求自 vue 中引入 mqtt 报错问题

[dongtingyue]

npm run 起的开发环境
```
import mqtt from 'mqtt'
....
Vue.prototype.mqtt = mqtt
```
为了全局使用 mqtt ，不用每个地方都用 import
mqtt client.jst
```
const nextTick = process ? process.nextTick : function (callback) {
setTimeout(callback, 0);
};
```
报错
process is not defined