Nodejs Content-Type: application/json 跨域问题

预期

发送Content-Type为application/json的post请求不触发跨域错误

• 客户端请求

 const params = {
      a: 'a',
      b: 'b',
  }
//直接发送对象则Content-Type为application/json 
axios.post(API_URL, params)
     .then((res) => {
          console.log(res)
        })

• 后端配置

如果请求为简单请求则不会出发 CORS 预检请求，可直接在请求内配置Access-Control-Allow-Origin,Access-Control-Allow-Methods,Access-Control-Allow-Headers,来解决跨域问题

// 简单请求
app.get('/API',(req,res,next) => {
 res.header("Access-Control-Allow-Origin", "*");
 res.header("Access-Control-Allow-Methods","PUT,POST,GET,DELETE,OPTIONS");
 res.header("Access-Control-Allow-Headers", "Content-Type,Content-Length, Authorization, Accept,X-Requested-With");
...

})

如果Content-Type 的值仅限于下列三者之一视为简单请求：
text/plain
multipart/form-data
application/x-www-form-urlencoded

本例Content-Type为application/json，则不属于简单请求，需要先发送预检请求，所有直接在请求内设置以下配置是无效的.

设置了Access-Control-Allow-Origin", "*"， 仍然会报错：

Access to XMLHttpRequest at 'http://localhost:portA/API' from origin 'http://localhost:portB' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

app.post('/API',(req,res,next) => {
 // res.header("Access-Control-Allow-Origin", "*");
 // res.header("Access-Control-Allow-Methods","PUT,POST,GET,DELETE,OPTIONS");
 // res.header("Access-Control-Allow-Headers", "Content-Type,Content-Length, Authorization, Accept,X-Requested-With");
 //无效
...

})

nodejs 解决方案

当触发预检请求时让options请求快速结束，直接执行请求

const express = require("express");
const app = express(); 

//跨域配置需放在接口前面
app.use("/API", (req,res,next) => {
    console.log('checking');
    
    //设置允许跨域的域名，*代表允许任意域名跨域
    res.header("Access-Control-Allow-Origin","http://localhost:port");
    //允许的header类型
    res.header("Access-Control-Allow-Headers","content-type");
    //跨域允许的请求方式 
    res.header("Access-Control-Allow-Methods","DELETE,PUT,POST,GET,OPTIONS");
    if (req.method.toLowerCase() == 'options')
        res.send(200);  //让options尝试请求快速结束
    else
        next();
})

app.get('/API',(req,res,next) => {...})
//或者
app.post('/API',(req,res,next) => {...})