JavaScript function which mimics PHP’s `similar_text()`.

similarText.js

// Source: ChatGPT 4
function similarText(first, second) {
    // Check for null, undefined, or empty string inputs
    if (first === null || second === null || typeof first === 'undefined' || typeof second === 'undefined' || first.trim().length === 0 || second.trim().length === 0) {
        return { matchingCharacters: 0, similarityPercentage: 0 };
    }
    
    // Type coercion to ensure inputs are treated as strings
    first += '';
    second += '';

    // Initialize variables for tracking positions and maximum match length
    let pos1 = 0, pos2 = 0, max = 0,
        firstLength = first.length, secondLength = second.length, p, q, l, sum;

    // Iterate over each character of the first string
    for (p = 0; p < firstLength; p++) {
        // Iterate over each character of the second string
        for (q = 0; q < secondLength; q++) {
            // Count the length of the matching substring
            for (l = 0; (p + l < firstLength) && (q + l < secondLength) && (first[p + l] === second[q + l]); l++);
            // Update the maximum matching substring length and positions
            if (l > max) {
                max = l;
                pos1 = p;
                pos2 = q;
            }
        }
    }

    // Initialize sum with the maximum matching substring length
    sum = max;

    // Recursively find matching characters in the remaining substrings
    if (sum) {
        if (pos1 && pos2) {
            sum += similarText(first.substr(0, pos2), second.substr(0, pos2)).matchingCharacters;
        }

        if ((pos1 + max < firstLength) && (pos2 + max < secondLength)) {
            sum += similarText(first.substr(pos1 + max, firstLength - pos1 - max), second.substr(pos2 + max, secondLength - pos2 - max)).matchingCharacters;
        }
    }

    // Calculate the similarity percentage
    let longerLength = Math.max(firstLength, secondLength);
    let percentage = (sum / longerLength) * 100;

    // Return the total count of matching characters and the similarity percentage
    return {matchingCharacters: sum, similarityPercentage: percentage};
}

For the use case of a simple (Scots) spellchecking tool I’m building, in terms of offering suggestions for words not in the dictionary (a JavaScript Set).