Count Words with Given Prefix in DSA Typescript

Mental Model

We want to find how many words start with a certain beginning part. We look at each word and check if it begins with that part.

Analogy: Imagine you have a box of books and you want to count how many titles start with the word 'The'. You look at each title and count if it starts with 'The'.

words: [apple, app, ape, bat, ball]
prefix: app

Check each word:
apple -> starts with app
app -> starts with app
ape -> does not
bat -> does not
ball -> does not

Dry Run Walkthrough

Input: words: ['apple', 'app', 'ape', 'bat', 'ball'], prefix: 'app'

Goal: Count how many words start with 'app'

Step 1: Check 'apple' if it starts with 'app'

apple (starts with 'app') -> count = 1

Why: We found one word starting with the prefix

Step 2: Check 'app' if it starts with 'app'

apple, app (starts with 'app') -> count = 2

Why: Another word matches the prefix

Step 3: Check 'ape' if it starts with 'app'

apple, app, ape (does not start with 'app') -> count = 2

Why: This word does not match the prefix, so count stays

Step 4: Check 'bat' if it starts with 'app'

apple, app, ape, bat (does not start with 'app') -> count = 2

Why: No match, count unchanged

Step 5: Check 'ball' if it starts with 'app'

apple, app, ape, bat, ball (does not start with 'app') -> count = 2

Why: No match, count unchanged

Result:

Final count = 2

Annotated Code

DSA Typescript

class WordPrefixCounter {
  words: string[];

  constructor(words: string[]) {
    this.words = words;
  }

  countWordsStartingWith(prefix: string): number {
    let count = 0;
    for (const word of this.words) {
      if (word.startsWith(prefix)) {
        count++;
      }
    }
    return count;
  }
}

// Driver code
const words = ['apple', 'app', 'ape', 'bat', 'ball'];
const prefix = 'app';
const counter = new WordPrefixCounter(words);
console.log(counter.countWordsStartingWith(prefix));

for (const word of this.words) {

iterate over each word in the list

if (word.startsWith(prefix)) {

check if current word begins with the prefix

count++;

increment count when prefix matches

Output

2

Complexity Analysis

Time: O(n * m) because we check each of the n words and compare up to m characters of the prefix

Space: O(n) for storing the list of words

vs Alternative: A trie data structure can speed up prefix queries to O(m) after building, but building the trie costs O(n * m)

Edge Cases

empty words list

count returns 0 because no words to check

DSA Typescript

for (const word of this.words) {

empty prefix string

all words start with empty prefix, so count equals total words

DSA Typescript

if (word.startsWith(prefix)) {

no words start with prefix

count returns 0 as no matches found

DSA Typescript

if (word.startsWith(prefix)) {

When to Use This Pattern

When you need to count how many strings start with a certain beginning, use a simple loop with prefix check or a trie for faster repeated queries.

Common Mistakes

Mistake: Checking if prefix is inside the word anywhere instead of only at the start
Fix: Use word.startsWith(prefix) instead of word.includes(prefix)

Summary

Counts how many words in a list start with a given prefix.

Use when you want to find how many words share the same beginning letters.

The key is to check each word's start against the prefix exactly.