Question

React 错误：无法读取未定义的属性“用户”

2019-12-10

167

javascript reactjs

我正在尝试运行此代码（从某个来源获取），其中动态创建输入并在提交时记录值。我收到此错误：

Cannot read property 'users' of undefined while calling createUI.

有人可以帮我吗？

import React, { useState } from "react";
import ReactDOM from "react-dom";

export default function App() {
  const [state, setState] = useState({
    users: [{ firstName: "", lastName: "" }]
  });

  const addClick = () => {
    setState(prevState => {
      users: [...prevState.users, { firstName: "", lastName: "" }];
    });
  };
  const createUI = () => {
    state.users.map((el, i) => {
      return (
        <div key={i}>
          <input
            placeholder="First Name"
            name="firstName"
            value={el.firstName || ""}
            onChange={handleChange}
          />
          <input
            placeholder="Last Name"
            name="lastName"
            value={el.lastName || ""}
            onChange={handleChange}
          />
          <input type="button" value="remove" onClick={removeClick} />
        </div>
      );
    });
  };

  const handleChange = (i, e) => {
    const { name, value } = e.target;
    let users = [...state.users];
    users[i] = { ...users[i], [name]: value };
    setState({ users });
  };

  const removeClick = i => {
    let users = [...state.users];
    users.splice(i, 1);
    setState({ users });
  };

  const handleSubmit = event => {
    alert("A name was submitted: " + JSON.stringify(this.state.users));
    event.preventDefault();
  };
  return (
    <form onSubmit={handleSubmit}>
      {createUI()}
      <input type="button" value="add more" onClick={addClick()} />
      <input type="submit" value="Submit" />
    </form>
  );
}

ReactDOM.render(<App />, document.getElementById("root"));

// const rootElement = document.getElementById("root");
// ReactDOM.render(<App />, rootElement)

沙盒在这里 ——> https://codesandbox.io/s/dynamic-fields-23cw4?fontsize=14&hidenavigation=1&theme=dark

Answer 1

该错误的原因是以下行：

<input type="button" value="add more" onClick={addClick()} />

从技术上讲，您正在调用 addClick 函数，该函数会更改 state 对象的值。有效的解决方案如下：

<input type="button" value="add more" onClick={() => addClick()} />

或更短的方式：

<input type="button" value="add more" onClick={addClick} />

希望对您有所帮助！

Answer 2

您的代码存在 3 个问题，

您执行了 onclick 处理程序 addClick，这触发了无限循环
createUI 方法未返回所创建组件的数组
addClick 函数的 setState 语法不正确。

const {useState} = React;

function App() {
  const [state, setState] = useState({
    users: [{ firstName: "", lastName: "" }]
  });

  const addClick = () => {
    setState(prevState => {
      return { users: [...prevState.users, { firstName: "", lastName: "" }] };
    });
  };
  const createUI = () => {
    return state.users.map((el, i) => {
      return (
        <div key={i}>
          <input
            placeholder="First Name"
            name="firstName"
            value={el.firstName || ""}
            onChange={(e)=>handleChange(e, i)}
          />
          <input
            placeholder="Last Name"
            name="lastName"
            value={el.lastName || ""}
            onChange={(e)=>handleChange(e, i)}
          />
          <input type="button" value="remove" onClick={removeClick} />
        </div>
      );
    });
  };

  const handleChange = (e, i) => {
    const { name, value } = e.target;
    let users = [...state.users];
    users[i] = { ...users[i], [name]: value };
    setState({ users });
  };

  const removeClick = i => {
    let users = [...state.users];
    users.splice(i, 1);
    setState({ users });
  };

  const handleSubmit = event => {
    alert("A name was submitted: " + JSON.stringify(this.state.users));
    event.preventDefault();
  };
  return (
    <form onSubmit={handleSubmit}>
      {createUI()}
      <input type="button" value="add more" onClick={addClick} />
      <input type="submit" value="Submit" />
    </form>
  );
}

ReactDOM.render(<App />, document.getElementById("root"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>

<div id="root"></div>

Answer 3

除了 @norbiial 的答案（应该是可接受的答案）之外：您的 addClick 函数不正确。您的 setState 调用未返回任何内容，这意味着只要您单击添加按钮，它就会再次中断。

更改为：

const addClick = () => {
  setState(prevState => ({
    users: [...prevState.users, { firstName: "", lastName: "" }]
  }));
};

在 createUI 中会出现相同的问题。