fix: ensure babel-preset-gatsby can be used with unit tests (#9629)

This PR does a few things, namely:

- Fixes a few things with the test stage, e.g. uses commonjs modules for
test running, uses the current node version for @babel/preset-env, etc.
- Fixes up docs slightly for the unit testing guide
- Fixes #9614

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Author: DSchau

docs/docs/unit-testing.md

@@ -26,32 +26,25 @@ npm install --save-dev jest babel-jest react-test-renderer identity-obj-proxy 'b
 ```
 
 Because Gatsby handles its own Babel configuration, you will need to manually
-tell Jest to use `babel-jest`. The easiest way to do this is to add a `"jest"`
-section in your `package.json`. You can set up some useful defaults at the same
-time:
-
-```json:title=package.json
-  "jest": {
-    "transform": {
-      "^.+\\.jsx?$": "<rootDir>/jest-preprocess.js"
-    },
-    "testRegex": "/.*(__tests__\\/.*)|(.*(test|spec))\\.jsx?$",
-    "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__/fileMock.js"
-    },
-    "testPathIgnorePatterns": ["node_modules", ".cache"],
-    "transformIgnorePatterns": [
-      "node_modules/(?!(gatsby)/)"
-    ],
-    "globals": {
-      "__PATH_PREFIX__": ""
-    },
-    "testURL": "http://localhost",
-    "setupFiles": [
-      "<rootDir>/loadershim.js"
-    ]
-  }
+tell Jest to use `babel-jest`. The easiest way to do this is to add a `jest.config.js`. You can set up some useful defaults at the same time:
+
+```json:title=jest.config.js
+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__/fileMock.js"
+  },
+  "testPathIgnorePatterns": ["node_modules", ".cache"],
+  "transformIgnorePatterns": ["node_modules/(?!(gatsby)/)"],
+  "globals": {
+    "__PATH_PREFIX__": ""
+  },
+  "testURL": "http://localhost",
+  "setupFiles": ["<rootDir>/loadershim.js"]
+}
 ```
 
 The `transform` section tells Jest that all `js` or `jsx` files need to be
@@ -128,11 +121,47 @@ needed at first, but will make things a lot easier if you want to test
 components that use `Link` or GraphQL.
 
 ```js:title=__mocks__/gatsby.js
+const React = require("react")
 const gatsby = jest.requireActual("gatsby")
-module.exports = { ...gatsby, graphql: jest.fn(), Link: "Link" }
+
+module.exports = {
+  ...gatsby,
+  graphql: jest.fn(),
+  Link: jest.fn().mockImplementation(({ to, ...rest }) =>
+    React.createElement("a", {
+      ...rest,
+      href: to,
+    })
+  ),
+  StaticQuery: jest.fn(),
+}
+```
+
+This mocks the `graphql()` function, `Link` component, and `StaticQuery` component.
+
+One more issue that you may encounter is that some components expect to be able
+to use the `location` prop that is passed in by `Router`. You can fix this by
+manually passing in the prop:
+
+```js:title=src/__tests__/index.js
+import React from "react"
+import renderer from "react-test-renderer"
+import BlogIndex from "../pages/index"
+
+describe("BlogIndex", () => {
+  it("renders correctly", () => {
+    const location = {
+      pathname: "/",
+    }
+
+    const tree = renderer.create(<BlogIndex location={location} />).toJSON()
+    expect(tree).toMatchSnapshot()
+  }))
+})
 ```
 
-This mocks the `graphql()` function and `Link` component.
+For more information on testing page components, be sure to read the docs on
+[testing components with GraphQL](/docs/testing-components-with-graphql/)
 
 ## Writing tests
 
@@ -150,11 +179,12 @@ import React from "react"
 import renderer from "react-test-renderer"
 import Bio from "./Bio"
 
-describe("Bio", () =>
+describe("Bio", () => {
   it("renders correctly", () => {
     const tree = renderer.create(<Bio />).toJSON()
     expect(tree).toMatchSnapshot()
-  }))
+  })
+})
 ```
 
 This is a very simple snapshot test, which uses `react-test-renderer` to render
@@ -220,95 +250,30 @@ config. First install `ts-jest`:
 npm install --save-dev ts-jest
 ```
 
