Firebase项目在本地工作，但部署的版本输出CORS错误

问题描述

我有一个Firebase项目，其中托管了前端，还有一个云功能来处理所有后端请求。当我执行firebase服务并在localhost上运行项目时，一切都很好。但是，部署后，当我尝试在线访问它时，出现以下错误。

CORS策略已阻止对XMLHttpRequest的访问：对 预检请求未通过访问控制检查：否 请求中出现“ Access-Control-Allow-Origin”标头 资源。

我已经尝试了所有在firebase中启用CORS的解决方案，但是错误并没有消失。是什么导致此错误，我该怎么办？

app.js中与云功能相关的代码（等效于index.js）

const functions = require('firebase-functions');

var express = require('express');
var cors = require("cors");

// These contain all the POSTS for the backend
var routes = require('./server/routes/routes');
var api = require('./server/routes/api');

var app = express();

app.use('/routes',routes);
app.use('/api',api);
app.use(cors({ origin: true }));

exports.app = functions.region('europe-west2').https.onRequest(app);

firebase.json

{
  "hosting": {
    "public": "public/client/dist","ignore": [
      "firebase.json","**/.*","**/node_modules/**"
    ],"rewrites": [
      {
        "source": "/api/**","function": "app"
      },{
        "source": "/routes/**",{
        "source": "**","destination": "/index.html"
      }
    ],"headers": [
      {
        "source": "/**","headers": [
          {
            "key": "Cache-Control","value": "no-cache,no-store,must-revalidate"
          }
        ]
      },{
        "source":
          "**/*.@(jpg|jpeg|gif|png|svg|webp|js|css|eot|otf|ttf|ttc|woff|woff2|font.css)","value": "max-age=604800"
          }
        ]
      }
    ]
  }
}

解决方法

跨域资源共享（CORS）允许AJAX请求跳过同域策略并从远程主机访问资源。

The * wildcard allows access from any origin
app.use(cors({
  origin: '*'
}));

If you want to restrict AJAX access to a single origin,you can use the origin
app.use(cors({
  origin: 'http://yourapp.com'
}));

要通过CORS启用HTTP cookie

app.use(cors({
  credentials: true,origin: '*'
}));

OR

app.use(cors({
  credentials: true,origin: 'http://yourapp.com'
}));