如何解决TypeError: Cannot read properties of undefined (reading 'fName')错误

下面是导致错误的核心问题及修正方案：

• 路由回调参数顺序颠倒：app.post("/")的回调函数参数顺序写反了，Express路由回调的标准顺序是(req, res)，你写成了(res, req)，导致req被赋值为响应对象，实际请求对象变成了res，自然读取不到req.body.fName。
  修正代码：

  app.post("/", function(req, res){
      const fName = req.body.fName;
      const lName = req.body.lName;
      const email = req.body.email;
      // 后续逻辑
  });
  

• 变量名不匹配：你定义的变量是fName和lName，但在merge_fields里误用了未定义的firstName和lastName，这会引发新的ReferenceError，需修正为对应变量：

  merge_fields:{
      FNAME: fName,
      LNAME: lName
  }
  

• 拼写错误：merge_feilds拼写错误，正确拼写是merge_fields（注意是fields而非feilds）。

• JSON序列化方法错误：JSONstringify(data)缺少点号，正确写法是JSON.stringify(data)。

• 变量重名冲突：你已引入request模块，又在回调中声明同名变量const request = https.request(...)，会覆盖模块引用，建议修改为其他名称，比如mailchimpRequest：

  const mailchimpRequest = https.request(url, options, function(response){
      response.on("data", function(data){
          console.log(JSON.parse(data))
      })
  })
  mailchimpRequest.write(Jsondata);
  mailchimpRequest.end();
  

修正后的完整代码 #

const express = require("express");
const bodyParser = require("body-parser");
const request = require("request");
const https = require("https");

const app = express();

app.use(express.static("public"));
app.use(bodyParser.urlencoded({extended: true}));

app.get("/", function(req, res){
    res.sendFile(__dirname + "/signup.html")
});

app.post("/", function(req, res){
    const fName = req.body.fName;
    const lName = req.body.lName;
    const email = req.body.email;

    const data = {
        members : [
            {
                email_address: email,
                status: "subscribed",
                merge_fields:{
                    FNAME: fName,
                    LNAME: lName
                }
            }
        ]
    };

    const Jsondata = JSON.stringify(data);

    const url = "https://us21.api.mailchimp.com/3.0/lists/eca64732d1";

    const options= {
        method: "POST",
        auth:"string1:60ca81db08a41da8672a40261c9d7afd-us21"
    };

    const mailchimpRequest = https.request(url, options, function(response){
        response.on("data", function(data){
            console.log(JSON.parse(data))
        })
    })
    mailchimpRequest.write(Jsondata);
    mailchimpRequest.end();
});

app.listen(3000, function(){
    console.log("server is running on port 3000");
});

内容的提问来源于stack exchange，提问作者Hamzacreator