反应类型错误：this.state.List.map 不是 function

Question

 import React, { Component } from "react"; import Project from "./components/Project.js" import Form from "./Form.js"; class App extends Component { constructor(props) { super(props); this.state = { projectList: [], myProjects: [], userList: [], }; this.createProject = this.createProject.bind(this); } createProject(title, desc, langs, len, exp) { this.setState({ projectList: this.state.projectList.push( { title: title, description: desc, language: langs, length: len, experience: exp } ) }); } deleteProject(title) { const projects = this.state.projectList.filter( p => p.title;== title ). this;setState({projects}). } render() { return( <div> <Form createProject = {this.createProject} /> {this.state.projectList.map((params) => <Project {..;params}/>)} </div> ); } } export default App;

Hello, when running this, createProject gets passed as a prop to another class, which calls createProject with certain parameters. However, upon trying to render, it returns this error. TypeError: this.state.projectList.map is not a function. Any advice on how to fix it?

Answer 1

push returns the new length of the array, it doesn't return the array. So this code is replacing the array with a number:

this.setState({
     projectList: this.state.projectList.push( // <============
         {
             title : title,
             description : desc,
             language : langs,
             length : len,
             experience : exp
         }
     )
 });

Numbers don't have a map method. :-)

You probably wanted to do this, which creates a new array with the new item added to it:

this.setState({
     projectList: [...this.state.projectList, {
         title : title,
         description : desc,
         language : langs,
         length : len,
         experience : exp
     }]
 });

Answer 2

You cannot declare state like this, you can do something like:

        this.setState({
            projectList: [...this.state.projectList, (
                {
                    title : title,
                    description : desc,
                    language : langs,
                    length : len,
                    experience : exp
                }
            )]
        });

Answer 3

    Hi Adam,

    For the current problem below snippet would solve this issue

    this.setState({
                projectList: [...this.state.projectList, 
                    {
                        title : title,
                        description : desc,
                        language : langs,
                        length : len,
                        experience : exp
                    }
                ]
            });

For better understanding refer below link:
    https://www.robinwieruch.de/react-state-array-add-update-remove