-Then edit the Jest config in your `package.json` to match this:
-
-```json:title=package.json
-  "jest": {
-    "transform": {
-        "^.+\\.tsx?$": "ts-jest",
-        "^.+\\.jsx?$": "<rootDir>/jest-preprocess.js"
-    },
-    "testRegex": "(/__tests__/.*\\.([tj]sx?)|(\\.|/)(test|spec))\\.([tj]sx?)$",
-    "moduleNameMapper": {
-      ".+\\.(css|styl|less|sass|scss)$": "identity-obj-proxy",
-      ".+\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$": "<rootDir>/__mocks__/fileMock.js"
-    },
-    "moduleFileExtensions": [
-        "ts",
-        "tsx",
-        "js",
-        "jsx",
-        "json",
-        "node"
-    ],
-    "testPathIgnorePatterns": ["node_modules", ".cache"],
-    "transformIgnorePatterns": [
-      "node_modules/(?!(gatsby)/)"
-    ],
-    "globals": {
-      "__PATH_PREFIX__": ""
-    },
-    "testURL": "http://localhost",
-    "setupFiles": [
-      "<rootDir>/loadershim.js"
-    ]
-  }
-```
-
-## Testing components with Router
-
-When you test components they are not in a `Router`, meaning they don't have
-access to some context and props that they may be expecting. The most common of
-these is the `Link` component. In the example above we mock the `Link` component
-as a string, which is the simplest solution and works for most uses. However
-sometimes you might want to test with the real `Link` component. As of v2,
-Gatsby uses `@reach/router` for navigation, which is good at handling test
-environments, and unlike React Router is happy to render `Link`s outside of a
-`Router` context. However there is a small issue related to the `gatsby` mock.
-We can use a small workaround to avoid an error.
-
-First, remove the `Link` mock from `gatsby`:
-
-```js:title=__mocks__/gatsby.js
-const gatsby = jest.requireActual("gatsby")
-module.exports = { ...gatsby, graphql: jest.fn() }
-```
-
-While the `Link` component is exported by the main `gatsby` package, it is
-actually defined in `gatsby-link`. That in turn uses `parsePath()` from
-`gatsby`, which causes module resolution issues. Fortunately it's an easy fix.
-You need to create a mock for `gatsby-link`, even though it will actually be the
-real module. You do this so that you can tell it to not try and use the mock
-`gatsby`:
-
-```js:title=__mocks__/gatsby-link.js
-jest.unmock("gatsby")
-module.exports = jest.requireActual("gatsby-link")
-```
-
-One more issue that you may encounter is that some components expect to be able
-to use the `location` prop that is passed in by `Router`. You can fix this by
-manually passing in the prop:
-
-```js:title=src/__tests__/index.js
-import React from "react"
-import renderer from "react-test-renderer"
-import BlogIndex from "../pages/index"
-
-describe("BlogIndex", () =>
-  it("renders correctly", () => {
-    const location = {
-      pathname: "/",
-    }
-
-    const tree = renderer.create(<BlogIndex location={location} />).toJSON()
-    expect(tree).toMatchSnapshot()
-  }))
+Then update the configuration in `jest.config.js`, like so:
+
+```json:title=jest.config.js
+module.exports = {
+  "transform": {
+    "^.+\\.tsx?$": "ts-jest",
+    "^.+\\.jsx?$": "<rootDir>/jest-preprocess.js"
+  },
+  "testRegex": "(/__tests__/.*\\.([tj]sx?)|(\\.|/)(test|spec))\\.([tj]sx?)$",
+  "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__/fileMock.js"
+  },
+  "moduleFileExtensions": ["ts", "tsx", "js", "jsx", "json", "node"],
+  "testPathIgnorePatterns": ["node_modules", ".cache"],
+  "transformIgnorePatterns": ["node_modules/(?!(gatsby)/)"],
+  "globals": {
+    "__PATH_PREFIX__": ""
+  },
+  "testURL": "http://localhost",
+  "setupFiles": ["<rootDir>/loadershim.js"]
+}
 ```
 
-For more information on testing page components, be sure to read the docs on
