String comparisons

levenshtein.js

/**
 * @description calculates the levenshtein distance between words
 * @param {string} str a string
 * @param {string} str2 another string
 * @returns {object} with properties 'steps' and 'transitions' 
 */
function levenshtein(str, str2) {
    if (typeof str !== 'string' || typeof str2 !== 'string') return;

    let [shorter, longer] = [str, str2].sort((a, b) => a.length - b.length);
    shorter = shorter.trim();
    longer = longer.trim();
    
    const sizeDiff = longer.length - shorter.length;
    const truncatedLonger = longer.substring(0, shorter.length);
    let steps = sizeDiff;

    const transitions = [];

    for (let i = 0; i < truncatedLonger.length; i++) {
        const sChar = shorter.charAt(i);
        const lChar = truncatedLonger.charAt(i);
        const isStart = i === 0;

        if (isStart) {
            transitions.push(shorter);
        }

        if (sChar !== lChar) {
            steps++;

            const prevForm = transitions[transitions.length - 1];
            const beforeReplace = isStart ? '' : prevForm.substring(0, i);
            const afterReplace = prevForm.substring(i + 1);
            const newForm = `${beforeReplace}${lChar}${afterReplace}`;

            transitions.push(newForm);
        }

    }
    if (sizeDiff) {
        for (let i = 0; i < sizeDiff; i++) {
            const lastTransition = transitions[transitions.length - 1];
            const nextChar = longer.charAt(truncatedLonger.length + i);
            const newForm = `${lastTransition}${nextChar}`;
            transitions.push(newForm);
        }
    }
    return {
        steps,
        transitions
    };
}