61. What is the output of console.log([] + {})?
- a) "[object Object]"
- b) "{}"
- c) TypeError
- d) 0
Answer: A - The array converts to an empty string, and the object to "[object Object]", resulting in concatenation.
62. Which method executes a function after a specified delay?
- a) setTimeout()
- b) setInterval()
- c) queueMicrotask()
- d) process.nextTick()
Answer: A -
setTimeout() schedules a one-time execution after the delay (in milliseconds).
63. What is the prototype of an object created with Object.create(null)?
- a) null (no prototype)
- b) Object.prototype
- c) undefined
- d) Array.prototype
Answer: A -
Object.create(null) creates an object with no prototype chain.
64. What does console.log(+"Infinity") output?
- a) Infinity (number type)
- b) "Infinity" (string)
- c) NaN
- d) TypeError
Answer: A - The unary
+ converts the string "Infinity" to the number Infinity.
65. Which method returns the primitive value of an object?
- a) valueOf()
- b) toString()
- c) toPrimitive()
- d) Symbol.toPrimitive
Answer: A -
valueOf() is called when an object needs to be converted to a primitive value.
66. What is the output of console.log(1 == "1")?
- a) true
- b) false
- c) NaN
- d) "11"
Answer: A - Loose equality (
==) performs type coercion, converting the string "1" to the number 1.
67. Which statement about let and const is true?
- a) Both are block-scoped
- b) Both are hoisted to the function scope
- c)
constallows reassignment - d)
letdoesn't support temporal dead zone
Answer: A -
let and const are block-scoped and hoisted but cannot be accessed before declaration (temporal dead zone).
68. What is the output of console.log(typeof (function(){}()))?
- a) "undefined"
- b) "function"
- c) "object"
- d) "null"
Answer: A - The IIFE (Immediately Invoked Function Expression) returns nothing, so its type is
undefined.
69. Which method creates a new array with all sub-array elements concatenated recursively?
- a) flat()
- b) flatMap()
- c) concat()
- d) join()
Answer: A -
flat(depth) flattens nested arrays up to the specified depth (default: 1).
70. What does console.log(new String("test") === "test") output?
- a) false
- b) true
- c) TypeError
- d) undefined
Answer: A - The
String object and primitive string are of different types (object vs string).
71. Which operator has the highest precedence?
- a) () (grouping)
- b) . (member access)
- c) new (with argument list)
- d) ++ (postfix increment)
Answer: A - Grouping
() has the highest precedence, followed by member access . and new.
72. What is the output of console.log(!!new Boolean(false))?
- a) true
- b) false
- c) "false"
- d) TypeError
Answer: A -
new Boolean(false) is an object (truthy), so !! converts it to true.
73. Which method prevents extensions to an object?
- a) Object.preventExtensions()
- b) Object.seal()
- c) Object.freeze()
- d) All of the above
Answer: A -
Object.preventExtensions() disables adding new properties (but allows modifying/deleting existing ones).
74. What is the output of console.log(+"")?
- a) 0
- b) NaN
- c) ""
- d) TypeError
Answer: A - The unary
+ coerces an empty string to 0.
75. Which method returns the first element that satisfies a condition?
- a) find()
- b) filter()
- c) some()
- d) indexOf()
Answer: A -
find() returns the first matching element (or undefined if none).
76. What is the output of console.log(3 > 2 > 1)?
- a) false
- b) true
- c) TypeError
- d) undefined
Answer: A - Evaluated as
(3 > 2) > 1 → true > 1 → 1 > 1 → false.
77. Which symbol is used as a placeholder for awaiting promises?
- a) await
- b) async
- c) then
- d) yield
Answer: A -
await pauses execution until a promise settles (must be inside an async function).
78. What is the output of console.log(Number("123abc"))?
- a) NaN
- b) 123
- c) "123abc"
- d) TypeError
Answer: A -
Number() returns NaN for strings with non-numeric characters.
79. Which method returns a string representation of an array?
- a) toString()
- b) join()
- c) String()
- d) All of the above
Answer: D - All three methods convert arrays to strings, but
join() allows custom separators.
80. What is the output of console.log(typeof class {})?
- a) "function"
- b) "class"
- c) "object"
- d) "undefined"
Answer: A - Classes are syntactic sugar over constructor functions, so their type is
"function".