Controlled Components

In HTML, form elements such as <input>, <textarea>, and <select> typically maintain their own state and update it based on user input. In React, mutable state is typically kept in the state property of components, and only updated with setState().

We can combine the two by making the React state be the “single source of truth”. Then the React component that renders a form also controls what happens in that form on subsequent user input. An input form element whose value is controlled by React in this way is called a “controlled component”.

For example, if we want to make the previous example log the name when it is submitted, we can write the form as a controlled component:

class NameForm extends React.Component {

constructor(props) {

super(props);

this.state = {value: ”};

this.handleChange = this.handleChange.bind(this);

this.handleSubmit = this.handleSubmit.bind(this);

}

handleChange(event) {

this.setState({value: event.target.value});

}

handleSubmit(event) {

alert(‘A name was submitted: ‘ + this.state.value);

event.preventDefault();

}

render() {

return (

<form onSubmit={this.handleSubmit}>

<label>

Name:

<input type=”text” value={this.state.value} onChange={this.handleChange} />

</label>

<input type=”submit” value=”Submit” />

</form>

);

}

}

Since the value attribute is set on our form element, the displayed value will always be this.state.value, making the React state the source of truth. Since handleChange runs on every keystroke to update the React state, the displayed value will update as the user types.

With a controlled component, every state mutation will have an associated handler function. This makes it straightforward to modify or validate user input. For example, if we wanted to enforce that names are written with all uppercase letters, we could write handleChange as:

handleChange(event) {

this.setState({value: event.target.value.toUpperCase()});

}