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React State

Binding this keyword

In React class components, it is common to pass event handler functions to elements in the render() method. If those methods update the component state, this must be bound so that those methods correctly update the overall component state.

In the example code, we bind this.changeName() so that our event handler works.

class MyName extends React.Component {
  constructor(props) {
    super(props);
    this.state = { name: 'Jane Doe' };
    this.changeName = this.changeName.bind(this);
  }


  changeName(newName) {
    this.setState({ name: newName });
  }


  render() {
    return (
      <h1>My name is {this.state.name}</h1>
      <NameChanger handleChange={this.changeName} />
    )
  }
}

Call super() in the Constructor

React class components should call super(props) in their constructors in order to properly set up their this.props object.

// WRONG!
class BadComponent extends React.Component {
  constructor() {
    this.state = { favoriteColor: 'green' };
  }
  // ...
}


// RIGHT!
class GoodComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = { favoriteColor: 'green' };
  }
  // ...
}

this.setState()

React class components can change their state with this.setState(). this.setState() should always be used instead of directly modifying the this.state object.

this.setState() takes an object which it merges with the component’s current state. If there are properties in the current state that aren’t part of that object, then those properties are unchanged.

In the example code, we see this.setState() used to update the Flavor component’s state from 'chocolate' to 'vanilla'.

class Flavor extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      favorite: 'chocolate',
    };
  }


  render() {
    return (
      <button
        onClick={(event) => {
          event.preventDefault();
          this.setState({ favorite: 'vanilla' });
        }}
      >
        No, my favorite is vanilla
      </button>
    );
  }
}

Dynamic Data in Components

React components can receive dynamic information from props, or set their own dynamic data with state. Props are passed down by parent components, whereas state is created and maintained by the component itself.

In the example, you can see this.state set up in the constructor, used in render(), and updated with this.setState(). this.props refers to the props, which you can see in the render() method.

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = { showPassword: false };
  }


  render() {
    let text;
    if (this.state.showPassword) {
      text = `The password is ${this.props.password}`;
    } else {
      text = 'The password is a secret';
    }


    return (
      <div>
        <p>{text}</p>
        <button
          onClick={(event) => {
            event.preventDefault();
            this.setState((oldState) => ({
              showPassword: !oldState.showPassword,
            }));
          }}
        >
          Toggle password
        </button>
      </div>
    );
  }
}

Component State in Constructor

React class components store their state as a JavaScript object. This object is initialized in the component’s constructor().

In the example, the component stores its state in this.state.

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      favoriteColor: 'green',
      favoriteMusic: 'Bluegrass',
    };
  }


  render() {
    // ...
  }
}

Don’t Change State While Rendering

When you update a React component’s state, it will automatically re-render. That means you should never update the state in a render function because it will cause an infinite loop.

In the example, we show some bad code that calls this.setState() inside of its render() method.

class BadComponent extends React.Component {
  constructor(props) {
    super(props);
    this.count = 0;
  }
  render() {
    // Don't do this! This is bad!
    this.setState({ count: this.state.count + 1 });
    return <div>The count is {this.state.count}</div>;
  }
}
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