💥 Add jest tips

docs/engineering/javascript/jest/helpful-tips.md

@@ -0,0 +1,45 @@
+# Helpful Jest Tips
+
+## Debug the DOM
+
+```typescript
+import { render, screen } from '@testing-library/react'
+
+describe('debug the dom', () => {
+  it('shows the DOM tree', () => {
+    // Render something
+    render(<div>Hello</div>)
+
+    // Debug the DOM tree in the console
+    screen.debug()
+
+    // Test something
+    expect(screen.getByText('Hello')).toBeInTheDocument()
+  })
+})
+```
+
+## Increase the number of debug lines printed
+
+This is helpful if you can't see the full output for a test failure.
+
+```bash
+DEBUG_PRINT_LIMIT=1000000 npx jest
+```
+
+## Force tests to run in series
+
+This is handy when you're debugging test logs but they're out of sync.
+
+```bash
+npx jest --runInBand
+```
+
+## Improve test speeds
+
+Use this flag to run across more available cores.
+
+```bash
+# You get a worker per core, so this uses 8 cores:
+npx jest --max-workers=8
+```

mkdocs.yml

@@ -61,6 +61,8 @@ nav:
               - Default .eslintrc: engineering/javascript/node/eslintrc.md
               - Colourful Console Logging: engineering/javascript/node/colour-logging.md
               - NPKill - node_modules: engineering/javascript/node/node-cleanup.md
+          - Jest:
+              - Helpful Tips: engineering/javascript/jest/helpful-tips.md
       - Ruby:
           - Handling JSON: engineering/ruby/handling-json.md
       - Git: