refactor: simplify config loading by skip fs.existsSync check (vuejs#5305)

Use error code thrown by `require` directly.
This also simplifies module mocking in unit test.

Author: haoqunjiang

packages/@vue/cli-service/__tests__/Service.spec.js

@@ -1,5 +1,4 @@
 jest.mock('fs')
-jest.mock('/vue.config.js', () => ({ lintOnSave: false }), { virtual: true })
 jest.mock('vue-cli-plugin-foo', () => () => {}, { virtual: true })
 
 const fs = require('fs')
@@ -125,32 +124,25 @@ test('keep publicPath when empty', () => {
 })
 
 test('load project options from vue.config.js', () => {
-  process.env.VUE_CLI_SERVICE_CONFIG_PATH = `/vue.config.js`
-  fs.writeFileSync('/vue.config.js', `module.exports = { lintOnSave: false }`)
+  jest.mock(path.resolve('/', 'vue.config.js'), () => ({ lintOnSave: false }), { virtual: true })
   mockPkg({
     vue: {
       lintOnSave: 'default'
     }
   })
   const service = createMockService()
-  fs.unlinkSync('/vue.config.js')
-  delete process.env.VUE_CLI_SERVICE_CONFIG_PATH
   // vue.config.js has higher priority
   expect(service.projectOptions.lintOnSave).toBe(false)
 })
 
-test('load project options from vue.config.js', () => {
-  process.env.VUE_CLI_SERVICE_CONFIG_PATH = `/vue.config.js`
-  fs.writeFileSync('/vue.config.js', '')  // only to ensure fs.existsSync returns true
+test('load project options from vue.config.js as a function', () => {
   jest.mock('/vue.config.js', () => function () { return { lintOnSave: false } }, { virtual: true })
   mockPkg({
     vue: {
       lintOnSave: 'default'
     }
   })
   const service = createMockService()
-  fs.unlinkSync('/vue.config.js')
-  delete process.env.VUE_CLI_SERVICE_CONFIG_PATH
   // vue.config.js has higher priority
   expect(service.projectOptions.lintOnSave).toBe(false)
 })

packages/@vue/cli-service/lib/Service.js

@@ -1,4 +1,3 @@
-const fs = require('fs')
 const path = require('path')
 const debug = require('debug')
 const merge = require('webpack-merge')
@@ -305,21 +304,21 @@ module.exports = class Service {
       process.env.VUE_CLI_SERVICE_CONFIG_PATH ||
       path.resolve(this.context, 'vue.config.js')
     )
-    if (fs.existsSync(configPath)) {
-      try {
-        fileConfig = require(configPath)
+    try {
+      fileConfig = require(configPath)
 
-        if (typeof fileConfig === 'function') {
-          fileConfig = fileConfig()
-        }
+      if (typeof fileConfig === 'function') {
+        fileConfig = fileConfig()
+      }
 
-        if (!fileConfig || typeof fileConfig !== 'object') {
-          error(
-            `Error loading ${chalk.bold('vue.config.js')}: should export an object or a function that returns object.`
-          )
-          fileConfig = null
-        }
-      } catch (e) {
+      if (!fileConfig || typeof fileConfig !== 'object') {
+        error(
+          `Error loading ${chalk.bold('vue.config.js')}: should export an object or a function that returns object.`
+        )
+        fileConfig = null
+      }
+    } catch (e) {
+      if (e.code !== 'MODULE_NOT_FOUND') {
         error(`Error loading ${chalk.bold('vue.config.js')}:`)
         throw e
       }