14.3 跨域

a.html

 <!--
 a和b是同域的：http://localhost:3000
 c是独立的：http://localhost:4000
 a获取c的数据

 a先引用c c把值放到window.name上，把a引用的地址改到b
 -->
<iframe src="http://localhost:4000/c.html" frameborder="0" onload="load()" id="iframe"></iframe>
<script>
    let first = true
    function load() {
      if (first) {
        let iframe = document.getElementById('iframe')
        iframe.src = 'http://localhost:3000/b.html'
        first = false
      } else {
        console.log(iframe.contentWindow.name)
      }
    }
</script>

b.html　　空页面

c.html

<script>
    window.name = '我不爱你'
</script>

服务端

a.js

let express = require('express')
let app  = express()
app.use(express.static(__dirname))
app.listen(3000)

b.js

let express = require('express')
let app  = express()
app.use(express.static(__dirname))
app.listen(4000)