all setIntersections tests passing

src/setIntersection.js

@@ -0,0 +1,8 @@
+export default function setIntersecion(firstSet, secondSet) {
+
+  const intersection = new Set([...firstSet].filter(x => secondSet.includes(x)))
+
+  if (!firstSet || !secondSet) {
+    return undefined
+  } return Array.from(intersection)
+}

test/setIntersection_test.js

@@ -0,0 +1,24 @@
+import { expect } from 'chai';
+import setIntersection from '../src/setIntersection';
+
+describe('setIntersection()', () => {
+  it('should be a function', () => {
+    expect(setIntersection).to.be.a('function');
+  });
+
+  it('Return the intersection of two sets.', () => {
+    const firstSet = [1, 2, 3, 4];
+    const secondSet = [2, 4, 6, 8];
+    expect(setIntersection(firstSet, secondSet)).to.deep.equal([2, 4]);
+  });
+
+  it('Return the intersection of two sets.', () => {
+    const firstSet = [3, 6, 9, 12];
+    const secondSet = [6, 7, 8, 9];
+    expect(setIntersection(firstSet, secondSet)).to.deep.equal([6, 9]);
+  });
+
+  it('Will not try to evaluate empty things', () => {
+    expect(setIntersection('', '')).to.equal(undefined);
+  });
+});