JavaScript quiz

Test your JavaScript knowledge with a free interactive quiz — 104 questions with answers and explanations. No signup needed to play.

What is the main difference between `for...in` and `for...of`?

1. What is the main difference between `for...in` and `for...of`?
2. How do you check if a variable is an array in JavaScript?
3. What does `typeof undeclaredVar` return (when `undeclaredVar` was never declared)?
4. What does `"hello world".includes("world")` return?
5. Which array method creates a new array with all elements that pass a test function?
6. What does this print? ```js const obj = {a: 1}; obj.a = 2; console.log(obj.a); ```
7. What is the difference between `null` and `undefined` in JavaScript?
8. What is the output of `typeof function(){}`?
9. What is the value of `x` after `const {a: x} = {a: 1};`?
10. Which of these correctly creates an object using shorthand property notation (ES6)?
11. Which statement is used to handle exceptions in JavaScript?
12. Which collects all remaining arguments into a true `Array` inside a function?

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