React useState Hook setter has no result with array variable

Question

I'm trying to adapt an example seen on an udemy course from a class-based stateful component to a function based component using the useState hook with React 16.7.0-alpha.2

While setter functions for primitive datatypes work fine (for example setUsername), calling a setter for an array variable has no effect/result. At least it doesn't reset the state variable back to an empty array.

On the other hand, setting a new copy of the array from state by using the concat method works as expected.

I'm still new to React hooks and wonder what I've missed?

import React, {useState} from 'react';
import { Grid, Form, Segment, Button, Header, Message, Icon } from 'semantic-ui-react';
import { Link } from 'react-router-dom';

import { registerUser } from './authFunctions';
import { isRegisterFormEmpty } from './validatorFunctions';

const Register = () => {

  //defining state properties and setters:
  const [username, setUsername] = useState('');
  const [email, setEmail] = useState('');  
  const [password, setPassword] = useState('');
  const [passwordConfirmation, setPasswordConfirmation] = useState('');
  const [registerErrors, setRegisterErrors] = useState([]);  

  //defining handlers:
  const onUsernameChange = e =>             setUsername(e.target.value);
  const onEmailChange = e =>                setEmail(e.target.value);  
  const onPasswordChange = e =>             setPassword(e.target.value);
  const onPasswordConfirmationChange = e => setPasswordConfirmation(e.target.value);

  const onFormSubmit = e => {
    e.preventDefault(); //prevent a page reload

    //set registerErrors to empty array in case that the user clicks on submit again
    setRegisterErrors([]); // DOES NOT WORK

    setUsername('JDoe'); //works as expected

    if( isRegisterFormEmpty(username, email, password, passwordConfirmation) ) {
      let error = {message: 'Please fill in all fields'};
      setRegisterErrors( registerErrors.concat(error) ); //THIS WORKS FINE, THOUGH...
    } else {
      //registerUser(username, email, password, passwordConfirmation);
    }//if

  }//onFormSubmit

  const showErrors = () => registerErrors.map( (error, idx) => <p key={idx}>{error.message}</p> );

  return (
    <Grid textAlign='center' verticalAlign='middle' className='app'>
      <Grid.Column style={{ maxWidth: 450 }}>
        <Header as='h2' icon color='teal' textAlign='center'>
          <Icon name='puzzle piece' color='teal' />
          Register to DevChat
        </Header>
        <Form size='large' onSubmit={onFormSubmit}>
          <Segment stacked>
            <Form.Input 
              fluid
              type='text'              
              icon='user'
              iconPosition='left'
              placeholder='Username'
              onChange={onUsernameChange}
              value={username}
            />
            <Form.Input 
              fluid
              type='email'              
              icon='mail'
              iconPosition='left'
              placeholder='Email'
              onChange={onEmailChange}
              value={email}
            />
            <Form.Input 
              fluid
              type='password'              
              icon='lock'
              iconPosition='left'
              placeholder='Password'
              onChange={onPasswordChange}
              value={password}
            />
            <Form.Input 
              fluid
              type='password'              
              icon='lock'
              iconPosition='left'
              placeholder='Password Confirmation'
              onChange={onPasswordConfirmationChange}
              value={passwordConfirmation}
            />  
            <Button
              color='teal'
              fluid
              size='large'
              content='Submit'
            />          
          </Segment>
        </Form>
        {
          registerErrors.length > 0 && 
            <Message error>
              <h3>Please note</h3>
              {showErrors()}
            </Message>
        }
        <Message>
            Already a user? <Link to='/login'>Login</Link>
        </Message>
      </Grid.Column>
    </Grid>
  )
}

export default Register;

Estus Flask · Accepted Answer · 2019-01-12 21:21:45Z

This is common useState pitfall.

setRegisterErrors([]) works, there's no chance for it to not work because it's called. It triggers synchronous component update. Since onFormSubmit doesn't exit after that, setRegisterErrors(registerErrors.concat(error)) is called after that, where registerErrors is previous state that was defined outside onFormSubmit.

onFormSubmit results in 2 register state updates, where second update (concatenated original array) overrides first update (empty array).

A way to fix this is same as with setState, to use state updater function that provides current state to be updated:

setRegisterErrors(registerErrors => [...registerErrors, error]);

Alternatively, register state updates can be merged:

e.preventDefault();

const registerErrors = [];

setUsername('JDoe');

if( isRegisterFormEmpty(username, email, password, passwordConfirmation) ) {
  registerErrors.push({message: 'Please fill in all fields'});
} else {
  ...
}

setRegisterErrors(registerErrors);

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