Math.random() returns a random decimal between 0 (inclusive) and 1 (exclusive).

    let randomNum = Math.floor(Math.random() * 10) + 1;
    console.log(randomNum);
   

3. What is the difference between map() and forEach()?

   map() → Transforms elements and creates a new array

   foreach() → Iterates but does not return a new array

    let arr = [1, 2, 3];
   
    let newArr = arr.map(x => x * 2);
    console.log(newArr); // [2, 4, 6]
   
    arr.forEach(x => console.log(x * 2)); // Logs: 2, 4, 6
   

4. What will be the output ?

    let numbers = [1, 2, 3, 4, 5];
    let result = numbers.splice(2, 2, 99);
    console.log(numbers);
    console.log(result);
    /* output
    [1, 2, 99, 5]
    [3, 4]
   

   splice(2, 2, 99) removes 2 elements from index 2 ([3, 4]) and replaces them with 99.

5. How do you format a JavaScript Date object into a readable string?

    let date = new Date();
    console.log(date.toDateString());  // Example: "Sat Feb 08 2025"
    console.log(date.toISOString());   // Example: "2025-02-08T10:30:00.000Z"
    console.log(date.toLocaleDateString()); // Example: "2/8/2025"
   

6. What will be the output ?

    console.log(typeof NaN); // number
    console.log(isNaN("Hello")); // true // "Hello" is not a number, so isNaN("Hello") → true.
    console.log(Number("  123  "));// 123
    console.log(parseInt("0123", 10)); //123
   

Next Topic :

12. DOM Manipulation - Javascript