Question 1: What will be the output of the following code? console.log(typeof null);
- null
- undefined
- object
- number
Explanation: Due to a historical bug, typeof null returns 'object'.
JavaScript Quiz (Advanced – Output Based) - All Questions
This JavaScript Advanced Output-Based Quiz is designed for experienced developers who want to master JavaScript quirks and edge cases. The quiz focuses entirely on predicting the output of tricky JavaScript code snippets involving type coercion, equality checks, closures, operators, and internal language behavior—commonly asked in senior-level interviews.
Question 2: What will be the output? console.log([] + []);
- []
- 0
- ""
- undefined
Explanation: Both arrays are converted to empty strings and concatenated.
Question 3: What will be the output? console.log([] + {});
- [object Object]
- {}
- NaN
- undefined
Explanation: Empty array becomes empty string, object becomes '[object Object]'.
Question 4: What will be the output? console.log({} + []);
- [object Object]
- 0
- NaN
- Error
Explanation: {} is treated as a block, +[] evaluates to 0.
Question 5: What will be the output? console.log(1 + "2" + 3);
- 6
- 123
- 15
- Error
Explanation: Number is coerced to string, resulting in string concatenation.
Question 6: What will be the output? console.log(1 + 2 + "3");
- 33
- 123
- 6
- Error
Explanation: 1 + 2 = 3, then '3' + '3' = '33'.
Question 7: What will be the output? console.log(0 == false);
- true
- false
- undefined
- Error
Explanation: Loose equality converts both to numbers.
Question 8: What will be the output? console.log(0 === false);
- true
- false
- undefined
- Error
Explanation: Strict equality checks type and value.
Question 9: What will be the output? console.log(NaN == NaN);
- true
- false
- NaN
- Error
Explanation: NaN is not equal to itself.
Question 10: What will be the output? console.log(typeof NaN);
- NaN
- undefined
- number
- object
Explanation: NaN is of type number.
Question 11: What will be the output? console.log([] == false);
- true
- false
- Error
- undefined
Explanation: [] becomes 0 and false becomes 0 during comparison.
Question 12: What will be the output? console.log([1,2] + [3,4]);
- [1,2,3,4]
- 10
- "1,23,4"
- Error
Explanation: Arrays are converted to strings and concatenated.
Question 13: What will be the output? console.log(true + false);
- true
- false
- 1
- NaN
Explanation: true becomes 1 and false becomes 0.
Question 14: What will be the output? console.log(!!"hello");
- true
- false
- "hello"
- undefined
Explanation: Non-empty strings are truthy.
Question 15: What will be the output? console.log(!!0);
- true
- false
- 0
- undefined
Explanation: 0 is a falsy value.
Question 16: What will be the output? console.log(typeof typeof 1);
- number
- string
- object
- undefined
Explanation: typeof 1 is 'number', and typeof 'number' is 'string'.
Question 17: What will be the output? console.log("5" - 3);
- 2
- "53"
- NaN
- Error
Explanation: The string is coerced to a number.
Question 18: What will be the output? console.log("5" + - - "2");
- 52
- 7
- NaN
- Error
Explanation: Double negation converts string to number, then concatenation occurs.
Question 19: What will be the output? console.log([] === []);
- true
- false
- undefined
- Error
Explanation: Arrays are compared by reference.
Question 20: What will be the output? console.log({} === {});
- true
- false
- undefined
- Error
Explanation: Objects are compared by reference.
JavaScript Quiz (Advanced – Output Based)
This JavaScript Advanced Output-Based Quiz is designed for experienced developers who want to master JavaScript quirks and edge cases. The quiz focuses entirely on predicting the output of tricky JavaScript code snippets involving type coercion, equality checks, closures, operators, and internal language behavior—commonly asked in senior-level interviews.
10
Questions
~10
Minutes
10
Min Limit