ran npm run tdd & generated TDD

test/elo/elo.js

@@ -0,0 +1,18 @@
+module.exports = ([...ratings], kFactor = 32, selfRating) => {
+const [a, b] = ratings;
+const expectedScore = (self, opponent) => 1 / (1 + 10 ** ((opponent - self) / 400));
+const newRating = (rating, i) =>
+(selfRating || rating) + kFactor * (i - expectedScore(i ? a : b, i ? b : a));
+if (ratings.length === 2) {
+return [newRating(a, 1), newRating(b, 0)];
+} else {
+for (let i = 0; i < ratings.length; i++) {
+let j = i;
+while (j < ratings.length - 1) {
+[ratings[i], ratings[j + 1]] = elo([ratings[i], ratings[j + 1]], kFactor);
+j++;
+}
+}
+}
+return ratings;
+};