ran npm run tdd & generated TDD

test/standardDeviation/standardDeviation.js

@@ -0,0 +1,7 @@
+module.exports = (arr, usePopulation = false) => {
+const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
+return Math.sqrt(
+arr.reduce((acc, val) => acc.concat((val - mean) ** 2), []).reduce((acc, val) => acc + val, 0) /
+(arr.length - (usePopulation ? 0 : 1))
+);
+};