Array length property in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code?

Consider the following JavaScript code. What will be logged to the console?

Javascript

const arr = [1, 2, 3, 4];
arr.length = 2;
console.log(arr.length, arr);

A 4 [1, 2, 3, 4]

B 2 [1, 2, 3, 4]

C 4 [1, 2]

D 2 [1, 2]

💡 Hint

Changing the length property can truncate the array.

Setting arr.length to 2 cuts off elements after index 1, so arr becomes [1, 2] and length is 2.

❓ Predict Output

intermediate

What is the output of this code?

What will this code print to the console?

Javascript

const arr = [];
arr[3] = 'hello';
console.log(arr.length);

A 3

B undefined

C 4

D 0

💡 Hint

Array length is one more than the highest index.

Assigning to index 3 sets length to 4 because length is highest index + 1.

❓ Predict Output

advanced

What is the output of this code?

What will this code output?

Javascript

const arr = [1, 2, 3];
arr.length = 5;
console.log(arr[3], arr[4], arr.length);

A undefined undefined 5

B 3 5 5

C undefined undefined 3

D 3 denifednu denifednu

💡 Hint

Increasing length adds empty slots, but does not add values.

Setting length to 5 adds empty slots at indexes 3 and 4, which are undefined.

❓ Predict Output

advanced

What error does this code raise?

What happens when this code runs?

Javascript

const arr = [1, 2, 3];
Object.defineProperty(arr, 'length', {writable: false});
arr.length = 1;
console.log(arr.length);

A undefined

B TypeError

C 1

D 3

💡 Hint

Trying to change a non-writable property causes an error.

Setting length when it is non-writable throws a TypeError in strict mode.

🧠 Conceptual

expert

How many elements does this array have?

Given this code, how many elements does the array have?

Javascript

const arr = [,,];
console.log(arr.length);

A 2

B 1

C undefined

D 0

💡 Hint

Empty slots count towards length.

The array has two empty slots, so length is 2.