Count Words with Given Prefix in DSA Javascript - Build from Scratch

Count Words with Given Prefix

📖 Scenario: You are working on a simple text analysis tool. You want to find out how many words in a list start with a certain prefix. This can help in searching or filtering words quickly.

🎯 Goal: Build a small program that counts how many words in a given list start with a specific prefix.

📋 What You'll Learn

Create an array of words with exact values

Create a variable to hold the prefix string

Use a loop to count how many words start with the prefix

Print the count as the final output

💡 Why This Matters

🌍 Real World

Counting words with a prefix is useful in search engines, autocomplete features, and filtering lists quickly.

💼 Career

This skill helps in text processing tasks common in software development, data analysis, and building user-friendly search tools.

1

Create the list of words

Create an array called words with these exact strings: "apple", "app", "application", "banana", "apricot"

DSA Javascript

// Your code here

Hint

Use square brackets [] to create an array and put the words inside quotes separated by commas.

2

Set the prefix to search

Create a variable called prefix and set it to the string "app"

DSA Javascript

const words = ["apple", "app", "application", "banana", "apricot"]
// Your code here

Hint

Use const prefix = "app" to create the prefix variable.

3

Count words starting with the prefix

Create a variable called count and set it to 0. Then use a for loop with variable word to go through words. Inside the loop, if word.startsWith(prefix) is true, increase count by 1.

DSA Javascript

const words = ["apple", "app", "application", "banana", "apricot"]
const prefix = "app"
// Your code here

Hint

Use startsWith method to check the prefix and increase count inside the loop.

4

Print the count

Use console.log(count) to print the number of words starting with the prefix.

DSA Javascript

const words = ["apple", "app", "application", "banana", "apricot"]
const prefix = "app"
let count = 0
for (const word of words) {
    if (word.startsWith(prefix)) {
        count++;
    }
}
// Your code here

Hint

Use console.log(count) to show the result.

Practice

1. What does the function countWordsWithPrefix(words, prefix) do when given a list of words and a prefix?

easy

A. Counts how many words contain the prefix anywhere

B. Counts the total number of words in the list

C. Counts how many words end with the given prefix

D. Counts how many words start with the given prefix

Solution

1. Step 1: Understand the function purpose

   The function checks each word to see if it starts with the given prefix.

2. Step 2: Count matching words

   It counts only those words that begin exactly with the prefix, ignoring others.

3. Final Answer:

   Counts how many words start with the given prefix -> Option D

4. Quick Check:

   Starts with prefix = Count [OK]

Hint: Look for words starting exactly with prefix [OK]

Common Mistakes:

• Counting words containing prefix anywhere
• Counting words ending with prefix
• Counting total words instead of filtered ones

2. Which of the following is the correct syntax to check if a string word starts with a prefix prefix in JavaScript?

easy

A. word.beginWith(prefix)

B. word.hasPrefix(prefix)

C. word.startsWith(prefix)

D. word.starts(prefix)

Solution

1. Step 1: Recall JavaScript string methods

   JavaScript strings have a method called startsWith() to check prefix.

2. Step 2: Match method name

   Only startsWith() is the correct method; others do not exist.

3. Final Answer:

   word.startsWith(prefix) -> Option C

4. Quick Check:

   Correct method name = startsWith [OK]

Hint: Use startsWith() method for prefix check [OK]

Common Mistakes:

• Using non-existent methods like hasPrefix
• Using incorrect method names like beginWith
• Confusing startsWith with other string methods

3. What is the output of the following code?

const words = ['apple', 'app', 'application', 'banana'];
const prefix = 'app';
function countWordsWithPrefix(words, prefix) {
  let count = 0;
  for (const word of words) {
    if (word.startsWith(prefix)) {
      count++;
    }
  }
  return count;
}
console.log(countWordsWithPrefix(words, prefix));

medium

A. 3

B. 2

C. 1

D. 4

Solution

1. Step 1: Check each word against prefix 'app'

   'apple', 'app', and 'application' all start with 'app'. 'banana' does not.

2. Step 2: Count matching words

   There are 3 words starting with 'app', so count is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Words starting with 'app' = 3 [OK]

Hint: Count words starting exactly with prefix [OK]

Common Mistakes:

• Counting words containing prefix anywhere
• Missing one of the matching words
• Counting all words regardless of prefix

4. The following code is intended to count words starting with a prefix, but it has a bug. What is the bug?

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

medium

A. startsWith method is used incorrectly

B. Loop goes out of bounds causing an error

C. count variable is not initialized

D. Prefix is not passed as a parameter

Solution

1. Step 1: Analyze loop condition

   The loop runs with i from 0 to words.length inclusive, but array indices go from 0 to words.length - 1.

2. Step 2: Identify out of bounds access

   At i = words.length, words[i] is undefined, so calling startsWith causes an error.

3. Final Answer:

   Loop goes out of bounds causing an error -> Option B

4. Quick Check:

   Loop index must be less than length [OK]

Hint: Use i < words.length, not <= [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Not checking array bounds before access
• Assuming startsWith works on undefined

5. Given the array words = ['Test', 'testing', 'Tester', 'tent', 'team'] and prefix 'te', which code snippet correctly counts words starting with the prefix ignoring case sensitivity?

hard

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

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

C. words.filter(w => w.includes(prefix)).length

D. words.filter(w => w.endsWith(prefix)).length

Solution

1. Step 1: Understand case sensitivity issue

   Direct startsWith is case sensitive, so 'Test' and 'test' differ.

2. Step 2: Use toLowerCase() to normalize

   Convert both word and prefix to lowercase before checking startsWith to ignore case.

3. Final Answer:

   words.filter(w => w.toLowerCase().startsWith(prefix.toLowerCase())).length -> Option A

4. Quick Check:

   Normalize case before prefix check [OK]

Hint: Convert both strings to lowercase before startsWith [OK]

Common Mistakes:

• Ignoring case sensitivity
• Using includes instead of startsWith
• Using endsWith instead of startsWith