Add dependencies and configs for unit tests

__mocks__/file-mock.js

@@ -0,0 +1 @@
+module.exports = "test-file-stub"

__mocks__/gatsby.js

@@ -0,0 +1,27 @@
+const React = require("react")
+const gatsby = jest.requireActual("gatsby")
+
+module.exports = {
+  ...gatsby,
+  graphql: jest.fn(),
+  Link: jest.fn().mockImplementation(
+    // these props are invalid for an `a` tag
+    ({
+      activeClassName,
+      activeStyle,
+      getProps,
+      innerRef,
+      partiallyActive,
+      ref,
+      replace,
+      to,
+      ...rest
+    }) =>
+      React.createElement("a", {
+        ...rest,
+        href: to,
+      })
+  ),
+  StaticQuery: jest.fn(),
+  useStaticQuery: jest.fn(),
+}

jest-preprocess.js

@@ -0,0 +1,5 @@
+const babelOptions = {
+  presets: ["babel-preset-gatsby"],
+}
+
+module.exports = require("babel-jest").createTransformer(babelOptions)

jest.config.js

@@ -0,0 +1,15 @@
+module.exports = {
+  transform: {
+    "^.+\\.jsx?$": `<rootDir>/jest-preprocess.js`,
+  },
+  moduleNameMapper: {
+    ".+\\.(css|styl|less|sass|scss)$": `identity-obj-proxy`,
+    ".+\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$": `<rootDir>/__mocks__/file-mock.js`,
+  },
+  testPathIgnorePatterns: [`node_modules`, `\\.cache`, `<rootDir>.*/public`],
+  transformIgnorePatterns: [`node_modules/(?!(gatsby)/)`],
+  globals: {
+    __PATH_PREFIX__: ``,
+  },
+  setupFiles: [`<rootDir>/loadershim.js`],
+}

loadershim.js

@@ -0,0 +1,3 @@
+global.___loader = {
+  enqueue: jest.fn(),
+}

package.json

@@ -25,12 +25,17 @@
     "react-helmet": "^6.1.0"
   },
   "devDependencies": {
+    "babel-jest": "26.3.0",
+    "babel-preset-gatsby": "0.5.11",
     "gatsby-plugin-postcss": "^2.3.11",
     "gatsby-plugin-purgecss": "^5.0.0",
     "gatsby-plugin-twitter": "^2.3.10",
     "gatsby-remark-embedder": "^3.0.0",
     "gh-pages": "^3.1.0",
+    "identity-obj-proxy": "3.0.0",
+    "jest": "26.4.2",
     "prettier": "2.0.5",
+    "react-test-renderer": "16.13.1",
     "tailwindcss": "^1.7.6"
   },
   "keywords": [
@@ -44,7 +49,7 @@
     "start": "npm run develop",
     "serve": "gatsby serve",
     "clean": "gatsby clean",
-    "test": "echo \"Write tests! -> https://gatsby.dev/unit-testing\" && exit 1",
+    "test": "jest",
     "deploy": "gatsby build --prefix-paths && gh-pages -d public -b master"
   },
   "repository": {