20 JavaScript Practice Questions

Here are 20 JavaScript practice questions focused on variables and data types. These questions range from beginner to intermediate level to help reinforce understanding.

🔰 Basic Level Questions (1–10)

1. Declare a variable name and assign your full name to it. What is the data type?
2. Create a variable age and assign your age. Is it a string or a number?
3. Declare a variable isStudent and assign it the value true. What is the type of this variable?
4. What will be the value and type of the following variable?

   let score;
   

5. What is the output of the following code?

   let city = "Delhi";
   console.log(typeof city);
   

6. Write a program that declares 3 variables: firstName (string), age (number), and isMarried (boolean). Log them all to the console.
7. What is the output and type of:

   let x = 42 + "7";
   console.log(x);
   

8. Change the value of the variable x below and log the result.

   let x = 10;
   x = "ten";
   console.log(x);
   

9. Declare a constant PI with the value 3.14. Try to reassign it. What happens?
10. Identify the error:

    const myName;
    myName = "John";
    

🔄 Intermediate Level Questions (11–20)

11. Check the type of the following values using typeof:
    • null
    • undefined
    • NaN
    • "123"
    • 123
12. Convert the string "100" to a number and add 50 to it.
13. What is the output of:

    let a = "5";
    let b = 2;
    console.log(a * b);
    

14. Create a variable and assign the result of true + false. What do you get and why?
15. Write a script that asks the user for their name using prompt() and then logs: "Hello, <name>!".
16. What is the difference between var, let, and const in variable declarations? Give an example.
17. Predict the output:

    let a = "10";
    let b = 10;
    console.log(a == b);   // ?
    console.log(a === b);  // ?
    

18. Write a program to swap the values of two variables without using a third variable.
19. Use template literals to log a sentence: My name is <name> and I am <age> years old.
20. Write a function describePerson(name, age, isStudent) that returns a sentence like: "John is 25 years old and is a student" or "Sarah is 30 years old and is not a student" based on the boolean.