如何将地图函数用于钩子 useState 属性

Question

我尝试使用 map 函数在 react-hooks useState 中循环我的列表数据，但我遇到了一个错误，即“TypeError：无法读取未定义的属性 'map'”

//1.Initial declaration
const App = props=>  {
const [state, changeState]= useState ({
    name:"",
    eventTitle:"",
    details:"",
    objdata:{},
    list:[],
    toggleIndex:"",
    editName: "",
    editEventTitle: "",
    editDetails: "",
    editObj: {}
});


//2.logic comes here


//3.tried using map


{(state.list.map((data,id)=>{
console.log ('loop data',data)
}))}

Answer 1

我们怀疑您没有以正确的方式设置您的状态。我试图在我的评论中解释，当你设置你的状态时，它不会将更新的属性与当前的属性合并。所以，你应该考虑一下。现在你正在这样设置你的状态：

const handleName = name => {
  changeState({
    name: name.target.value
  });
};

在这里，您正在设置 name 属性并丢失您状态的其他部分。因此，当您设置您的状态时，您会丢失 list 以及您状态的其他部分。你应该这样做：

const handleName = name => {
  const { target } = name;

  changeState(state => ({
    ...state,
    name: target.value,
  }));
};

你取旧状态，通过spreading它保留其他属性，然后更新相关部分。我会在这里使用event 而不是name。这不是“名称”，实际上它毕竟是“事件”:)

const handleName = event => {
  const { target } = event;

  changeState(state => ({
    ...state,
    name: target.value,
  }));
};

此外，您的代码中还有一些其他问题和不必要的部分。例如，您在处理提交并将对象添加到您的list 时非常费力。您不需要在您的州内使用额外的objdata 将其推送到list。如果你想构造一个额外的对象，你可以在函数本身中完成。

这是一个非常简单的方法：

const submitHandle = () => {
  const { name, eventTitle, details } = state;
  const obj = { name, eventTitle, details };

  changeState(state => ({
    ...state,
    list: [ ...state.list, obj ],
  }))
};

同样，我们使用扩展运算符来保留状态的其他部分，并在更新list 时保留其他对象。不要像在 submitHandle 函数中那样设置状态。试着把它想得简单:)

此外，您不需要在不需要时绑定函数。您可以在下面找到代码的工作副本。我只是删除了不必要的部分并修复了问题。

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

const App = props => {
  const [state, changeState] = useState({
    name: "",
    eventTitle: "",
    details: "",
    list: [],
    toggleIndex: "",
    editName: "",
    editEventTitle: "",
    editDetails: "",
    editObj: {}
  });

  const handleName = event => {
    const { target } = event;

    changeState(state => ({
      ...state,
      name: target.value
    }));
  };

  const handleEventTitle = event => {
    const { target } = event;

    changeState(state => ({
      ...state,
      eventTitle: target.value
    }));
  };

  const handleDetails = event => {
    const { target } = event;

    changeState(state => ({
      ...state,
      details: target.value
    }));
  };

  const submitHandle = () => {
    const { name, eventTitle, details } = state;
    const obj = { name, eventTitle, details };

    changeState(state => ({
      ...state,
      list: [...state.list, obj]
    }));
  };

  const resetHandle = () =>
    changeState(state => ({
      ...state,
      name: "",
      eventTitle: "",
      details: ""
    }));

  return (
    <div>
      <div className="jumbotron jumbotron-fluid">
        <div className="container">
          <h1 className="display-5 text-center">Let's set your reminders</h1>
        </div>
      </div>
      <div className="bg-dark container-fluid">
        <div className="row">
          <div className="col-sm-12 col-md-4 col-lg-4 " />

          <div className="col-sm-12 col-md-4 col-lg-4 ">
            <div className="card login-card ">
              <div className=" card-header ">
                <h3 className="text-center"> TO-DO LIST FORM</h3>
              </div>

              <div className="card-body">
                <form className="form-elements">
                  <input
                    value={state.name}
                    className="form-control form-inputs form-elements"
                    type="text"
                    onChange={handleName}
                    placeholder="user name"
                  />
                  <input
                    value={state.eventTitle}
                    className="form-control form-inputs form-elements"
                    type="text"
                    onChange={handleEventTitle}
                    placeholder="Event Title"
                  />
                  <input
                    value={state.details}
                    className="form-control form-inputs form-elements"
                    type="text"
                    onChange={handleDetails}
                    placeholder="Details "
                  />
                </form>
              </div>

              <div className="card-footer ">
                <button
                  type="submit"
                  onClick={submitHandle}
                  className="btn-primary offset-lg-1 offset-md-0 btn-sm "
                >
                  Create
                </button>

                <button
                  type="reset"
                  onClick={resetHandle}
                  className="btn-primary offset-lg-5 offset-md-0 btn-sm"
                >
                  cancel
                </button>
              </div>
            </div>
          </div>
          <div className="col-sm-12 col-md-4 col-lg-4 " />
        </div>

        <div className="container-fluid bg-dark">
          <div className="row ">
            {state.list.map(data => (
              <div style={{ border: "1px black solid" }}>
                <p>{data.name}</p>
                <p>{data.eventTitle}</p>
                <p>{data.details}</p>
              </div>
            ))}
          </div>
        </div>
      </div>

      <div
        className="footer footer-copyright"
        style={{ background: "#e9ecef" }}
      >
        <div className="container">
          <h6 className=" text-center">Just make it work ;)</h6>
        </div>
      </div>
    </div>
  );
};

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

Answer 2

如果您的列表为空，您将收到此错误，只需检查列表是否为空即可解决问题：

{if(state.list){
state.list.map((data,id)=>{
console.log ('loop data',data)
})
}}