ReactJS error warning

I'm creating my first application with ReactJS and I found this warning when I run my code :

> Warning: Failed form propType: You provided a checked prop to a form > field without an onChange handler. This will render a read-only > field. If the field should be mutable use defaultChecked. Otherwise, > set either onChange or readOnly. Check the render method of > Login.

Can someone tell me how I fix it please ?

React has 2 ways of working with form controls - Controlled Components and Uncontrolled Components

You get this warning when you don't supply the element neither the attributes needed for controlled nor those needed for an uncontrolled component:

> Warning: Failed form propType: You provided a checked prop to a form > field without an onChange handler. This will render a read-only field. > If the field should be mutable use defaultChecked. Otherwise, set > either onChange or readOnly. Check the render method of Login.

Controlled Components

---

Attributes needed:

1. value - <input> (not checkbox or radio), <select>, <textbox> or checked for (checkbox or radio).
2. onChange

---

React handles the condition of the element by updating the value or checked attribute (depending on the element) from the props or the state. We need to notify react when we make a change, like inserting data, or checking the box, so react can update the element's condition when it rerenders the component. To do so, we must include an onChange handler, in which we will update the state or notify the component's parent, so it will update the props.

<input
  type="checkbox"
  checked={ this.props.checked }
  onChange={ this.checkboxHandler } 
/>

const { render } = ReactDOM;

class Demo extends React.Component {
  constructor(props) {
    super(props);
    
    this.state = {
      checked: true
    };
    
    this.checkboxHandler = this.checkboxHandler.bind(this);
  }
  
  checkboxHandler(e) {
    this.setState({
      checked: e.target.checked
    });
  }
  
  render() {
    return (
      <input
        type="checkbox"
        checked={ this.state.checked }
        onChange={ this.checkboxHandler } 
      />
    );
  }
}

render(
  <Demo />,
  document.getElementById('demo')
);

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.4.1/react.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.4.1/react-dom.js"></script>

<h1>The Checkbox</h1>

<div id="demo"></div>

Uncontrolled Components

---

Attributes needed:

defaultValue - <input> (not checkbox or radio), <select>, <textbox> or defaultChecked for (checkbox or radio).

---

React sets the initial value using defaultValue or defaultChecked, and the update of the element's state is controlled by the user, usually via the DOM using refs.

<input
  type="checkbox"
  defaultChecked={ this.props.checked } 
/>

The defaultChecked may not be updated if the component is re-rendered in future so use this approach with caution.

You may be better off just using a blank function to remove the warning. Especially if you want to handle click on the whole div which includes the checkbox and the associated text.

<div onClick={this.handleClick}>
  <input type="checkbox" checked={this.props.checked} onChange={()=>{}}/>
  {this.props.text}
</div>

You need to add defaultChecked attribute to your checkbox:

<div>
    <input type='checkbox' defaultChecked />
</div>

For those that prefer a Functional Component instead of a Class Component this Controlled Component approach is simple and easy to implement. If you don't know what a Controlled Component is then please refer to @Ori Drori's well explained answer in this thread.

import {useState} from "react";

export default function YourComponentName(){

  const [checked, setChecked] = useState(true);

  return (
    <>
          <input
            type="checkbox"
            checked={checked}
            onChange={() => setChecked(!checked)}
          />
    </>
  );
};