Merge pull request #2 from cypress-io/issue-1

fix mutations

index.js

@@ -101,7 +101,15 @@ Xvfb.prototype = {
   },
 
   _restoreDisplayEnvVariable: function () {
-    process.env.DISPLAY = this._oldDisplay
+    // https://github.com/cypress-io/xvfb/issues/1
+    // only reset truthy backed' up values
+    if (this._oldDisplay) {
+      process.env.DISPLAY = this._oldDisplay
+    } else {
+      // else delete the values to get back
+      // to undefined
+      delete process.env.DISPLAY
+    }
   },
 
   _spawnProcess: function (lockFileExists, onAsyncSpawnError) {

package.json

@@ -13,12 +13,14 @@
   "dependencies": {},
   "license": "MIT",
   "scripts": {
-    "test": "standard --fix --verbose *.js",
+    "test": "standard --fix --verbose *.js && mocha",
     "semantic-release": "semantic-release pre && npm publish --access public && semantic-release post",
     "commit": "commit-wizard"
   },
   "devDependencies": {
+    "chai": "^4.1.2",
     "condition-circle": "^1.5.0",
+    "mocha": "^3.5.0",
     "pre-git": "^3.15.0",
     "semantic-release": "^6.3.6",
     "simple-commit-message": "^3.0.2",

test/xvfb_spec.js

@@ -0,0 +1,22 @@
+const { expect } = require('chai')
+
+const Xvfb = require('../')
+
+describe('xvfb', function(){
+  beforeEach(function(){
+    this.xvfb = new Xvfb()
+  })
+
+  context('issue: #1', function(){
+    it('issue #1: does not mutate process.env.DISPLAY', function(){
+      delete process.env.DISPLAY
+
+      expect(process.env.DISPLAY).to.be.undefined
+
+      this.xvfb._setDisplayEnvVariable()
+      this.xvfb._restoreDisplayEnvVariable()
+
+      expect(process.env.DISPLAY).to.be.undefined
+    })
+  })
+})