Inject the fetch function too. Now we have control over all the dependencies of this function.

src/load-student.spec.js

 const { assertThat, equalTo } = require('hamjest');
 // const { fetch } = require('./backend');
-let fetch;
 
 const loadStudent = ({ studentId }, dependencies) => {
   const cache = dependencies.cache;
+  const fetch = dependencies.fetch;
   if (!cache[studentId]) {
     cache[studentId] = fetch({ studentId });
   }
@@ -17,9 +17,9 @@ describe('Load a student', () => {
     assertThat(student, equalTo('student 42'));
   });
   it('fetch the student if not in the cache', () => {
-    let emptyCache = {};
-    fetch = () => 'student 23';
-    const student = loadStudent({ studentId: 23 }, { cache: emptyCache });
+    const emptyCache = {};
+    const dependencies = { cache: emptyCache, fetch: () => 'student 23' };
+    const student = loadStudent({ studentId: 23 }, dependencies);
     assertThat(student, equalTo('student 23'));
   });
 });