Support de-duping issues (#65)

* De-dupe against open issues when dedupe_issues = true

* Update README

* Fix compile errors

* Add unit tests for issue.ts

__tests__/issue.test.ts

@@ -42,3 +42,72 @@ describe('getIssueOption', () => {
     expect(issue.getIssueOption('hi')).toEqual(expected)
   })
 })
+
+describe('getExistingIssueNumber', () => {
+  const expectedTitle = 'expected title'
+  const expectedIssueNumber = 12345
+  const repo = 'testRepo'
+  const owner = 'testOwner'
+
+  beforeEach(() => {
+    process.env.INPUT_ISSUE_TITLE = expectedTitle
+  })
+
+  test('gets existing open issue', async () => {
+    const getIssues = jest.fn()
+    getIssues.mockResolvedValue({
+      data: [
+        {
+          title: expectedTitle,
+          number: expectedIssueNumber
+        }
+      ]
+    })
+    const result = await issue.getExistingIssueNumber(getIssues, {repo, owner})
+
+    expect(getIssues).toHaveBeenCalledWith({
+      repo,
+      owner,
+      state: 'open'
+    })
+
+    expect(result).toBe(expectedIssueNumber)
+  })
+
+  test('returns null when there is no open issue', async () => {
+    const getIssues = jest.fn()
+    getIssues.mockResolvedValue({data: []})
+
+    const result = await issue.getExistingIssueNumber(getIssues, {repo, owner})
+
+    expect(getIssues).toHaveBeenCalledWith({
+      repo,
+      owner,
+      state: 'open'
+    })
+
+    expect(result).toBe(null)
+  })
+
+  test('returns null when no issues match the issue title', async () => {
+    const getIssues = jest.fn()
+    getIssues.mockResolvedValue({
+      data: [
+        {
+          title: 'some random other issue',
+          number: 54321
+        }
+      ]
+    })
+
+    const result = await issue.getExistingIssueNumber(getIssues, {repo, owner})
+
+    expect(getIssues).toHaveBeenCalledWith({
+      repo,
+      owner,
+      state: 'open'
+    })
+
+    expect(result).toBe(null)
+  })
+})