Array creation in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Array.from with mapping function

What is the output of this JavaScript code?

Javascript

const arr = Array.from({ length: 4 }, (_, i) => i * 2);
console.log(arr);

A [2, 4, 6, 8]

B [0, 1, 2, 3]

C [0, 2, 4, 6]

D [undefined, undefined, undefined, undefined]

💡 Hint

Array.from creates an array from an object with length and applies the function to each index.

Array.from creates an array of length 4. The mapping function multiplies each index by 2, so the array is [0, 2, 4, 6].

❓ Predict Output

intermediate

Output of new Array with fill and map

What is the output of this JavaScript code?

Javascript

const arr = new Array(3).fill(1).map((x, i) => x + i);
console.log(arr);

A [1, 2, 3]

B [2, 3, 4]

C [1, 1, 1]

D [NaN, NaN, NaN]

💡 Hint

fill sets all elements to 1, then map adds the index to each element.

new Array(3) creates an array with 3 empty slots. fill(1) sets all to 1. map adds index to each element: 1+0=1, 1+1=2, 1+2=3.

❓ Predict Output

advanced

Output of spread operator with Array constructor

What is the output of this JavaScript code?

Javascript

const arr = [...Array(5).keys()];
console.log(arr);

A []

B [0, 1, 2, 3, 4]

C [undefined, undefined, undefined, undefined, undefined]

D [1, 2, 3, 4, 5]

💡 Hint

Array(5).keys() returns an iterator of indexes from 0 to 4.

Array(5) creates an array with 5 empty slots. keys() returns an iterator over indexes 0 to 4. Spread operator converts iterator to array [0,1,2,3,4].

❓ Predict Output

advanced

Output of Array constructor with string argument

What is the output of this JavaScript code?

Javascript

const arr = new Array('3');
console.log(arr.length);

A 1

B 3

C TypeError

D undefined

💡 Hint

A single numeric string argument like '3' to the Array constructor is converted to number 3 and used as the array length.

new Array('3') creates an array with one element '3' because the argument is a string, not a number.

🧠 Conceptual

expert

Which option creates an array of length 4 filled with zeros?

Which of the following JavaScript code snippets creates an array of length 4 where every element is the number 0?

A Array.from({ length: 4 }, () => 0)

B [0, 0, 0, 0]

C new Array(4).map(() => 0)

D Array(4).fill(0)

💡 Hint

map on a new Array with empty slots does not run the function.

Array(4).fill(0) creates an array of length 4 filled with zeros. [0,0,0,0] is also correct but is a literal, not a creation method. new Array(4).map(() => 0) returns an array of 4 empty slots because map does not run on empty slots. Array.from works but the question asks for the method that creates it directly.