如何使用'；体解析器'；用JavaScript？

我正在通过Node.js学习Web服务器。 当我尝试使用主体解析器时，我再也不能前进了。 状态代码成功。 这似乎是成功的。我收到了回复信息。但它不会显示在浏览器上

有什么问题？ 我的代码如下

//basic-server.js

const express = require('express')
const cors = require('cors');
const app = express()

const bodyParser = require('body-parser')
const jsonParser = bodyParser.json()

const PORT = 5000;
const ip = 'localhost';

app.use(cors())

app.get('/', (req, res) =>{
  res.send("hello")
})

app.post('/lower', jsonParser, (req, res) =>{
  
 res.send(req.body.body.toLowerCase())
  
})
app.post('/upper', jsonParser, (req, res) =>{
  
  res.send(req.body.body.toUpperCase())
})



app.listen(PORT, ip, () => {
  console.log(`http server listen on ${ip}:${PORT}`);
});

开发人员工具控制台上的错误消息

'Uncaught (in promise) SyntaxError: Unexpected token A in JSON at position 0
Promise.then (async)
post @ App.js:21
toUpperCase @ App.js:31'.

以下信息来自“网络”选项卡

-preflight
Request URL: http://localhost:5000/upper
Request Method: OPTIONS
Status Code: 204 No Content
Remote Address: 127.0.0.1:5000
Referrer Policy: strict-origin-when-cross-origin
Access-Control-Allow-Headers: content-type
Access-Control-Allow-Methods: GET,HEAD,PUT,PATCH,POST,DELETE
Access-Control-Allow-Origin: *
Connection: keep-alive
Content-Length: 0
Date: Fri, 28 May 2021 10:15:31 GMT
Keep-Alive: timeout=5
Vary: Access-Control-Request-Headers
X-Powered-By: Express
Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: ko-KR,ko;q=0.9,en-US;q=0.8,en;q=0.7
Access-Control-Request-Headers: content-type
Access-Control-Request-Method: POST
Cache-Control: no-cache
Connection: keep-alive
Host: localhost:5000
Origin: null
Pragma: no-cache
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: cross-site
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.128 Safari/537.36

---

您正在正确使用

bodyparser

。问题在于您的代码中，您只返回了一个符号

a

，而客户端试图解析这个不正确的JSON，这就是它失败的原因

---

您可以将

res.send（req.body.body.toUpperCase（））

更改为

res.send（JSON.stringify（req.body.body.toUpperCase（））

，以解决此问题。

您得到的响应不是JSON格式，因此请尝试将其添加到server.js中请求上方的代码中

app.use（bodyparser.urlencoded（{extended:false}））；
use（bodyparser.json（））如何调用
post
？
-preflight
Request URL: http://localhost:5000/upper
Request Method: OPTIONS
Status Code: 204 No Content
Remote Address: 127.0.0.1:5000
Referrer Policy: strict-origin-when-cross-origin
Access-Control-Allow-Headers: content-type
Access-Control-Allow-Methods: GET,HEAD,PUT,PATCH,POST,DELETE
Access-Control-Allow-Origin: *
Connection: keep-alive
Content-Length: 0
Date: Fri, 28 May 2021 10:15:31 GMT
Keep-Alive: timeout=5
Vary: Access-Control-Request-Headers
X-Powered-By: Express
Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: ko-KR,ko;q=0.9,en-US;q=0.8,en;q=0.7
Access-Control-Request-Headers: content-type
Access-Control-Request-Method: POST
Cache-Control: no-cache
Connection: keep-alive
Host: localhost:5000
Origin: null
Pragma: no-cache
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: cross-site
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.128 Safari/537.36

-fetch
Request URL: http://localhost:5000/upper
Request Method: POST
Status Code: 200 OK
Remote Address: 127.0.0.1:5000
Referrer Policy: strict-origin-when-cross-origin
Access-Control-Allow-Origin: *
Connection: keep-alive
Content-Length: 1
Content-Type: text/html; charset=utf-8
Date: Fri, 28 May 2021 10:15:31 GMT
ETag: W/"1-bc1M4j2I4u6VaLpUbAB8Y9kTHBs"
Keep-Alive: timeout=5
X-Powered-By: Express
Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: ko-KR,ko;q=0.9,en-US;q=0.8,en;q=0.7
Cache-Control: no-cache
Connection: keep-alive
Content-Length: 12
Content-Type: application/json
Host: localhost:5000
Origin: null
Pragma: no-cache
sec-ch-ua: "Google Chrome";v="89", "Chromium";v="89", ";Not A Brand";v="99"
sec-ch-ua-mobile: ?0
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: cross-site
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.128 Safari/537.36
{body: "a"}
body: "a"

-Response
A