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How to Create a Function to Count Vowels in a String

Counting characters in a string is a common task in programming that can be both educational and useful. In this tutorial, we’ll be more specific and will walk through how to create a function to count the number of vowels in a given string. This guide is perfect for beginners and anyone looking to refresh their coding skills.

We’ll start by identifying the problem. Our function will be receiving a string, count how many characters a, e, i, o and u are present and return that value. Seems easy, so let’s get to work.

The plan

1. Define a function that takes a string as an input parameter;
2. Initialize a counter variable that keeps track of the number of vowels;
3. Loop through the string and if a vowel is found, update the counter;
4. Return the count value.

The implementation

I chose to use Javascript to solve this problem. You can use another language;; although the code will differ, the logic remains the same.

Begin by creating a JavaScript file. (I named mine index.js but you can name yours however you want.) Then, let’s start by step 1 of our plan; defining a function that takes a string as an input. Write the following code into your file.

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function countVowels(str) {
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}

We can implement steps 2 and 4 at the same time, since both involve the counter variable. Initialize a variable called counter with a value of 0 and then return it.

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function countVowels(str) {
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    let counter = 0;
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    return counter;
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}

Now we get to the best part, step 3.

First we’ll identify the characters we want to find (in our case are the vowels aeiou) and assign them to a variable vowels. Then, since a string in Javascript is iterable, we can loop through each character in the input string and check if that character exists in vowels.

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function countVowels(str) {
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    let counter = 0;
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    const vowels = "aeiou";
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    for (let character of str) {
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        if(vowels.includes(character)) counter++;
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    }
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    return counter;
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}

The implemention of the function is almost complete; we just need to modify it for an edge case. What if the input string contains uppercase characters? The solution is simple: convert every character of the string to lowercase using the toLowerCase method. Update the following highlighted line.

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function countVowels(str) {
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    let counter = 0;
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    const vowels = "aeiou";
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    for (let character of str.toLowerCase()) {
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        if(vowels.includes(character)) counter++;
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    }
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    return counter;
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}

We are now ready to test our function. You can use different inputs to ensure it works as expected. Here are a few examples.

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// ... (previous code)
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console.log(countVowels("hello world!")); // Output: 3
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console.log(countVowels("Hedegare's Blog")); // Output: 5
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console.log(countVowels("AEIOU are the vowels")); // Output: 10

The recap

We got to the end of our post; let’s recap everything we’ve done:

• Define the function: we defined a function called countVowels with a str parameter;
• Initialize the counter: we started with a vowel counter set to 0;
• Define the vowels: we defined a string containing all the vowels for easy comparison;
• Convert to lowercase: before iterating through the string, we converted it to lowercase to ensure uppercase characters are counted;
• Iterate through the string: we used a for ... of loop to iterate through each character in the string;
• Check and count: we checked if the character is a vowel by using the includes method and incremented the counter if it is;
• Return count: finally, we returned the value of the counter.

Last updated on 2025-06-01