Adds a check for the diff existing on local

CHANGELOG.md

@@ -13,6 +13,7 @@
 * Fix --base options for danger local. [@peterjgrainger][]
 * Fix a minor typo in Semaphore CI setup. [@hongrich][]
 * Fix for capitalized Dangerfiles in CI environment. [@wizardishungry][]
+* Fix `danger local` crashing when comparing master to HEAD with no changes. [@orta][]
 
 ## 3.1.4
 

source/commands/danger-local.ts

@@ -25,5 +25,11 @@ setSharedArgs(program).parse(process.argv)
 const app = (program as any) as App
 const base = app.base || "master"
 const localPlatform = new LocalGit({ base, staged: app.staging })
-const fakeSource = new FakeCI(process.env)
-runRunner(app, { source: fakeSource, platform: localPlatform, additionalArgs: ["--local"] })
+localPlatform.validateThereAreChanges().then(changes => {
+  if (changes) {
+    const fakeSource = new FakeCI(process.env)
+    runRunner(app, { source: fakeSource, platform: localPlatform, additionalArgs: ["--local"] })
+  } else {
+    console.log("No git changes detected between head and master.")
+  }
+})

source/platforms/LocalGit.ts

@@ -15,20 +15,36 @@ export interface LocalGitOptions {
 
 export class LocalGit implements Platform {
   public readonly name: string
+  private gitDiff: string
 
   constructor(public readonly options: LocalGitOptions) {
     this.name = "local git"
   }
 
+  async getGitDiff(): Promise<string> {
+    if (this.gitDiff) {
+      return this.gitDiff
+    }
+    const base = this.options.base || "master"
+    const head = "HEAD"
+
+    this.gitDiff = await localGetDiff(base, head)
+    return this.gitDiff
+  }
+
+  async validateThereAreChanges(): Promise<boolean> {
+    const diff = await this.getGitDiff()
+    return diff.trim().length > 0
+  }
+
   async getPlatformDSLRepresentation(): Promise<any> {
     return null
   }
 
   async getPlatformGitRepresentation(): Promise<GitDSL> {
     const base = this.options.base || "master"
     const head = "HEAD"
-
-    const diff = await localGetDiff(base, head)
+    const diff = await this.getGitDiff()
     const commits: GitCommit[] = await localGetCommits(base, head)
     const gitJSON = diffToGitJSONDSL(diff, commits)