What could be causing the error "Cannot read the state property of undefined in react-native?"

Question

What could be causing the error "Cannot read the state property of undefined in react-native?"

I really need some assistance. I am attempting to create a JSON object called users in my state properties to test the functionality of my authentication system. However, when I tried to access it, I encountered the error "Cannot read property 'state' of undefined" with the error pointing to this part of the code const { users, textInputEmail, textInputPassword } = this.state. Additionally, when I checked the users variable, it showed 'undefined'. What did I do incorrectly?

import React, { Component } from 'react'
import { View, TextInput } from 'react-native'
import { MyButton, ErrorMessage } from '../uikit'
import { FormStyle, InputStyle } from '../constants/styles'
import { SIGN_IN, SUCCESS } from '../routes'

export class LogIn extends Component {

    state = {
        users: {
            user: [
                {
                    name: '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="177d786773676c72725b66737279797b6275387a7674">[email protected]</a>',
                    password: '12345678'
                },

                {
                    name: '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="466f6a60657a686f48787e61636275233a3634">[email protected]</a>',
                    password: '87654321'
                }

            ],
        },
        textInputEmail: '',
        textInputPassword: ''
    }

    isUser() {
        console.log(users)
        const { users, textInputEmail, textInputPassword } = this.state
        let currentUser, password;

        currentUser = users.map((user) => user.name == textInputEmail ? user : 'unknown')

        if (currentUser == 'unknown')
            return alert('Incorrect user or password!')
        else {
            if (currentUser.password == textInputPassword)
                this.props.navigation.navigate(SUCCESS)
        }
    }

    render() {
        const { mainContainer, buttons } = FormStyle
        const { container, text } = InputStyle

        return (
            <View>
                <View style={mainContainer}>
                    <View style={container}>
                        <TextInput
                            style={text}
                            placeholder={'Email/Login'}
                            onChangeText={(value) => this.setState({ textInputEmail: value })}
                        >
                        </TextInput>
                        <ErrorMessage errorText={'Incorrect email'} />
                    </View>

                    <View style={container}>
                        <TextInput
                            style={text}
                            placeholder={'Password'}
                            secureTextEntry={true}
                            onChangeText={(value) => this.setState({ textInputPassword: value })}
                        >
                        </TextInput>
                        <ErrorMessage errorText={'Incorrect password'} />
                    </View>

                    <View style={buttons}>
                        <MyButton
                            name={'Log in'.toUpperCase()}
                            onPress={this.isUser} />
                        <MyButton
                            name={'Sign in'.toUpperCase()}
                            onPress={() => this.props.navigation.navigate(SIGN_IN)} />
                    </View>
                </View>
            </View>
        )
    }
}

javascript json dictionary react-native

Answer 1

Answer №1

To ensure the correct 'this' value in React class components, it is necessary to bind the function. There are multiple ways to achieve this, which can be found here.

One approach is to update the declaration of your 'isUser' method from:

isUser() {
  ...
}

To:

isUser = () => {
  ...
}

Alternatively, you can bind it within a constructor like so:

export class LogIn extends Component {
  constructor(props) {
    super(props);
    this.isUser = this.useUser.bind(this);
  }
  ...
}

Another option is to bind the function directly in the 'render' method, as shown below:

<MyButton
  name={'Log in'.toUpperCase()}
  onPress={this.isUser.bind(this)} />

Answer 2

To ensure the correct 'this' value in React class components, it is necessary to bind the function. There are multiple ways to achieve this, which can be found here.

One approach is to update the declaration of your 'isUser' method from:

isUser() {
  ...
}

To:

isUser = () => {
  ...
}

Alternatively, you can bind it within a constructor like so:

export class LogIn extends Component {
  constructor(props) {
    super(props);
    this.isUser = this.useUser.bind(this);
  }
  ...
}

Another option is to bind the function directly in the 'render' method, as shown below:

<MyButton
  name={'Log in'.toUpperCase()}
  onPress={this.isUser.bind(this)} />

Answer 3

Answer №2

When you utilize the following code:

onPress={this.isUser}

You're disconnecting isUser from the component's scope, resulting in "this" becoming undefined when it is called. To resolve this issue, you can create an arrow function that simply invokes this.isUser():

onPress={() => this.isUser()}

Alternatively, you can turn isUser itself into an arrow function:

isUser = () => {
  ...
}

If you are interested in a more detailed explanation of why this change is necessary and how it impacts the code, I have discussed it further here.

Answer 4

When you utilize the following code:

onPress={this.isUser}

You're disconnecting isUser from the component's scope, resulting in "this" becoming undefined when it is called. To resolve this issue, you can create an arrow function that simply invokes this.isUser():

onPress={() => this.isUser()}

Alternatively, you can turn isUser itself into an arrow function:

isUser = () => {
  ...
}

If you are interested in a more detailed explanation of why this change is necessary and how it impacts the code, I have discussed it further here.

What could be causing the error "Cannot read the state property of undefined in react-native?"

Answer №1

Answer №2

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