test: add unitTest for sub exercise

js/tests/sub_test.js

@@ -0,0 +1,14 @@
+export const tests = []
+const t = (f) => tests.push(f)
+
+// Sub is declared and is a function and take 2 argument
+t(() => typeof Sub === 'function')
+t(() => Sub.length === 2)
+
+// Sub works with positive numbers
+t(() => Sub(5, 2) === 3)
+t(() => Sub(Sub(Sub(5, 2), 2), 2) === -1) // Sub Sub Sub !!
+
+// sub works with negative numbers
+t(() => Sub(-5, -2) === -3)
+t(() => Sub(Sub(Sub(-5, -2), -2), -2) === 1) // Sub Sub Sub !!