Add mean calculation test

Author: HoussamLh

prep/mean.js

@@ -0,0 +1,6 @@
+function calculateMean(list) {
+  const sum = list.reduce((acc, num) => acc + num, 0);
+  return sum / list.length;
+}
+
+module.exports = calculateMean;

prep/mean.test.js

@@ -0,0 +1,9 @@
+const calculateMean = require("./mean");
+
+test("calculates the mean of a list of numbers", () => {
+  const list = [3, 50, 7];
+  const currentOutput = calculateMean(list);
+  const targetOutput = 20;
+
+  expect(currentOutput).toEqual(targetOutput); // 20 is (3 + 50 + 7) / 3
+});