score:637

Accepted answer

I believe my input is controlled since it has a value.

For an input to be controlled, its value must correspond to that of a state variable.

That condition is not initially met in your example because this.state.name is not initially set. Therefore, the input is initially uncontrolled. Once the onChange handler is triggered for the first time, this.state.name gets set. At that point, the above condition is satisfied and the input is considered to be controlled. This transition from uncontrolled to controlled produces the error seen above.

By initializing this.state.name in the constructor:

e.g.

this.state = { name: '' };

the input will be controlled from the start, fixing the issue. See React Controlled Components for more examples.

Unrelated to this error, you should only have one default export. Your code above has two.

score:0

For dynamically setting state properties for form inputs and keeping them controlled you could do something like this:

const inputs = [
    { name: 'email', type: 'email', placeholder: "Enter your email"},
    { name: 'password', type: 'password', placeholder: "Enter your password"},
    { name: 'passwordConfirm', type: 'password', placeholder: "Confirm your password"},
]

class Form extends Component {
  constructor(props){
    super(props)
    this.state = {} // Notice no explicit state is set in the constructor
  }

  handleChange = (e) => {
    const { name, value } = e.target;

    this.setState({
      [name]: value
    }
  }

  handleSubmit = (e) => {
    // do something
  }

  render() {
     <form onSubmit={(e) => handleSubmit(e)}>
       { inputs.length ?
         inputs.map(input => {
           const { name, placeholder, type } = input;
           const value = this.state[name] || ''; // Does it exist? If so use it, if not use an empty string

           return <input key={name}  type={type} name={name} placeholder={placeholder} value={value} onChange={this.handleChange}/>
       }) :
         null
       }
       <button type="submit" onClick={(e) => e.preventDefault }>Submit</button>
     </form>    
  }
}

score:1

In short, if you are using class component you have to initialize the input using state, like this:

this.state = { the_name_attribute_of_the_input: "initial_value_or_empty_value" };

and you have to do this for all of your inputs you'd like to change their values in code.

In the case of using functional components, you will be using hooks to manage the input value, and you have to put initial value for each input you'd like to manipulate later like this:

const [name, setName] = React.useState({name: 'initialValue'});

If you'd like to have no initial value, you can put an empty string.

score:2

This generally happens only when you are not controlling the value of the filed when the application started and after some event or some function fired or the state changed, you are now trying to control the value in input field.

This transition of not having control over the input and then having control over it is what causes the issue to happen in the first place.

The best way to avoid this is by declaring some value for the input in the constructor of the component. So that the input element has value from the start of the application.

score:2

Please try this code

import React from "react";

class MyForm extends React.Component {
    constructor(props) {
        super(props);
        this.state = { name: "" };
        this.onFieldChange = this.onFieldChange.bind(this);
    }

    onFieldChange(e) {
        this.setState({[e.target.name]: e.target.value});
    }

    render() {
        return (
            <form className="add-support-staff-form">
                <input name="name" type="text" value={this.state.name} onChange={this.onFieldChange} />
        </form>
        );
    }
}

export default MyForm;

score:3

In my case component was rerendering and throwing A component is changing an uncontrolled input of type checkbox to be controlled error. It turned out that this behaviour was a result of not keeping true or false for checkbox checked state (sometimes I got undefined). Here what my faulty component looked like:

import * as React from 'react';
import { WrappedFieldProps } from 'redux-form/lib/Field';

type Option = {
  value: string;
  label: string;
};

type CheckboxGroupProps = {
  name: string;
  options: Option[];
} & WrappedFieldProps;


const CheckboxGroup: React.FC<CheckboxGroupProps> = (props) => {

  const {
    name,
    input,
    options,
  } = props;

  const [value, setValue] = React.useState<string>();
  const [checked, setChecked] = React.useState<{ [name: string]: boolean }>(
    () => options.reduce((accu, option) => {
      accu[option.value] = false;
      return accu;
    }, {}),
  );

  React.useEffect(() => {
    input.onChange(value);

    if (value) {
      setChecked({
        [value]: true, // that setChecked argument is wrong, causes error
      });
    } else {
      setChecked(() => options.reduce((accu, option) => {
        accu[option.value] = false;
        return accu;
      }, {}));
    }

  }, [value]);

  return (
    <>
      {options.map(({ value, label }, index) => {
        return (
          <LabeledContainer
            key={`${value}${index}`}
          >
            <Checkbox
              name={`${name}[${index}]`}
              checked={checked[value]}
              value={value}
              onChange={(event) => {
                if (event.target.checked) {
                  setValue(value);
                } else {
                  setValue(undefined);
                }
                return true;
              }}
            />
            {label}
          </LabeledContainer>
        );
      })}
    </>
  );
};

To fix that problem I changed useEffect to this

React.useEffect(() => {
  input.onChange(value);

  setChecked(() => options.reduce((accu, option) => {
      accu[option.value] = option.value === value;
      return accu;
    }, {}));

}, [value]);

That made all checkboxes keep their state as true or false without falling into undefined which switches control from React to developer and vice versa.

score:3

For people using Formik, you need to add a default value for the specific field name to the form's initialValues.

score:3

I had the same problem. the problem was when i kept the state info blank

  const [name, setName] = useState()

I fixed it by adding empty string like this

  const [name, setName] = useState('')

score:5

An update for this. For React Hooks use const [name, setName] = useState(" ")

score:5

Simply create a fallback to '' if the this.state.name is null.

