Introduced JEST test framework with eg test

Docs that helped with introducing jest into nextjs
https://nextjs.org/docs/app/building-your-application/testing/jest
https://github.com/vercel/next.js/tree/canary/examples/with-jest

Docs that helped with issues
testing-library/jest-dom#546

__test__/page.test.tsx

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+/**
+ * @jest-environment jsdom
+ */
+import { render, screen } from "@testing-library/react";
+import Home from "@/app/page";
+
+describe('Home', () => {
+    it('renders a heading', () => {
+        render(<Home />)
+
+        const heading = screen.getByRole("heading", {
+            name: "Welcome, please sign in.",
+        });
+
+        expect(heading).toBeInTheDocument();
+    })
+})

jest.config.js

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+const nextJest = require("next/jest");
+
+const createJestConfig = nextJest({
+  // Provide the path to your Next.js app to load next.config.js and .env files in your test environment
+  dir: "./",
+});
+
+// Add any custom config to be passed to Jest
+const customJestConfig = {
+  setupFilesAfterEnv: ["<rootDir>/jest.setup.js"],
+  testEnvironment: "jsdom",
+};
+
+module.exports = createJestConfig(customJestConfig);
+
+// All imported modules in your tests should be mocked automatically
+// automock: false,
+
+// Stop running tests after `n` failures
+// bail: 0,
+
+// The directory where Jest should store its cached dependency information
+// cacheDirectory: "/private/var/folders/vj/3fwtldh57v7bq0c7kgmthw1c0000gp/T/jest_dy",
+
+// Automatically clear mock calls, instances, contexts and results before every test
+// clearMocks: false,
+
+// Indicates whether the coverage information should be collected while executing the test
+// collectCoverage: false,
+
+// An array of glob patterns indicating a set of files for which coverage information should be collected
+// collectCoverageFrom: undefined,
+
+// The directory where Jest should output its coverage files
+// coverageDirectory: undefined,
+
+// An array of regexp pattern strings used to skip coverage collection
+// coveragePathIgnorePatterns: [
+//   "/node_modules/"
+// ],
+
+// Indicates which provider should be used to instrument code for coverage
+// coverageProvider: "v8",
+
+// A list of reporter names that Jest uses when writing coverage reports
+// coverageReporters: [
+//   "json",
+//   "text",
+//   "lcov",
+//   "clover"
+// ],
+
+// An object that configures minimum threshold enforcement for coverage results
+// coverageThreshold: undefined,
+
+// A path to a custom dependency extractor
+// dependencyExtractor: undefined,
+
+// Make calling deprecated APIs throw helpful error messages
+// errorOnDeprecated: false,
+
+// The default configuration for fake timers
+// fakeTimers: {
+//   "enableGlobally": false
+// },
+
+// Force coverage collection from ignored files using an array of glob patterns
+// forceCoverageMatch: [],
+
+// A path to a module which exports an async function that is triggered once before all test suites
+// globalSetup: undefined,
+
+// A path to a module which exports an async function that is triggered once after all test suites
+// globalTeardown: undefined,
+
+// A set of global variables that need to be available in all test environments
+// globals: {},
+
+// The maximum amount of workers used to run your tests. Can be specified as % or a number. E.g. maxWorkers: 10% will use 10% of your CPU amount + 1 as the maximum worker number. maxWorkers: 2 will use a maximum of 2 workers.
+// maxWorkers: "50%",
+
+// An array of directory names to be searched recursively up from the requiring module's location
+// moduleDirectories: [
+//   "node_modules"
+// ],
+
+// An array of file extensions your modules use
+// moduleFileExtensions: [
+//   "js",
+//   "mjs",
+//   "cjs",
+//   "jsx",
+//   "ts",
+//   "tsx",
+//   "json",
+//   "node"
+// ],
+
+// A map from regular expressions to module names or to arrays of module names that allow to stub out resources with a single module
+// moduleNameMapper: {},
+
+// An array of regexp pattern strings, matched against all module paths before considered 'visible' to the module loader
+// modulePathIgnorePatterns: [],
+
+// Activates notifications for test results
+// notify: false,
+
+// An enum that specifies notification mode. Requires { notify: true }
+// notifyMode: "failure-change",
+
+// A preset that is used as a base for Jest's configuration
+// preset: undefined,
+
+// Run tests from one or more projects
+// projects: undefined,
+
+// Use this configuration option to add custom reporters to Jest
+// reporters: undefined,
+
+// Automatically reset mock state before every test
+// resetMocks: false,
+
+// Reset the module registry before running each individual test
+// resetModules: false,
+
+// A path to a custom resolver
+// resolver: undefined,
+
+// Automatically restore mock state and implementation before every test
+// restoreMocks: false,
+
+// The root directory that Jest should scan for tests and modules within
+// rootDir: undefined,
+
+// A list of paths to directories that Jest should use to search for files in
+// roots: [
+//   "<rootDir>"
+// ],
+
+// Allows you to use a custom runner instead of Jest's default test runner
+// runner: "jest-runner",
+
+// The paths to modules that run some code to configure or set up the testing environment before each test
+// setupFiles: [],
+
+// A list of paths to modules that run some code to configure or set up the testing framework before each test
+// setupFilesAfterEnv: ["<rootDir>/jest.setup.js"],
+
+// The number of seconds after which a test is considered as slow and reported as such in the results.
+// slowTestThreshold: 5,
+
+// A list of paths to snapshot serializer modules Jest should use for snapshot testing
+// snapshotSerializers: [],
+
+// The test environment that will be used for testing
+// testEnvironment: "jsdom",
+
+// Options that will be passed to the testEnvironment
+// testEnvironmentOptions: {},
+
+// Adds a location field to test results
+// testLocationInResults: false,
+
+// The glob patterns Jest uses to detect test files
+// testMatch: [
+//   "**/__tests__/**/*.[jt]s?(x)",
+//   "**/?(*.)+(spec|test).[tj]s?(x)"
+// ],
+
+// An array of regexp pattern strings that are matched against all test paths, matched tests are skipped
+// testPathIgnorePatterns: [
+//   "/node_modules/"
+// ],
+
+// The regexp pattern or array of patterns that Jest uses to detect test files
+// testRegex: [],
+
+// This option allows the use of a custom results processor
+// testResultsProcessor: undefined,
+
+// This option allows use of a custom test runner
+// testRunner: "jest-circus/runner",
+
+// A map from regular expressions to paths to transformers
+// transform: undefined,
+
+// An array of regexp pattern strings that are matched against all source file paths, matched files will skip transformation
+// transformIgnorePatterns: [
+//   "/node_modules/",
+//   "\\.pnp\\.[^\\/]+$"
+// ],
+
+// An array of regexp pattern strings that are matched against all modules before the module loader will automatically return a mock for them
+// unmockedModulePathPatterns: undefined,
+
+// Indicates whether each individual test should be reported during the run
+// verbose: undefined,
+
+// An array of regexp patterns that are matched against all source file paths before re-running tests in watch mode
+// watchPathIgnorePatterns: [],
+
+// Whether to use watchman for file crawling
+// watchman: true,
+
+// createJestConfig is exported this way to ensure that next/jest can load the Next.js config which is async

jest.setup.js

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+// Learn more: https://github.com/testing-library/jest-dom
+import "@testing-library/jest-dom";