How to count the occurrence of a specific string in a string in JavaScript

In JavaScript, counting occurrences of a substring within a string is a common task. There are several approaches to accomplish this, from simple loops to built-in string methods.

For example, counting how many times "is" appears in "this is a string" should return 2.

count('this is a string', 'is') should return 2;

Method 1: Using indexOf() with Loop

The most reliable approach uses indexOf() to find each occurrence and increment a counter:

const str1 = 'this is a string';
const str2 = 'is';

function countOccurrences(mainStr, subStr) {
    if (subStr.length === 0) return 0;
    
    let count = 0;
    let position = 0;
    
    while ((position = mainStr.indexOf(subStr, position)) !== -1) {
        count++;
        position += subStr.length; // Move past current match
    }
    
    return count;
}

console.log(countOccurrences(str1, str2));

2

Method 2: Using split() Method

Another approach splits the string by the substring and counts the resulting array length:

function countWithSplit(mainStr, subStr) {
    if (subStr.length === 0) return 0;
    return mainStr.split(subStr).length - 1;
}

const str1 = 'this is a string';
const str2 = 'is';

console.log(countWithSplit(str1, str2));

2

Method 3: Using Regular Expression

Regular expressions provide a flexible solution, especially for complex patterns:

function countWithRegex(mainStr, subStr) {
    // Escape special regex characters
    const escapedSubStr = subStr.replace(/[.*+?^${}()|[\]\]/g, '\$&');
    const regex = new RegExp(escapedSubStr, 'g');
    const matches = mainStr.match(regex);
    return matches ? matches.length : 0;
}

const str1 = 'this is a string';
const str2 = 'is';

console.log(countWithRegex(str1, str2));

2

Handling Overlapping Occurrences

For overlapping matches (like "aa" in "aaa"), modify the position increment:

function countOverlapping(mainStr, subStr) {
    if (subStr.length === 0) return 0;
    
    let count = 0;
    let position = 0;
    
    while ((position = mainStr.indexOf(subStr, position)) !== -1) {
        count++;
        position += 1; // Move by 1 for overlapping matches
    }
    
    return count;
}

console.log(countOverlapping('aaa', 'aa')); // Non-overlapping: 1, Overlapping: 2

2

Comparison

Conclusion

The indexOf() method with a loop provides the most control and performance for counting substring occurrences. Use split() for simple cases, and regular expressions when pattern matching is needed.