Filter method in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this filter method?

Look at this JavaScript code. What will it print to the console?

Javascript

const numbers = [1, 2, 3, 4, 5];
const result = numbers.filter(num => num > 3);
console.log(result);

A [4, 5]

B [1, 2, 3]

C []

D [3, 4, 5]

💡 Hint

Filter keeps only items where the condition is true.

The filter method keeps numbers greater than 3, so it returns [4, 5].

❓ Predict Output

intermediate

What does this filter return?

What will be the output of this code snippet?

Javascript

const words = ['apple', 'banana', 'cherry', 'date'];
const filtered = words.filter(word => word.length === 5);
console.log(filtered);

A ['apple', 'banana']

B ['apple']

C ['apple', 'cherry']

D ['banana', 'date']

💡 Hint

Check the length of each word carefully.

Only 'apple' has exactly 5 letters, so the result is ['apple'].

❓ Predict Output

advanced

What is the output of this filter with objects?

What will this code print to the console?

Javascript

const people = [
  {name: 'Alice', age: 25},
  {name: 'Bob', age: 17},
  {name: 'Carol', age: 19}
];
const adults = people.filter(person => person.age >= 18);
console.log(adults);

A [{name: 'Bob', age: 17}]

B [{name: 'Alice', age: 25}]

C [{name: 'Alice', age: 25}, {name: 'Carol', age: 19}]

D []

💡 Hint

Filter keeps people who are 18 or older.

Alice and Carol are 18 or older, so they are included in the result.

❓ Predict Output

advanced

What happens with this filter callback?

What will this code output?

Javascript

const arr = [0, 1, 2, 3];
const filtered = arr.filter(x => x);
console.log(filtered);

A [1, 2, 3]

B [0, 1, 2, 3]

C []

D [0]

💡 Hint

Filter keeps items where the callback returns a truthy value.

0 is falsy, so it is removed. The rest are truthy and kept.

🧠 Conceptual

expert

Which option causes a runtime error?

Which of these filter uses will cause a runtime error when run?

A ['a', 'b', 'c'].filter((x, i) => i > 1)

B [1, 2, 3].filter(x => x > 1)

C [true, false].filter(x => x === true)

D [null, undefined].filter(x => x.length > 0)

💡 Hint

Check if the callback tries to access a property on null or undefined.

Accessing x.length on null or undefined causes a TypeError at runtime.