Revised gitignore to add test back

.gitignore

@@ -2,6 +2,6 @@
 !index.js
 node_modules/
 .DS_Store
-gatsby-node.js
-internals.js
+./gatsby-node.js
+./internals.js
 !src/*.js

src/__tests__/gatsby-node.js

@@ -0,0 +1,103 @@
+jest.mock(`fs-extra`)
+const fs = require(`fs-extra`)
+const path = require(`path`)
+const { onPostBuild } = require(`../gatsby-node`)
+
+describe("gatsby-plugin-guru-export", () => {
+  describe(`onPostBuild`, () => {
+    beforeEach(() => {
+      fs.exists = jest.fn().mockResolvedValue(true);
+      fs.writeFile = jest.fn().mockResolvedValue();
+      fs.mkdirp = jest.fn().mockResolvedValue();
+    });
+
+    it(`exports`, async () => {
+      fs.writeFile = jest.fn();
+      fs.writeFile.mockResolvedValue(true);
+      const graphql = jest.fn();
+      graphql.mockResolvedValue({
+        data: {
+          site: {
+            siteMetadata: {
+              title: `a sample title`,
+              description: `a description`,
+              siteUrl: `http://dummy.url/`,
+            },
+          },
+          allMarkdownRemark: {
+            edges: [
+              {
+                node: {
+                  frontmatter: {
+                    id: 1,
+                    title: "sample title",
+                    description: "sample description",
+                    slug: 'sample1'
+                  },
+                },
+              },
+              {
+                node: {
+                  frontmatter: {
+                    id: 2,
+                    title: "sample title 2",
+                    description: "sample description 2",
+                    slug: 'sample2'
+                  },
+                },
+              },
+            ],
+          },
+        },
+      });
+      const customQuery = `
+      {
+        allMarkdownRemark(
+          limit: 1000,
+        ) {
+          edges {
+            node {
+              frontmatter {
+                id
+                title
+                description
+                slug
+              }
+            }
+          }
+        }
+      }
+    `;
+      const options = {
+        feeds: [
+          {
+            output: `/guru`,
+            serialize: ({ query: { site, allMarkdownRemark } }) =>
+              allMarkdownRemark.edges.map((edge) => {
+                return {site: site, ... edge.node.frontmatter};
+              }),
+            query: customQuery,
+            guruFieldMapping: {
+              Title: "title",
+              ExternalId: "id",
+              ExternalUrl: (serializedData) =>
+                `${serializedData.site.siteMetadata.siteUrl}${serializedData.slug}`,
+            },
+          },
+        ],
+      };
+      await onPostBuild({ graphql }, options);
+
+      expect(fs.writeFile).toHaveBeenCalledTimes(2);
+
+      const call0 = fs.writeFile.mock.calls[0];
+      const call1 = fs.writeFile.mock.calls[1];
+
+      expect(call0[0]).toEqual(path.join(`public/guru`, `card0.yaml`));
+      expect(call0[1]).toMatchSnapshot();
+
+      expect(call1[0]).toEqual(path.join(`public/guru`, `card1.yaml`));
+      expect(call1[1]).toMatchSnapshot();
+    });
+  });
+});