add tests

test/when/when.test.js

@@ -0,0 +1,18 @@
+const test = require('tape');
+const when = require('./when.js');
+
+test('Testing when', (t) => {
+  //For more information on all the methods supported by tape
+  //Please go to https://github.com/substack/tape
+  t.true(typeof when === 'function', 'when is a Function');
+
+  const doubleEvenNumbers = when(
+    (x) => x % 2 === 0,
+    (x) => x * 2
+  );
+
+  t.true(doubleEvenNumbers(2) === 4);
+  t.true(doubleEvenNumbers(1) === 1);
+
+  t.end();
+});