41. What is the output of console.log(typeof NaN)?
- a) "number"
- b) "NaN"
- c) "undefined"
- d) "string"
Answer: A - Despite its name ("Not a Number"),
NaN is technically of type "number".
42. Which method creates a new array with transformed elements?
- a) map()
- b) filter()
- c) reduce()
- d) forEach()
Answer: A -
map() applies a function to each element and returns a new array.
43. What does document.querySelector() return if no match is found?
- a) null
- b) undefined
- c) false
- d) throws an error
Answer: A - It returns
null when no matching element exists in the DOM.
44. Which operator is used for exponentiation in JavaScript?
- a) **
- b) ^
- c) Math.pow()
- d) //
Answer: A - The exponentiation operator (
**) raises a base to a power (e.g., 2 ** 3 → 8).
45. What is the output of console.log([] == ![])?
- a) true
- b) false
- c) TypeError
- d) undefined
Answer: A - Due to type coercion, both sides evaluate to
false ([] becomes "" then 0, ![] becomes false then 0).
46. Which method converts an array to a string?
- a) join()
- b) toString()
- c) String()
- d) All of the above
Answer: D - All three methods can convert arrays to strings, but
join() allows custom separators.
47. What is the purpose of the finally block in a try-catch?
- a) Executes regardless of try/catch outcome
- b) Handles uncaught errors
- c) Replaces the catch block
- d) Stops execution
Answer: A -
finally runs whether the try succeeds or fails, useful for cleanup.
48. What does console.log(!!0) output?
- a) false
- b) true
- c) 0
- d) "0"
Answer: A -
0 is falsy, so !!0 converts it to boolean false.
49. Which method returns the first index where an element is found in an array?
- a) indexOf()
- b) findIndex()
- c) search()
- d) Both A and B
Answer: D - Both
indexOf() (for primitive values) and findIndex() (for complex conditions) return the first matching index.
50. What is the output of console.log("b" + "a" + +"a" + "a")?
- a) "baNaNa"
- b) "baaa"
- c) "baundefineda"
- d) TypeError
Answer: A - The unary
+ coerces "a" to NaN, resulting in concatenation: "b" + "a" + "NaN" + "a".
51. Which statement about const is true?
- a) It must be initialized during declaration
- b) It can be reassigned
- c) It is hoisted to the global scope
- d) It allows duplicate declarations
Answer: A -
const requires initialization and cannot be reassigned (though object properties can be modified).
52. What is the output of console.log(0.1 + 0.2)?
- a) 0.30000000000000004
- b) 0.3
- c) NaN
- d) "0.30"
Answer: A - Floating-point precision issues cause this result (not exactly
0.3).
53. Which method checks if an object has a specific property?
- a) hasOwnProperty()
- b) in operator
- c) Object.keys()
- d) Both A and B
Answer: D -
hasOwnProperty() checks for non-inherited properties, while in includes inherited ones.
54. What does console.log(typeof (() => {})) output?
- a) "function"
- b) "object"
- c) "arrow"
- d) "undefined"
Answer: A - Arrow functions are still of type
"function".
55. Which method delays execution until the call stack is clear?
- a) setTimeout(fn, 0)
- b) setImmediate() (Node.js only)
- c) process.nextTick() (Node.js only)
- d) All of the above
Answer: D - All three delay execution, but prioritization differs (
nextTick() > setImmediate() > setTimeout() in Node.js).
56. What is the output of console.log(new Boolean(false))?
- a) Boolean object (truthy)
- b) false
- c) "false"
- d) TypeError
Answer: A -
new Boolean(false) creates a truthy object wrapper, not a primitive.
57. Which method creates a shallow copy of an object?
- a) Object.assign({}, obj)
- b) {...obj} (spread operator)
- c) JSON.parse(JSON.stringify(obj)) (deep copy)
- d) Both A and B
Answer: D - Both
Object.assign() and the spread operator create shallow copies.
58. What is the output of console.log(1 < 2 < 3)?
- a) true
- b) false
- c) TypeError
- d) undefined
Answer: A - Evaluated as
(1 < 2) < 3 → true < 3 → 1 < 3 → true.
59. Which method reverses an array in place?
- a) reverse()
- b) toReversed() (ES2023)
- c) sort() with a custom comparator
- d) None of the above
Answer: A -
reverse() mutates the original array. toReversed() (new in ES2023) returns a new reversed array.
60. What does console.log(+"") output?
- a) 0
- b) NaN
- c) ""
- d) TypeError
Answer: A - The unary
+ coerces an empty string to 0.