Update linting and testing process

__tests__/index.test.ts

@@ -0,0 +1,17 @@
+/**
+ * Unit tests for the action's entrypoint, src/index.ts
+ */
+
+import * as main from '../src/main'
+
+// Mock the action's entrypoint
+const runMock = jest.spyOn(main, 'run').mockImplementation()
+
+describe('index', () => {
+  it('calls run when imported', async () => {
+    // eslint-disable-next-line @typescript-eslint/no-require-imports
+    require('../src/index')
+
+    expect(runMock).toHaveBeenCalled()
+  })
+})

__tests__/main.test.ts

@@ -1,4 +1,68 @@
-describe('TODO - Add a test suite', () => {
-  it('TODO - Add a test', async () => {
-  });
-});
+/**
+ * Unit tests for the action's main functionality, src/main.ts
+ *
+ * These should be run as if the action was called from a workflow.
+ * Specifically, the inputs listed in `action.yml` should be set as environment
+ * variables following the pattern `INPUT_<INPUT_NAME>`.
+ */
+
+import * as core from '@actions/core'
+import * as main from '../src/main'
+
+// Mock the GitHub Actions core library
+//const debugMock = jest.spyOn(core, 'debug')
+const infoMock = jest.spyOn(core, 'info')
+const getInputMock = jest.spyOn(core, 'getInput')
+const setFailedMock = jest.spyOn(core, 'setFailed')
+const setOutputMock = jest.spyOn(core, 'setOutput')
+
+// Mock the action's main function
+const runMock = jest.spyOn(main, 'run')
+
+describe('action', () => {
+  beforeEach(() => {
+    jest.clearAllMocks()
+  })
+
+  it('sets the time output', async () => {
+    // Set the action's inputs as return values from core.getInput()
+    getInputMock.mockImplementation((name: string): string => {
+      switch (name) {
+        case 'milliseconds':
+          return '500'
+        default:
+          return ''
+      }
+    })
+
+    await main.run()
+
+    expect(runMock).toHaveReturned()
+    expect(infoMock).toHaveBeenCalledWith(expect.stringMatching(/.*/))
+    expect(infoMock).toHaveBeenCalledWith(expect.stringMatching(/.*/))
+    expect(setOutputMock).toHaveBeenCalledWith(
+      'time',
+      expect.stringMatching(/.*/)
+    )
+  })
+
+  it('sets a failed status', async () => {
+    // Set the action's inputs as return values from core.getInput()
+    getInputMock.mockImplementation((name: string): string => {
+      switch (name) {
+        case 'milliseconds':
+          return 'this is not a number'
+        default:
+          return ''
+      }
+    })
+
+    await main.run()
+
+    expect(runMock).toHaveReturned()
+    expect(setFailedMock).toHaveBeenNthCalledWith(
+      1,
+      'milliseconds not a number'
+    )
+  })
+})