Similarity Percentage between two strings in Javascript

string_similarity_percentage.js

var str1 = 'Hello World';
var str2 = 'World';
var similarity_percentage = Math.round(similarity(str1, str2) * 10000) / 100;

function similarity(s1, s2) {
	var longer = s1;
	var shorter = s2;

	if (s1.length < s2.length) {
		longer = s2;
		shorter = s1;
	}

	var longerLength = longer.length;

	if (longerLength === 0) {
		return 1.0;
	}

	return (longerLength - editDistance(longer, shorter)) / parseFloat(longerLength);
}

function editDistance(s1, s2) {
	s1 = s1.toLowerCase();
	s2 = s2.toLowerCase();

	var costs = new Array();

	for (var i = 0; i <= s1.length; i++) {
		var lastValue = i;

		for (var j = 0; j <= s2.length; j++) {
			if (i == 0) {
				costs[j] = j;
			}
			else {
				if (j > 0) {
					var newValue = costs[j - 1];

					if (s1.charAt(i - 1) != s2.charAt(j - 1)) {
						newValue = Math.min(Math.min(newValue, lastValue),
						costs[j]) + 1;
					}

					costs[j - 1] = lastValue;
					lastValue = newValue;
				}
			}
		}

		if (i > 0) {
			costs[s2.length] = lastValue;
		}
	}

	return costs[s2.length];
}