Question

我一直在尝试调用服务器端POST函数来对Web应用程序中的用户进行身份验证。我的ExpressJS和ReactJS环境都已正确配置并运行，一个在端口3000上，一个在3001上。

我能够调用函数并执行服务器端Express代码，但是当我尝试访问React中的返回值时，该值似乎未定义。希望有帮助！

反应代码：

import React, { Component } from 'react';

export default class tem extends Component {
  constructor(props) {
    super(props);

    var testVar = {
      key: "0000000000",
      status: false
    };
    this.state = {
      users: [],
      username: "",
      password: "",
      submitted: "yes",
      responseData: testVar
    };
    this.handleNameChange = this.handleNameChange.bind(this);
    this.handlePassChange = this.handlePassChange.bind(this);
    this.handleSubmit = this.handleSubmit.bind(this);
    this.getResponse = this.getResponse.bind(this);
  }

  handleNameChange(e) {
    this.setState({ username: e.target.value });
  }
  handlePassChange(e) {
    this.setState({ password: e.target.value });
  }

  getResponse(urlPath, user, pass) {
    let setResp = this;
    fetch(urlPath, {
      method: "POST",
      headers: {
        "Content-Type": "application/json"
      },
      body: JSON.stringify({
        username: this.state.username,
        password: this.state.password
      })
    })
      .then(function(response) {
        return response.json();
      })
      .then(function(response) {
        setResp.setState({ responseData: response });
      });
  }

  handleSubmit(event) {
    event.preventDefault();
    this.setState({ submitted: "YES" });
    this.setState({
      responseData: this.getResponse(
        "/login",
        this.state.username,
        this.state.password
      )
    });
  }

  render() {
    return (
      <div>
        <h1>login </h1>
        Enter your username
        <form onSubmit={this.handleSubmit}>
          <input
            type="text"
            placeholder="Search..."
            value={this.props.filterText}
            onChange={this.handleNameChange}
          />
          Enter your password
          <input
            type="password"
            placeholder="enter password"
            value={this.props.filterText}
            onChange={this.handlePassChange}
          />
          <button type="submit">Start</button>
        </form>
        {this.state.responseData.key}
        {this.state.submitted}
      </div>
    );
  }
}

我稍后尝试访问reponseData.key，但出现以下错误：

Error Screenshot

我输出的JSON是：

{
    "key": "408f51qk54",
    "status": true
}

我通过邮递员尝试过，并且工作正常。我不知道是什么错误！

Answer 1

response.json()返回一个Promise，因此在使用setState中的响应之前，请确保在Promise链中将其返回。

let setResp = this;
fetch(urlPath, {
  method: "POST",
  headers: {
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    username: this.state.username,
    password: this.state.password
  })
})
  .then(function(response) {
    return response.json();
  })
  .then(function(response) {
    setResp.setState({ responseData: response });
  });

您也不应该使用setState的结果来调用this.getResponse，因为您不会从this.getResponse返回任何信息，并且它是异步的。这样会使您的responseData成为undefined.

只需致电this.getResponse：

handleSubmit(event) {
  event.preventDefault();
  this.setState({ submitted: "YES" });
  this.getResponse(
    "/login",
    this.state.username,
    this.state.password
  );
}

Answer 2

例如，您要像这样从.post返回数据：

app.post('/', function (req, res) {
// server side code
    res.json(/*returning data*/);
});

因此，在React代码中，您可以这样检索它：

functionThatMakePostRequest = (args) => {
const data = {
    id: 555,
    name: "name",
};

fetch('/', {
    method: 'post',
    body: JSON.stringify(data),
    headers: {
        'Accept': 'application/json',
        'Content-Type': 'application/json',
    },
})
.then(res => res.json()) //returns array of data
.then(res => this.setState({ res })); //assign state to array res
};

在ReactJS中检索返回的POST数据

2 个答案: