TypeError: Cannot read property 'upgrade' of undefined

 INFO  Starting development server...
10% building 2/2 modules 0 active ERROR  TypeError: Cannot read property 'upgrade' of undefined
TypeError: Cannot read property 'upgrade' of undefined
    at Server.<anonymous> (D:\GIT\cloud-healthy-ui\node_modules\_webpack-dev-server@3.11.2@webpack-dev-server\lib\Server.js:135:47)
    at Array.forEach (<anonymous>)
    at new Server (D:\GIT\cloud-healthy-ui\node_modules\_webpack-dev-server@3.11.2@webpack-dev-server\lib\Server.js:134:27)
    at serve (D:\GIT\cloud-healthy-ui\node_modules\_@vue_cli-service@4.4.6@@vue\cli-service\lib\commands\serve.js:172:20)
    at process._tickCallback (internal/process/next_tick.js:68:7)
    at Function.Module.runMain (internal/modules/cjs/loader.js:832:11)
    at startup (internal/bootstrap/node.js:283:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:622:3)
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! healthy@2.4.0 dev: `vue-cli-service serve`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the healthy@2.4.0 dev script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

问题解决：排查vue.config.js中是否有引用的变量例如：VUE_BASE_API_HOST，未在.env或.env.development等环境变量定义文件中定义！