-[testing components with GraphQL](/docs/testing-components-with-graphql/)
-
 ## Other resources
 
 If you need to make changes to your Babel config, you can edit the config in
@@ -318,3 +283,8 @@ though remember you may need to install the Babel 7 versions. See
 
 For more information on Jest testing, visit
 [the Jest site](https://jestjs.io/docs/en/getting-started).
+
+For an example encapsulating all of these techniques--and a full unit test suite with [react-testing-library][react-testing-library], check out the [using-jest][using-jest] example.
+
+[using-jest]: https://github.com/gatsbyjs/gatsby/tree/master/examples/using-jest
+[react-testing-library]: https://github.com/kentcdodds/react-testing-library

examples/using-jest/.gitignore

@@ -0,0 +1,3 @@
+public
+.cache
+node_modules

examples/using-jest/.prettierrc

@@ -0,0 +1,5 @@
+{
+  "semi": false,
+  "singleQuote": true,
+  "trailingComma": "es5"
+}

examples/using-jest/LICENSE

@@ -0,0 +1,22 @@
+The MIT License (MIT)
+
+Copyright (c) 2015 gatsbyjs
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+

examples/using-jest/README.md

@@ -0,0 +1,17 @@
+<p align="center">
+  <a href="https://www.gatsbyjs.org">
+    <img alt="Gatsby" src="https://www.gatsbyjs.org/monogram.svg" width="60" />
+  </a>
+</p>
+<h1 align="center">
+  using Jest
+</h1>
+
+Kick off your next Gatsby app with some great testing practices enabled via [Jest][jest], [react-testing-library][react-testing-library], and of course, [Gatsby][gatsby] 💪
+
+Check out the [unit testing doc][unit-testing-doc] for further info!
+
+[jest]: https://jestjs.io/
+[react-testing-library]: https://github.com/kentcdodds/react-testing-library
+[gatsby]: https://gatsbyjs.org
+[unit-testing-doc]: https://www.gatsbyjs.org/docs/unit-testing/

examples/using-jest/__mocks__/fileMock.js

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

examples/using-jest/__mocks__/gatsby.js

@@ -0,0 +1,14 @@
+const React = require('react')
+const gatsby = jest.requireActual('gatsby')
+
+module.exports = {
+  ...gatsby,
+  graphql: jest.fn(),
+  Link: jest.fn().mockImplementation(({ to, ...rest }) =>
+    React.createElement('a', {
+      ...rest,
+      href: to,
+    })
+  ),
+  StaticQuery: jest.fn(),
+}

examples/using-jest/gatsby-config.js

@@ -0,0 +1,30 @@
+module.exports = {
+  siteMetadata: {
+    title: 'Gatsby Default Starter',
+  },
+  plugins: [
+    'gatsby-plugin-react-helmet',
+    {
+      resolve: `gatsby-source-filesystem`,
+      options: {
+        name: `images`,
+        path: `${__dirname}/src/images`,
+      },
+    },
+    'gatsby-transformer-sharp',
+    'gatsby-plugin-sharp',
+    {
+      resolve: `gatsby-plugin-manifest`,
+      options: {
+        name: 'gatsby-starter-default',
+        short_name: 'starter',
+        start_url: '/',
+        background_color: '#663399',
+        theme_color: '#663399',
+        display: 'minimal-ui',
+        icon: 'src/images/gatsby-icon.png', // This path is relative to the root of the site.
+      },
+    },
+    'gatsby-plugin-offline',
+  ],
+}

examples/using-jest/jest-preprocess.js

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

examples/using-jest/jest.config.js

@@ -0,0 +1,18 @@
+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__/fileMock.js',
+  },
+  testPathIgnorePatterns: ['node_modules', '.cache'],
+  transformIgnorePatterns: ['node_modules/(?!(gatsby)/)'],
+  globals: {
+    __PATH_PREFIX__: '',
+  },
+  testURL: 'http://localhost',
+  setupTestFrameworkScriptFile: '<rootDir>/jest.setup.js',
+  setupFiles: ['<rootDir>/loadershim.js'],
+}

examples/using-jest/jest.setup.js

@@ -0,0 +1,2 @@
+import 'jest-dom/extend-expect'
+import 'react-testing-library/cleanup-after-each'

examples/using-jest/loadershim.js

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