Count Words with Given Prefix in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of prefix count function with simple input

What is the output of the following JavaScript code that counts words starting with a given prefix?

DSA Javascript

function countWordsWithPrefix(words, prefix) {
  let count = 0;
  for (const word of words) {
    if (word.startsWith(prefix)) {
      count++;
    }
  }
  return count;
}

const words = ['apple', 'app', 'application', 'banana', 'apex'];
console.log(countWordsWithPrefix(words, 'app'));

A 2

B 3

C 4

D 1

💡 Hint

Count how many words start exactly with 'app'.

The words starting with 'app' are 'apple', 'app', and 'application', so the count is 3.

🧠 Conceptual

intermediate

Understanding prefix matching in strings

Which of the following statements correctly describes how to check if a word starts with a given prefix in JavaScript?

A Use word.indexOf(prefix) === word.length - prefix.length to check prefix.

B Use word.includes(prefix) to check if the prefix is anywhere in the word.

C Use word.endsWith(prefix) to check if the word starts with the prefix.

D Use word.startsWith(prefix) to check if the word begins with the prefix.

💡 Hint

Think about the method that checks the start of a string.

The startsWith() method returns true if the string begins with the specified prefix.

🔧 Debug

advanced

Identify the error in prefix counting code

What error will the following code produce when run, and why?

DSA Javascript

function countPrefix(words, prefix) {
  let count = 0;
  for (let i = 0; i <= words.length; i++) {
    if (words[i].startsWith(prefix)) {
      count++;
    }
  }
  return count;
}

console.log(countPrefix(['cat', 'car', 'dog'], 'ca'));

A SyntaxError due to missing semicolon.

B Returns 0 because prefix is not found.

C TypeError: Cannot read property 'startsWith' of undefined because loop goes out of bounds.

D Works correctly and returns 2.

💡 Hint

Check the loop condition and array indexing.

The loop uses i <= words.length which causes words[words.length] to be undefined, so calling startsWith on undefined throws a TypeError.

🚀 Application

advanced

Count words with prefix ignoring case

Which code snippet correctly counts words starting with a prefix ignoring case differences?

A words.filter(word => word.toLowerCase().startsWith(prefix.toLowerCase())).length

B words.filter(word => word.startsWith(prefix)).length

C words.filter(word => word.includes(prefix.toLowerCase())).length

D words.filter(word => word.toUpperCase() === prefix.toUpperCase()).length

💡 Hint

Convert both word and prefix to the same case before checking.

To ignore case, convert both strings to lower (or upper) case before using startsWith.

🧠 Conceptual

expert

Time complexity of counting words with prefix

What is the time complexity of counting how many words start with a given prefix in a list of n words, each of average length m?

A O(n * m) because each word is checked character by character for the prefix.

B O(n^2) because nested loops are needed.

C O(m) because only prefix length matters.

D O(n + m) because prefix check is constant time.

💡 Hint

Consider how many words and how many characters per word are checked.

Each of the n words is checked for prefix of length up to m, so total is O(n * m).