[tests] Add Jest runner and unit tests for src/utils (0% -> 97% coverage)

.github/workflows/checks.yml

@@ -21,6 +21,9 @@ jobs:
       - name: Check CONTRIBUTING versions
         run: npm run check:contributing-versions
 
+      - name: Run unit tests
+        run: npm test
+
       - name: Build
         run: npm run build
 

.lintstagedrc.js

@@ -1,7 +1,7 @@
 module.exports = {
   // Lint & Prettify TS and JS files - only the staged files
   "**/*.(ts|tsx|js|jsx)": (filenames) => [
-    `cross-env NODE_ENV=test npx eslint --fix --max-warnings=0 ${filenames.map((f) => `'${f.replace(/'/g, "'\\''")}'`).join(" ")}`,
+    `cross-env NODE_ENV=test npx eslint --fix --max-warnings=0 --no-warn-ignored ${filenames.map((f) => `'${f.replace(/'/g, "'\\''")}'`).join(" ")}`,
     `npx prettier --write ${filenames.map((f) => `'${f.replace(/'/g, "'\\''")}'`).join(" ")}`,
   ],
 

jest.config.js

@@ -0,0 +1,16 @@
+module.exports = {
+  testEnvironment: "jsdom",
+  // Target the running node version so babel-preset-gatsby does not inject
+  // browser core-js polyfills, which the site does not install.
+  transform: {
+    "\\.[jt]sx?$": [
+      "babel-jest",
+      { presets: [["babel-preset-gatsby", { targets: { node: "current" } }]] },
+    ],
+  },
+  // Only pick up the maintained suites under __tests__ directories. The legacy
+  // enzyme `*.test.js` files next to components target React 16 and are not run.
+  testMatch: ["**/__tests__/**/*.test.js"],
+  collectCoverageFrom: ["src/utils/**/*.js"],
+  coveragePathIgnorePatterns: ["/node_modules/"],
+};