Reduce method in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this reduce example?

Consider the following JavaScript code using the reduce method. What will be printed to the console?

Javascript

const numbers = [1, 2, 3, 4];
const sum = numbers.reduce((acc, val) => acc + val, 0);
console.log(sum);

A undefined

B 24

C 10

D TypeError

💡 Hint

Think about how reduce adds each number starting from 0.

The reduce method adds all numbers starting from 0, so 1+2+3+4 equals 10.

❓ Predict Output

intermediate

What does this reduce code output?

Look at this code that uses reduce to find the longest word in an array. What will it print?

Javascript

const words = ['cat', 'elephant', 'dog', 'hippopotamus'];
const longest = words.reduce((a, b) => a.length > b.length ? a : b);
console.log(longest);

A "hippopotamus"

B TypeError

C "dog"

D "elephant"

💡 Hint

Reduce compares lengths and keeps the longer word.

The reduce compares each word and keeps the longest. 'hippopotamus' is longest.

❓ Predict Output

advanced

What is the output of this reduce with objects?

This code uses reduce to count how many times each color appears. What is the output?

Javascript

const colors = ['red', 'blue', 'red', 'green', 'blue', 'blue'];
const count = colors.reduce((acc, color) => {
  acc[color] = (acc[color] || 0) + 1;
  return acc;
}, {});
console.log(count);

A {"red":3,"blue":2,"green":1}

B {"red":1,"blue":1,"green":1}

C TypeError

D {"red":2,"blue":3,"green":1}

💡 Hint

Each color is counted by adding 1 to its current count.

The reduce counts each color's occurrences correctly: red 2, blue 3, green 1.

❓ Predict Output

advanced

What error does this reduce code cause?

What error will this code produce when run?

Javascript

const arr = [];
const result = arr.reduce((a, b) => a + b);
console.log(result);

A SyntaxError

B TypeError: Reduce of empty array with no initial value

C undefined

D 0

💡 Hint

Think about what happens when reduce has no initial value and the array is empty.

Reduce throws a TypeError if the array is empty and no initial value is given.

🧠 Conceptual

expert

Which option produces the product of all numbers using reduce?

You want to multiply all numbers in an array using reduce. Which option correctly does this?

A arr.reduce((acc, val) => acc * val, 1)

B arr.reduce((acc, val) => acc + val, 1)

C arr.reduce((acc, val) => acc * val)

D arr.reduce((acc, val) => val * val, 1)

💡 Hint

Start with 1 and multiply each value to the accumulator.

Option A multiplies all numbers starting from 1. Option A fails on empty arrays. B adds instead of multiplies. D multiplies val by itself, ignoring acc.