Chapter quiz · Data & Types

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8 questions on Data & Types. Pass 70% to clear the chapter.

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Quiz

Data & Types — Chapter Quiz

Eight questions covering strings, numbers, booleans, and arrays.

0/ 9

Question 1
1
What does typeof null return?
A"null"B"object"C"undefined"D"primitive"
typeof null returns "object" — a historical bug that can't be fixed without breaking the web. Check for null with value === null.
Question 2
2
What does this code log?
```
console.log("5" + 3);
```
A8B"53"CNaNDTypeError
+ with a string on either side concatenates. "5" + 3 converts 3 to a string and joins → "53".
Question 3
3
What does this code log?
```
console.log(0.1 + 0.2 === 0.3);
```
AtrueBfalseC0.3DNaN
Floating-point precision. 0.1 + 0.2 is actually 0.30000000000000004, so the strict equality returns false.
Question 4
4
Which of these values are **falsy**?
Select all that apply.
A0B""C"0"D[]EnullFundefined
The eight falsy values: false, 0, -0, 0n, "", null, undefined, NaN. "0" is a non-empty string (truthy), and [] is an object (truthy).
Question 5
5
To get the **last** element of an array arr cleanly in modern JS, you call arr._____. (Fill in the method call.)
arr.at(-1) returns the last element. (Before at(), you'd write the clunkier arr[arr.length - 1].)
Question 6
6
What does this code log?
```
const nums = [10, 1, 2];
nums.sort();
console.log(nums[0]);
```
A1B10C2Dundefined
Default sort converts items to strings. As strings, "1" sorts before "10" and "2", so nums[0] is 1. To sort numerically you'd write nums.sort((a, b) => a - b).
Question 7
7
Which array method returns a **new** array of items that pass a predicate?
AmapBfilterCreduceDforEach
filter(predicate) returns the items for which the predicate returned true. map transforms every item; reduce produces a single value; forEach returns undefined.
Question 8
8
After const [a, ...rest] = [1, 2, 3, 4];, what is rest?
A2B[2]C[2, 3, 4]D[1, 2, 3, 4]
The rest pattern ...rest collects everything not already assigned into an array. a gets 1; rest gets [2, 3, 4].