Java Script Exam 2

1. What keyword creates a block-scoped variable that can be reassigned?

let score = 10;

A) var B) let C) const D) static

2. Which statement correctly creates a function named sum?

function sum(a, b) { return a + b; }

A) function sum(a,b) { return a + b; } B) def sum(a,b): return a + b C) fn sum(a,b) { a + b } D) let sum = (a, b) => a – b

3. What will this log?

console.log(typeof null);

A) ‘object’ B) ‘null’ C) ‘undefined’ D) ‘number’

4. Which method adds a value to the end of an array?

let a = [1,2]; a.push(3);

A) push B) pop C) shift D) unshift

5. What does === test for?

A) Value only (with type coercion) B) Type only C) Value and type (strict equality) D) Inequality

6. Which of these creates an object with a property name?

let obj = { name: 'Sam' };

A) { name: ‘Sam’ } B) [ ‘name’ : ‘Sam’ ] C) new Object(‘name’,’Sam’) D) Object(‘name’=’Sam’)

7. What will [1,2,3].map(x => x * 2) return?

[1,2,3].map(x => x * 2);

A) [1,2,3] B) [2,4,6] C) [1,4,9] D) undefined

8. Which is true about const?

A) const variables cannot be reassigned B) const is function-scoped only C) const values are immutable (deeply) D) const creates global variables

9. What is hoisting in JavaScript?

A) Moving declarations to top of scope during compile B) Garbage collection mechanism C) Sorting arrays automatically D) A type of event listener

10. Which method parses a JSON string into an object?

JSON.parse('{"a":1}');

A) JSON.stringify B) JSON.parse C) parseJSON D) eval

11. What will this expression return: [...'abc'] ?

[...'abc']

A) [‘abc’] B) [‘a’,’b’,’c’] C) [‘a’,’bc’] D) Error

12. How do you schedule a function to run after 2 seconds?

setTimeout(fn, 2000);

A) setInterval(fn,2000) B) setTimeout(fn,2000) C) delay(fn,2000) D) defer(fn,2000)

13. Which event fires when the DOM has been loaded (but before images load)?

A) load B) DOMContentLoaded C) readyState D) onrender

14. What does Array.prototype.filter return?

A) The first matched element B) A new array with elements that pass a test C) Nothing (undefined) D) The original array mutated

15. Which of these accesses the element with id ‘app’?

document.getElementById('app')

A) document.getElementById(‘app’) B) document.querySelectorAll(‘#app’) C) document.getElementsByClassName(‘app’) D) document.getElement(‘app’)

16. What is a Promise?

A) A pattern for synchronous loops B) An object representing eventual completion/failure of an async operation C) A type of callback only D) A strict equality operator

17. How do you catch errors from a Promise?

promise.then(...).catch(err => ...)

A) try { promise } catch(e) { } B) promise.then(…).catch(…) C) promise.finally(…) D) promise.retry(…)

18. What does async/await provide?

A) Tighter loops B) Syntactic sugar to write async code like synchronous code C) Replacement for Promises entirely D) A new datatype

19. Which built-in method returns the characters in a string as uppercase?

'hi'.toUpperCase()

A) toUpper() B) toUpperCase() C) upper() D) upcase()

20. How do you remove the first element from an array?

let a = [1,2,3]; a.shift();

A) pop B) shift C) slice D) splice

21. Which operator spreads an array into separate arguments?

Math.max(...[1,2,3])

A) … (spread) B) ++ C) && D) ||

22. What will 0 == '0' evaluate to?

A) true B) false C) TypeError D) undefined

23. Which method trims whitespace from both ends of a string?

'  hi  '.trim()

A) trim() B) strip() C) chomp() D) collapse()

24. What will typeof [] return?

A) ‘array’ B) ‘object’ C) ‘list’ D) ‘undefined’

25. Which regex flag makes matching case-insensitive?

/abc/i

A) g B) i C) m D) s

26. Which built-in object provides methods for rounding and generating random numbers?

A) Math B) Number C) Random D) Utils

27. What is the output of typeof NaN?

A) ‘NaN’ B) ‘number’ C) ‘undefined’ D) ‘object’

28. Which DOM API finds elements by class name?

document.getElementsByClassName('btn')

A) getElementById B) getElementsByClassName C) querySelector(‘#btn’) D) all()

29. What does event.preventDefault() do?

A) Stops event propagation B) Prevents default browser action (e.g., form submit) C) Removes the event listener D) Throws an error

30. Which of these creates a shallow copy of an object?

let copy = Object.assign({}, obj);

A) Object.assign({}, obj) B) JSON.parse(JSON.stringify(obj)) C) both A and B D) neither

31. What does document.createElement('div') do?

A) Creates and appends a div to body B) Creates a new div element but doesn’t insert it C) Throws an error D) Returns HTML string

32. Which method returns the index of the first match in an array?

A) lastIndexOf B) indexOf C) findIndex D) find

33. Which will parse integer from string?

A) Number(’10px’) B) parseInt(’10px’, 10) C) +’10px’ D) None of the above

34. Which of the following is a truthy value?

A) 0 B) ” (empty string) C) null D) ‘0’ (string containing zero)

35. What does Object.keys(obj) return?

A) An array of property names (keys) B) An array of values C) Iterable of entries D) Nothing

36. Which will stop propagation of an event so it doesn’t bubble up?

A) event.stopPropagation() B) event.preventDefault() C) event.cancel() D) return false

37. Which creates a new object that delegates to a specified prototype?

A) new B) Object.create C) =>> D) prototypeOf

38. Which will convert a value to a number, returning NaN on failure?

A) Number(’10’) B) +’10’ C) parseInt(’10’) D) All of the above (in different ways)

39. What will 0 === -0 evaluate to?

A) true B) false C) TypeError D) undefined

40. Which statement throws an exception?

A) raise B) throw C) error D) catch

41. Which returns milliseconds since 1970-01-01 UTC?

A) Date() B) Date.now() C) new Date().time() D) Date.getTime()

42. What does call() do on functions?

A) Calls function with specified this and arguments B) Creates a copy of the function C) Binds permanently D) Delays execution

43. Which removes a property from an object?

delete obj.prop;

A) remove obj.prop B) delete obj.prop C) obj.prop = null D) obj.remove(‘prop’)

44. Which is true about arrow functions?

A) They have their own this B) They do not bind their own this and inherit from surrounding scope C) They can be used as constructors with new D) They always return objects

45. What is event delegation?

A) Attaching many listeners to many elements B) Attaching a single listener to a parent and detecting target via event.target C) Removing events automatically D) Scheduling events

46. Which array method reduces values to a single value?

A) forEach B) reduce C) map D) filter

47. What does void 0 evaluate to?

A) undefined B) 0 C) null D) NaN

48. Which error type indicates invalid JSON or code syntax?

A) TypeError B) ReferenceError C) SyntaxError D) RangeError

49. Which returns true if at least one element passes a predicate?

arr.some(x => x > 5)

A) every B) some C) filter D) find

50. Which method adds an element as the last child of a node?

parent.appendChild(child)

A) appendChild B) insertBefore C) replaceChild D) removeChild

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