Chapter quiz · Objects & Classes

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8 questions on Objects & Classes. Pass 70% to clear the chapter.

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Quiz

Objects & Classes — Chapter Quiz

Eight questions on object literals, destructuring, classes, prototypes, and collections.

0/ 8

Question 1
1
What does this code log?
```
const a = { x: 1 };
const b = { x: 1 };
console.log(a === b);
```
AtrueBfalseC1DTypeError
Objects are compared by reference, not by content. a and b are two different objects, so === is false. To compare contents you'd compare property-by-property.
Question 2
2
What does this code log?
```
const user = { name: "Ada", age: 36 };
const { name: who } = user;
console.log(who);
```
A"Ada"B"name"CundefinedD{ name: "Ada" }
name: who means "extract the name property, but call it who". So who becomes "Ada".
Question 3
3
In a subclass constructor, when MUST you call super(...)?
AIt is optionalBBefore any reference to thisCAt the very end of the constructorDOnly if the parent has no constructor
A subclass constructor must call super(...) before referring to this. Accessing this before super throws ReferenceError.
Question 4
4
What does this code log?
```
class Counter {
  count = 0;
  inc() { this.count++; }
}
const c = new Counter();
c.inc(); c.inc(); c.inc();
console.log(c.count);
```
A0B3CundefinedDTypeError
Class fields like count = 0 initialize on every new instance. Three inc() calls give 3.
Question 5
5
A private class field #x can be read from outside the class with obj.#x.
TTrueFFalse
The # syntax is real privacy — #x is only accessible inside the class. Outside the class it's a SyntaxError.
Question 6
6
Which of these is the cleanest way to remove duplicates from an array tags?
AA for..of loop that builds a new arrayBtags.filter((t, i) => tags.indexOf(t) === i)C[...new Set(tags)]DYou can't deduplicate without writing a function
new Set(arr) ignores duplicates. Spreading it back into an array gives a deduplicated copy in one line. (Option b also works but is O(n²) and harder to read.)
Question 7
7
What does this code log?
```
const a = { x: 1 };
const b = { ...a, y: 2 };
a.x = 99;
console.log(b.x);
```
A1B99CundefinedDTypeError
{ ...a, y: 2 } copies a's properties into a new object. Mutating a afterward doesn't affect b — they're separate objects.
Question 8
8
When does a Map beat a plain object?
AWhen keys are non-string values (objects, functions)BWhen you need a guaranteed iteration orderCWhen you need cheap size lookupDAll of the above
All three. Map accepts any key type (plain objects coerce keys to strings), guarantees insertion-order iteration, and has a built-in .size.