MCQs on Array Methods

1. What is the output?

[1, 2, 3].forEach(num => num * 2);

Options:
A) [2, 4, 6]
B) undefined
C) [1, 2, 3]
D) Error

✅ Answer: B) undefined

Explanation:
forEach() does NOT return anything → returns undefined

2. What is the output?

let result = [1, 2, 3].map(num => {
    if (num > 1) return num * 2;
});

console.log(result);

Options:
A) [2, 4, 6]
B) [undefined, 4, 6]
C) [4, 6]
D) Error

✅ Answer: B) [undefined, 4, 6]

Explanation:
map() always returns same length
For num = 1, nothing returned → undefined

3. What is the output?

let arr = [1, 2, 3];
let result = arr.filter(num => num > 5);

console.log(result);

Options:
A) [undefined]
B) []
C) null
D) Error

✅ Answer: B) []

Explanation:
No elements match → empty array

4. What is the output?

let result = [1, 2, 3].reduce((acc, val) => acc + val);
console.log(result);

Options:
A) 6
B) 0
C) undefined
D) Error

✅ Answer: A) 6

Explanation:
Default acc = first element = 1
→ 1+2=3 → 3+3=6

5. What is the output?

let arr = [1, 2, 3];

let result = arr.some(num => num > 2);

console.log(result);

Options:
A) true
B) false

✅ Answer: A) true

One element (3) satisfies condition

6. What is the output?

let arr = [2, 4, 6];

let result = arr.every(num => num % 2 === 0);

console.log(result);

Options:
A) true
B) false

✅ Answer: A) true

All elements satisfy condition

7. What is the output?

let arr = [1, 2, 3];

arr.map(num => num * 2);
console.log(arr);

Options:
A) [2, 4, 6]
B) [1, 2, 3]
C) undefined
D) Error

✅ Answer: B) [1, 2, 3]

map() does NOT modify original array

8. What is the output?

let arr = [1, 2, 3];

arr.forEach(num => num * 2);
console.log(arr);

Options:
A) [2, 4, 6]
B) [1, 2, 3]

✅ Answer: B) [1, 2, 3]
forEach doesn’t change array unless you modify directly

9. What is the output?

let arr = [1, [2, 3]];

console.log(arr.flat());

Options:
A) [1, 2, 3]
B) [1, [2,3]]

✅ Answer: A) [1, 2, 3]

10. What is the output?

let arr = [1, 2, 3];

let result = arr.reduce((acc, val) => acc + val, 5);

console.log(result);

Options:
A) 6
B) 11
C) 10

✅ Answer: B) 11

5 + 1 + 2 + 3 = 11

11. What is the output?

console.log([..."hello"]);

Options:
A) "hello"
B) ["h","e","l","l","o"]

✅ Answer: B

12. What is the output?

let arr = [10, 20, 30];

console.log(arr.at(-1));

Options:
A) 10
B) 30
C) undefined

✅ Answer: B) 30