JavaScript Program to Check the Number of Occurrences of a Character in the String

If you check the number of occurrences of 'o' in the string 'school', the result is 2.

Example 1: Check Occurrence of a Character Using for Loop

// program to check the number of occurrence of a character

function countString(str, letter) {
    let count = 0;

    // looping through the items
    for (let i = 0; i < str.length; i++) {

        // check if the character is at that position
        if (str.charAt(i) == letter) {
            count += 1;
        }
    }
    return count;
}

// take input from the user
const string = prompt('Enter a string: ');
const letterToCheck = prompt('Enter a letter to check: ');

//passing parameters and calling the function
const result = countString(string, letterToCheck);

// displaying the result
console.log(result);

Output

Enter a string: school
Enter a  letter to check: o
2

In the above example, the user is prompted to enter a string and the character to check.

• In the beginning, the value of the count variable is 0.
• The for loop is used to iterate over the strings.
• The charAt() method returns a character at a specified index.
• During each iteration, if the character at that index matches the required character to match, then the count variable is increased by 1.

Example 2: Check occurrence of a character using a Regex

// program to check the occurrence of a character

function countString(str, letter) {

    // creating regex 
    const re = new RegExp(letter, 'g');

    // matching the pattern
    const count = str.match(re).length;

    return count;
}

// take input from the user
const string = prompt('Enter a string: ');
const letterToCheck = prompt('Enter a letter to check: ');

//passing parameters and calling the function
const result = countString(string, letterToCheck);

// displaying the result
console.log(result);

Output

Enter a string: school
Enter a  letter to check: o
2

In the above example, a regular expression (regex) is used to find the occurrence of a string.

• const re = new RegExp(letter, 'g'); creates a regular expression.
• The match() method returns an array containing all the matches. Here, str.match(re);gives ["o", "o"].
• The length property gives the length of an array element.