express处理跨域问题，中间件 CORS

CORS是一个W3C标准，全称是"跨域资源共享"（Cross-origin resource sharing）。

1、不用中间件的话可以这样写：

app.all('*', function (req, res, next) {
    res.header("Access-Control-Allow-Origin", "*");
    res.header("Access-Control-Allow-Headers", "Content-Type,Content-Length, Authorization, Accept,X-Requested-With");
    res.header("Access-Control-Allow-Methods","PUT,POST,GET,DELETE,OPTIONS");
    res.header("X-Powered-By",' 3.2.1')
    if(req.method=="OPTIONS") res.send(200);/*让options请求快速返回*/
    else  next();
})

2、使用CORS,和其他中间件的用法一样，app.use()即可:

var express = require('express')
var cors = require('cors')
var app = express()
 
app.use(cors())
 
app.get('/products/:id', function (req, res, next) {
  res.json({msg: 'This is CORS-enabled for all origins!'})
})
 
app.listen(80, function () {
  console.log('CORS-enabled web server listening on port 80')
})

如果要单独为某个接口实现允许跨域请求，在回调函数之前先用cors()方法进行跨域处理即可：

var express = require('express')
var cors = require('cors')
var app = express()
 
app.get('/products/:id', cors(), function (req, res, next) {
  res.json({msg: 'This is CORS-enabled for a Single Route'})
})
 
app.listen(80, function () {
  console.log('CORS-enabled web server listening on port 80')
})

还可以自己手动配置：

var express = require('express')
var cors = require('cors')
var app = express()
 
var corsOptions = {
  origin: 'http://example.com',
  optionsSuccessStatus: 200 // some legacy browsers (IE11, various SmartTVs) choke on 204
}
 
app.get('/products/:id', cors(corsOptions), function (req, res, next) {
  res.json({msg: 'This is CORS-enabled for only example.com.'})
})
 
app.listen(80, function () {
  console.log('CORS-enabled web server listening on port 80')
})

更多信息请参考官网：CORS