1.22 — Exercise Questions: Array Methods

Practice questions for all ten subtopics in Section 1.22. Mix of short-answer, prediction, code-writing, and debugging tasks.

How to use this material (instructions)

1. Read lessons in order — README.md, then 1.22.a → 1.22.j.
2. Answer closed-book first — then compare to the matching lesson.
3. Code each answer — type it out in a console or editor, do not just read.
4. Interview prep — 1.22-Interview-Questions.md.
5. Quick review — 1.22-Quick-Revision.md.

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1.22.a — map() (Q1–Q8)

Q1. What does map() return? Does it mutate the original array?

Q2. Predict the output:

const result = [1, 2, 3].map(n => { n * 2 });
console.log(result);

Q3. Write a map call that converts ["hello", "world"] to ["HELLO", "WORLD"].

Q4. Given const users = [{ name: "Alice", age: 25 }, { name: "Bob", age: 30 }], use map to create ["Alice (25)", "Bob (30)"].

Q5. Predict the output:

["1", "2", "3"].map(parseInt);

Why is this surprising? How do you fix it?

Q6. What is wrong with this code?

const pairs = [1, 2, 3].map(n => { value: n });

Q7. Write a map that adds a total property to each order: [{ item: "A", price: 10, qty: 3 }, ...].

Q8. Can map change the length of the resulting array? If you want fewer elements, what should you use?

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1.22.b — filter() (Q9–Q16)

Q9. What does filter() return if no elements pass the test? Can it ever return null?

Q10. Write a filter call to get even numbers from [1, 2, 3, 4, 5, 6, 7, 8].

Q11. Predict the output:

const result = [0, 1, "", "hello", null, undefined, false, 42].filter(Boolean);
console.log(result);

Q12. Given an array of products, write a filter to select items with price < 50 AND inStock === true.

Q13. Write a one-liner to remove duplicates from [1, 2, 2, 3, 3, 3, 4] using filter.

Q14. Why should you generally filter before map in a chain?

Q15. Write a search function that filters an array of { name, email } objects by a query string (case-insensitive, matches against both name and email).

Q16. What is the difference between filter(x => x) and filter(Boolean)?

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1.22.c — reduce() (Q17–Q26)

Q17. What happens when you call reduce on an empty array without an initial value?

Q18. Write a reduce to sum all numbers in [10, 20, 30, 40]. Show the step-by-step trace.

Q19. Write a reduce to find the maximum value in [3, 7, 2, 9, 5].

Q20. Write a reduce to count the frequency of each character in "hello world" (ignoring spaces).

Q21. Write a reduce that groups this array by category:

[{ name: "A", category: "x" }, { name: "B", category: "y" }, { name: "C", category: "x" }]

Q22. What is wrong with this reduce?

const sum = [1, 2, 3].reduce((acc, n) => {
  if (n > 1) acc + n;
  return acc;
});

Q23. Explain why reduce is called the "universal" array method.

Q24. Write a reduce that flattens [[1, 2], [3, 4], [5, 6]] into [1, 2, 3, 4, 5, 6].

Q25. What is reduceRight? Give one use case.

Q26. Write a reduce to convert [["name", "Alice"], ["age", "25"], ["city", "NYC"]] into { name: "Alice", age: "25", city: "NYC" }.

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1.22.d — forEach() (Q27–Q33)

Q27. What does forEach return?

Q28. Can you break out of a forEach loop? What about return?

Q29. What is wrong with this code?

const urls = ["/api/1", "/api/2"];
urls.forEach(async (url) => {
  const data = await fetch(url).then(r => r.json());
  console.log(data);
});
console.log("All done!");

Q30. A junior developer wrote const names = users.forEach(u => u.name). What is names?

Q31. When should you use forEach instead of map?

Q32. Rewrite this forEach as a map:

const doubled = [];
[1, 2, 3].forEach(n => doubled.push(n * 2));

Q33. What loop construct should you use if you need await inside a loop?

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1.22.e — some() and every() (Q34–Q40)

Q34. What does [].some(x => true) return? What about [].every(x => false)?

Q35. Predict the output:

const nums = [2, 4, 5, 6, 8];
console.log(nums.every(n => n % 2 === 0));
console.log(nums.some(n => n % 2 !== 0));

Q36. Write a some call to check if any user in an array has role === "admin".

Q37. Write an every call to check if all fields in a form object have non-empty values.

Q38. Why is arr.some(fn) more efficient than arr.filter(fn).length > 0?

Q39. Write a one-liner to check if an array has any duplicates.

