React Redux Reducer: 'this.props.tasks.map is

问题:

I am making a React Redux example; however, I ran into an issue and get the error below:

TypeError: this.props.tasks.map is not a function [Learn More]

I have tried many things and I cannot seem to understand why this is not working. I believe it is when the allReducers maps the tasks from the Tasks function. I have fixed this error back and forth but then it would complain it was undefined. I would fix that and loop back to this issue. Any help would be appreciated. Im sure I am making a simple mistake. Below are my following files

App.js

import React from 'react';
import TaskBoard from "../containers/task-board";
require('../../scss/style.scss');

const App = () => (
    <div>
        <h2>Task List</h2>
        <hr />
        <TaskBoard/>
    </div>
);

export default App;

index.js

    import {combineReducers} from 'redux';
    import {Tasks} from './reducer-tasks';
    const allReducers = combineReducers({
        tasks: Tasks
    });

    export default allReducers

task-board.js

    import React, {Component} from 'react';
    import {bindActionCreators} from 'redux';
    import {connect} from 'react-redux';
    import {deleteTaskAction} from '../actions/ActionIndex';
    import {editTaskAction} from '../actions/ActionIndex';
    class TaskBoard extends Component {
        renderList() {
            return this.props.tasks.map((task) => {
                if(task.status == "pending"){
                    return (<li key={task.id}>
                        {task.id} {task.description}
                        <button type="button">Finish</button>
                        <button type="button">Edit</button>
                        <button onClick={() => this.props.deleteTask(task)} type="button">Delete</button>
                    </li>
                );
            }
        });
    }
    render() {
        if (!this.props.tasks) {
            console.log(this.props.tasks);
            return (<div>You currently have no tasks, please first create one...</div>);
        }
        return (
            <div>
                {this.renderList()}
            </div>
        );
    }
}
    function mapStateToProps(state) {
        return {
            tasks: state.tasks
        };
    }
    function matchDispatchToProps(dispatch){
        return bindActionCreators(
        {
            deleteTask: deleteTaskAction,
            editTask: editTaskAction
        }, dispatch)
    }
    export default connect(mapStateToProps,matchDispatchToProps)(TaskBoard);

reducer-tasks.js

const initialState = {
	tasks: [
        {
            id: 1,
            description: "This is a task",
            status: "pending"
        },
        {
            id: 2,
            description: "This is another task",
            status: "pending"
        },
        {
            id: 3,
            description: "This is an easy task",
            status: "pending" 

        }
	]
}

export function Tasks (state = initialState, action) {
    switch (action.type) {
        case 'ADD_TASK':
            return Object.assign({}, state, {
            	tasks: [
            		...state.tasks,
            		{
            			description: action.text,
            			status: action.status
            		}
            	]
            })
            break;

        case 'EDIT_TASK':
            return action.payload;
            break;

        case 'DELETE_TASK':
            return Object.assign({}, state, {
            	status: action.status
            })
            break;
    }

    return state;
}

actionindex.js

    export const addTaskAction = (task) => {
        return {
            type: 'ADD_TASK',
            text: "Here is a sample description",
            status: "pending"
        }
    };
    export const deleteTaskAction = (task) => {
        return {
            type: 'DELETE_TASK',
            status: "deleted"
        }
    };
    export const editTaskAction = (task) => {
        return {
            type: 'EDIT_TASK',
            payload: task
        }
    };

回答1:

It's because the function 'map' can only be used for arrays, not for objects.

If you print out this.props.tasks in the render function of task-board.js you'll see that it's an OBJECT which contains the tasks array, not the actual tasks array itself.

So to fix this it's quite easy, instead of:

    return this.props.tasks.map((task) => {

it's

    return this.props.tasks.tasks.map((task) => {

Then it works

回答2:

According to your reducer composition, your initialState should be:

const initialState = [
    {
        id: 1,
        description: "This is a task",
        status: "pending"
    },
    {
        id: 2,
        description: "This is another task",
        status: "pending"
    },
    {
        id: 3,
        description: "This is an easy task",
        status: "pending" 

    }
]