add functionality to error if branch already exists

Author: peterjgrainger

__tests__/create-branch.test.ts

@@ -1,5 +1,6 @@
 import {createBranch} from '../src/create-branch'
 import { readFileSync } from 'fs';
+import { context } from '@actions/github';
 
 describe('Create a branch based on the input', () => {
 
@@ -12,6 +13,9 @@ describe('Create a branch based on the input', () => {
     octokitMock = {
       git: {
         createRef: jest.fn()
+      },
+      repos: {
+        getBranch: jest.fn()
       }
     }
   })
@@ -30,8 +34,20 @@ describe('Create a branch based on the input', () => {
     process.env.GITHUB_TOKEN = 'token'
   })
 
+  it('gets a branch', async () => {
+    octokitMock.repos.getBranch.mockRejectedValue(new HttpError())
+    process.env.GITHUB_REPOSITORY = 'peterjgrainger/test-action-changelog-reminder'
+    await createBranch(githubMock, context, branch)
+    expect(octokitMock.repos.getBranch).toHaveBeenCalledWith({
+      repo: 'test-action-changelog-reminder',
+      owner: 'peterjgrainger',
+      branch
+    })
+  })
+
 
-  it('Create new branch', async () => {
+  it('Create new branch if not already there', async () => {
+    octokitMock.repos.getBranch.mockRejectedValue(new HttpError())
     await createBranch(githubMock, contextMock, branch)
     expect(octokitMock.git.createRef).toHaveBeenCalledWith({
       ref: 'refs/heads/release-v1',
@@ -48,4 +64,18 @@ describe('Create a branch based on the input', () => {
       expect(error).toEqual(new ReferenceError('No token defined in the environment variables'))
     }
   })
+
+  it('fails if branch already exists', async() => {
+    try {
+      await createBranch(githubMock, contextMock, branch)
+      throw Error('should error')
+    } catch (error) {
+      expect(error).toEqual(new ReferenceError('Branch already exists'))
+    }
+  })
 });
+
+class HttpError extends Error {
+  name = 'HttpError'
+  status = 404
+}

src/create-branch.ts

@@ -2,12 +2,28 @@ import { Context } from "@actions/github/lib/context";
 import { GitHub } from "@actions/github";
 
 export async function createBranch(github: any, context: Context, branch: string) {
-  const toolkit : GitHub = new github(githubToken())
-  await toolkit.git.createRef({
-    ref: `refs/heads/${branch}`,
-    sha: context.sha,
-    ...context.repo
-  })
+  const toolkit : GitHub = new github(githubToken());
+    let branchExists;
+    // throws HttpError if branch already exists.
+    try {
+      await toolkit.repos.getBranch({
+        ...context.repo,
+        branch
+      })
+
+      branchExists = true;
+    } catch(error) {
+      if(error.name === 'HttpError' && error.status === 404) {
+        await toolkit.git.createRef({
+          ref: `refs/heads/${branch}`,
+          sha: context.sha,
+          ...context.repo
+        })
+      } else {
+        throw Error(error)
+      }
+    }
+    if (branchExists) throw ReferenceError('Branch already exists')
 }
 
 function githubToken(): string {