Q40. Explain what "short-circuit evaluation" means in the context of some and every.

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1.22.f — slice() and splice() (Q41–Q48)

Q41. Does slice mutate the original array? Does splice?

Q42. Predict the output:

const arr = [10, 20, 30, 40, 50];
console.log(arr.slice(1, 3));
console.log(arr.slice(-2));
console.log(arr);

Q43. Predict the output:

const arr = [10, 20, 30, 40, 50];
const removed = arr.splice(1, 2);
console.log(removed);
console.log(arr);

Q44. Write a splice call that inserts "x" at index 2 without removing any elements.

Q45. How do you remove an element at index 3 from an array without mutating the original?

Q46. What does arr.slice() (no arguments) do?

Q47. Write a pagination function using slice: given an array, page number, and page size, return the correct page.

Q48. What does splice return?

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1.22.g — sort() (Q49–Q55)

Q49. Predict the output:

console.log([10, 2, 1, 20, 3].sort());

Why is this result surprising?

Q50. Write a compare function to sort numbers in ascending order.

Q51. Write a compare function to sort objects by a name property alphabetically (case-insensitive).

Q52. Does sort() return a new array or the same array?

Q53. What is toSorted() and why would you prefer it over sort()?

Q54. Write a multi-level sort: sort by age ascending, then by name alphabetically as a tiebreaker.

Q55. What does "stable sort" mean, and since what ES version is it guaranteed?

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1.22.h — flat() and flatMap() (Q56–Q61)

Q56. Predict the output:

console.log([1, [2, [3, [4]]]].flat());
console.log([1, [2, [3, [4]]]].flat(Infinity));

Q57. What is the default depth for flat()?

Q58. How does flatMap differ from calling .map().flat()?

Q59. Write a flatMap that takes ["hello world", "foo bar"] and returns ["hello", "world", "foo", "bar"].

Q60. Use flatMap to double each number: [1, 2, 3] becomes [1, 1, 2, 2, 3, 3].

Q61. Does flat() remove holes in sparse arrays?

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1.22.i — Functional Thinking (Q62–Q67)

Q62. What is the difference between imperative and declarative code?

Q63. Name three non-mutating array methods and three mutating ones.

Q64. Why should you generally filter before map in a chain?

Q65. When is a for loop preferable to a functional chain?

Q66. Rewrite this imperative code using array methods:

const result = [];
for (let i = 0; i < nums.length; i++) {
  if (nums[i] > 10) {
    result.push(nums[i] * 2);
  }
}

Q67. What is the danger of using sort() inside a function that receives an array parameter?

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Comprehensive / Cross-topic (Q68–Q80)

Q68. Write a chain that takes an array of students, filters those with grade > 80, maps to names, and sorts alphabetically.

Q69. Given an array of transactions ({ type: "income"|"expense", amount }), calculate net income using reduce.

Q70. Write a pipeline that takes raw API user data, filters active users, transforms to { id, displayName }, and sorts by displayName.

Q71. What is the output?

const arr = [1, 2, 3];
const result = arr.map(n => n * 2).filter(n => n > 3).reduce((a, b) => a + b, 0);
console.log(result);

Q72. Implement Array.prototype.includes using some.

Q73. Write a function that takes an array of numbers and returns { min, max, sum, avg } using reduce.

Q74. Given nested arrays of tags, flatten them and return a sorted list of unique tags.

Q75. Write a one-liner to check if all strings in ["hello", "world", "foo"] have length >= 3.

Q76. What is the output?

const arr = [1, 2, 3, 4, 5];
console.log(arr.slice(1, -1));
console.log(arr.filter(n => n !== 3));
console.log(arr);

Q77. Why does [10, 9, 8].sort() produce [10, 8, 9]?

Q78. Write a reduce that implements flatMap functionality for [[1, 2], [3], [4, 5]].

Q79. What is the difference between find and filter? When would you use each?

Q80. Write a function that takes an array of objects and a key, and returns the objects grouped by that key (generic group-by).

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Answer hints

Q	Hint
Q2	Missing return — curly braces need explicit return
Q5	parseInt(element, index) — index is used as radix
Q11	filter(Boolean) removes all falsy values
Q17	TypeError: Reduce of empty array with no initial value
Q27	undefined — always
Q34	some([]) = false, every([]) = true (vacuous truth)
Q43	removed = [20, 30], arr = [10, 40, 50]
Q49	Lexicographic: [1, 10, 2, 20, 3]
Q71	map: [2, 4, 6] → filter: [4, 6] → reduce: 10
Q77	Default sort converts to strings: "10" < "8" < "9"

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