The five letters a, e, i, o and u are called vowels. All other alphabets except these 5 vowels are called consonants.

Example 1: Count the number of Vowels using Regex

// program to count the number of vowels in a string

function countVowel(str) { 

    // find the count of vowels
    let count = str.match(/[aeiou]/gi).length;

    // return number of vowels
    return count;
}

// take input
let string = prompt('Enter a string: ');

let result = countVowel(string);

console.log(result);

Output

Enter a string: JavaScript program
5

In the above program, the user is prompted to enter a string and that string is passed to the countVowel() function.

• The regular expression (RegEx) pattern is used with the match() method to find the number of vowels in a string.
• The pattern /[aeiou]/gi checks for all the vowels (case-insensitive) in a string. Here,
  str.match(/[aeiou]/gi); gives [“a”, “a”, “i”, “o”, “a”]
• The length property gives the length of the vowels present.

Example 2: Count the number of Vowels using for Loop

// program to count the number of vowels in a string

// defining vowels
const vowels = ["a", "e", "i", "o", "u"]

function countVowel(str) {
    // initialize count
    let count = 0;

    // loop through string to test if each character is a vowel
    for (let letter of str.toLowerCase()) {
        if (vowels.includes(letter)) {
            count++;
        }
    }

    // return number of vowels
    return count
}

// take input
let string = prompt('Enter a string: ');

let result = countVowel(string);

console.log(result);

Output

Enter a string: JavaScript program
5

In the above example,

• All the vowels are stored in a vowels variable.
• Initially, the value of the count variable is 0.
• The for...of loop is used to iterate over all the characters of a string.
• The toLowerCase() method converts all the characters of a string to lowercase.
• The includes() method checks if the vowel variable contains any of the characters of the string.
• If any character matches, the value of count is increased by 1.