JavaScript Variables and Data Types MCQs

1. Variables in JavaScript

In JavaScript, variables are used to store data that can be used and manipulated throughout your program. JavaScript allows you to declare variables using var, let, and const.

Declaring Variables

There are three main ways to declare variables in JavaScript:

var x = 10;   // Declares a variable globally or function-scoped
let y = 20;   // Block-scoped, preferred for variables that may change
const z = 30; // Block-scoped, used for constants (cannot be reassigned)

• var is function-scoped and can be redeclared and updated. It is hoisted, meaning it is moved to the top of its scope during execution.
• let is block-scoped and allows you to declare variables that can be reassigned. It is not hoisted in the same way as var.
• const is also block-scoped but must be assigned a value at the time of declaration and cannot be reassigned.

Differences Between var, let, and const

Keyword	Scope	Reassignable	Hoisted	Use Case
var	Function	Yes	Yes (but initialized as undefined)	Legacy code; avoid using in modern JavaScript
let	Block	Yes	No (in temporal dead zone until declared)	Use when value will change later
const	Block	No	No	Use for constants or fixed values

Examples

// Using let
let age = 25;
age = 26; // valid

// Using const
const pi = 3.14159;
pi = 3.14; // Error: Assignment to constant variable

// Using var
function example() {
  if (true) {
    var test = "hello";
  }
  console.log(test); // works due to function scope
}

JavaScript: Variables and Data Types

1. Which keyword is used to declare a variable that cannot be reassigned in JavaScript?

2. Which of the following is a primitive data type in JavaScript?

3. What will be the output of the following code?

    var x = 10;
    var x = 20;
    console.log(x);
    

4. Which operator is used to check the data type of a variable in JavaScript?

5. Which of the following is the correct way to declare a string variable in JavaScript?

6. What will be the output of the following code?

    let result = 3 + '5';
    console.log(result);
    

7. Which data type can represent very large integers in JavaScript?

8. What will happen when the following code is executed?

    const num = 5;
    num = 10;
    console.log(num);
    

9. What is the difference between 'null' and 'undefined' in JavaScript?

10. What will be the output of the following code?

    let value = null;
    console.log(typeof value);
    

11. Which of the following keywords is block-scoped in JavaScript?

12. Which data type was introduced in ES6 to create unique values?

13. What will be the output of the following code?

        let a = 10;
        let b = '10';
        console.log(a + b);
        

14. What does 'NaN' stand for in JavaScript?

15. What is the initial value of a variable that has been declared but not yet assigned a value in JavaScript?

16. What will be the output of the following code?

        let y = 10;
        if (true) {
            let y = 20;
        }
        console.log(y);
        

17. Which of the following declarations allows a variable to be accessible outside of a block?

18. Which statement is true about 'null' and 'undefined'?

19. What happens if you try to declare a 'const' variable without initializing it?

        const value;
        console.log(value);
        

20. Which data type in JavaScript can store large integers beyond the safe integer limit?