<input name="name" type="text" value={this.state.name || ''} onChange={this.onFieldChange('name').bind(this)}/>

This also works with the useState variables.

score:5

I believe my input is controlled since it has a value.

Now you can do this two ways the best way is to have a state key to each input with 1 onChange handler. If you have checkboxes you will need to write a separate onChange handler.

With a Class component you would want to write it like this 👇


import React from 'react';

export default class MyForm extends React.Component {
    constructor(props) {
        super(props);
        this.state = {
         myFormFields: {
           name: '',
           dob: '',
           phone: ''
         }
        }

      this.onFormFieldChange = this.onFormFieldChange.bind(this)
    }


// Always have your functions before your render to keep state batches in sync.
    onFormFieldChange(e) {
       // No need to return this function can be void
       this.setState({
         myFormFields: {
            ...this.state.myFormFields,
            [e.target.name]: e.target.value 
         }
       })
    }


    render() {
       // Beauty of classes we can destruct our state making it easier to place
       const { myFormFields } = this.state
      
        return (
            <form className="add-support-staff-form">
                <input name="name" type="text" value={myFormFields.name} onChange={this.onFormFieldChange}/>
                <input name="dob" type="date" value={myFormFields.dob} onChange={this.onFormFieldChange}/>
                <input name="phone" type="number" value={myFormFields.phone} onChange={this.onFormFieldChange}/>
            </form>
        )
    }

}

export default MyForm;

Hope that helps for a class but the most performative and what the newest thing the devs are pushing everyone to use is Functional Components. This is what you would want to steer to as class components don't intertwine well with the latest libraries as they all use custom hooks now.

To write as a Functional Component


import React, { useState } from 'react';

const MyForm = (props) => {
    // Create form initial state
    const [myFormFields, setFormFields] = useState({
       name: '',
       dob: '',
       phone: ''
    })


   // Always have your functions before your return to keep state batches in sync.
    const onFormFieldChange = (e) => {
       // No need to return this function can be void
       setFormFields({
         ...myFormFields,
         [e.target.name]: e.target.value 
       })
    }

      
       return (
            <form className="add-support-staff-form">
                <input name="name" type="text" value={myFormFields.name} onChange={onFormFieldChange}/>
                <input name="dob" type="date" value={myFormFields.dob} onChange={onFormFieldChange}/>
                <input name="phone" type="number" value={myFormFields.phone} onChange={onFormFieldChange}/>
            </form>
        )

}

export default MyForm;

Hope this helps! 😎

score:6

Set a value to 'name' property in initial state.

this.state={ name:''};

score:7

If the props on your component was passed as a state, put a default value for your input tags

<input type="text" placeholder={object.property} value={object.property ? object.property : ""}>

score:8

When you use onChange={this.onFieldChange('name').bind(this)} in your input you must declare your state empty string as a value of property field.

incorrect way:

this.state ={
       fields: {},
       errors: {},
       disabled : false
    }

correct way:

this.state ={
       fields: {
         name:'',
         email: '',
         message: ''
       },
       errors: {},
       disabled : false
    }

score:9

In my case, I was missing something really trivial.

<input value={state.myObject.inputValue} />

My state was the following when I was getting the warning:

state = {
   myObject: undefined
}

By alternating my state to reference the input of my value, my issue was solved:

state = {
   myObject: {
      inputValue: ''
   }
}

score:10

One potential downside with setting the field value to "" (empty string) in the constructor is if the field is an optional field and is left unedited. Unless you do some massaging before posting your form, the field will be persisted to your data storage as an empty string instead of NULL.

This alternative will avoid empty strings:

constructor(props) {
    super(props);
    this.state = {
        name: null
    }
}

... 

<input name="name" type="text" value={this.state.name || ''}/>

score:20

Simple solution to resolve this problem is to set an empty value by default :

<input name='myInput' value={this.state.myInput || ''} onChange={this.handleChange} />

score:28

I know others have answered this already. But a very important factor here that may help other people experiencing similar issue:

You must have an onChange handler added in your input field (e.g. textField, checkbox, radio, etc). Always handle activity through the onChange handler.

Example:

<input ... onChange={ this.myChangeHandler} ... />

When you are working with checkbox you may need to handle its checked state with !!.

Example:

<input type="checkbox" checked={!!this.state.someValue} onChange={.....} >

Reference: https://github.com/facebook/react/issues/6779#issuecomment-326314716

score:75

Another approach it could be setting the default value inside your input, like this:

 <input name="name" type="text" value={this.state.name || ''} onChange={this.onFieldChange('name').bind(this)}/>

score:165

When you first render your component, this.state.name isn't set, so it evaluates to undefined or null, and you end up passing value={undefined} or value={null}to your input.

When ReactDOM checks to see if a field is controlled, it checks to see if value != null (note that it's !=, not !==), and since undefined == null in JavaScript, it decides that it's uncontrolled.

So, when onFieldChange() is called, this.state.name is set to a string value, your input goes from being uncontrolled to being controlled.

If you do this.state = {name: ''} in your constructor, because '' != null, your input will have a value the whole time, and that message will go away.


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