w3resource
React Tutorial

State and Lifecycle

In the previous tutorials, we learnt that the only way to update the UI is by calling the ReactDOM.render() method.

In this tutorial, we will look at the concept of state, this we will do by rewriting the ticking clock example from the previous tutorial.

function tick() {
  const element = (
    <div€
      <h1>Hello, world!</h1>
      <h2>It is {new Date().toLocaleTimeString()}.</h2>
    </div>
  );
  ReactDOM.render(element, document.getElementById('root') );
}
setInterval(tick, 1000);

We can start by encapsulating how the clock looks by splitting the above in bits:

function Clock(props) {
  return (
    <div>
      <h1>Hello, world!</h1>
      <h2>It is {props.date.toLocaleTimeString()}.</h2>
    </div>
  );
}

function tick() {
  ReactDOM.render(
    <Clock date={new Date()} />,
    document.getElementById('root')
  );
}
setInterval(tick, 1000);

However, it misses a crucial requirement: the fact that the Clock sets up a timer and updates the UI every second should be an implementation detail of the Clock.

Ideally we want to write this once and have the Clock update itself:

ReactDOM.render(
  <Clock />,
  document.getElementById('root')
);

To implement this, we need to add "state" to the Clock component.

State is like props, but it is private and fully controlled by the component and its mutable, It can only be used in class components. Thus we have to convert the Clock function component above to a class component.

Converting a Function to a Class

You can convert a function component like Clock to a class in five steps:

  1. Create an ES6 class, with the same name, that extends React.Component.
  2. Add a single empty method to it called render().
  3. Move the body of the function into the render() method.
  4. Replace props with this.props in the render() body.
  5. Delete the remaining empty function declaration.
class Clock extends React.Component {
  render() {
    return (
      <div>
        <h1>Hello, world!</h1>
        <h2>It is {this.props.date.toLocaleTimeString()}.</h2>
      </div>
    );
  }
}

Clock is now defined as a class rather than a function.

The render method will be called each time an update happens, but as long as we render into the same DOM node, only a single instance of the Clock class will be used. This lets us use additional features such as local state and lifecycle methods.

Adding Local State to a Class

We will move the date from props to state in three steps:

  1. Replace this.props.date with this.state.date in the render() method:
  2. Add a class constructor that assigns the initial this.state:
  3. Remove the date prop from the <Clock /> element:

The final code sample looks like this:

class Clock extends React.Component {
  constructor(props) {
    super(props);
    this.state = {date: new Date()};
  }

  render() {
    return (
      <div€
        <h1>Hello, world!>/h1>
        <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
      </div>
    );
  }
}

ReactDOM.render(
  <Clock />,
  document.getElementById('root')
);

Next, we'll make the Clock set up its own timer and update itself every second.

Adding Lifecycle Methods to a Class

In applications with many components, it's very important to free up resources taken by the components when they are destroyed.

We want to set up a timer whenever the Clock is rendered to the DOM for the first time. This is called "mounting" in React.

We also want to clear that timer whenever the DOM produced by the Clock is removed. This is called "unmounting" in React.

We can declare special methods on the component class to run some code when a component mounts and unmounts:

class Clock extends React.Component {
  constructor(props) {
    super(props);
    this.state = {date: new Date()};
  }

  componentDidMount() {

  }

  componentWillUnmount() {

  }

  render() {
    return (
      <div>
        <h1>Hello, world!</h1>
        <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
      </div>
    );
  }
}

These methods are called "lifecycle methods".

The componentDidMount() method runs after the component output has been rendered to the DOM. This is a good place to set up a timer:

componentDidMount() {
    this.timerID = setInterval(
      () => this.tick(),
      1000
    );
  }

Note how we save the timer ID right on this.

While this.props is set up by React itself and this.state has a special meaning, you are free to add additional fields to the class manually if you need to store something that doesn't participate in the data flow (like a timer ID).

We will tear down the timer in the componentWillUnmount() lifecycle method:

componentWillUnmount() {
    clearInterval(this.timerID);
  }

Finally, we will implement a method called tick() that the Clock component will run every second.

It will use this.setState() to schedule updates to the component local state:

class Clock extends React.Component {
  constructor(props) {
    super(props);
    this.state = {date: new Date()};
  }

  componentDidMount() {
    this.timerID = setInterval(
      () => this.tick(),
      1000
    );
  }

  componentWillUnmount() {
    clearInterval(this.timerID);
  }

  tick() {
    this.setState({
      date: new Date()
    });
  }

  render() {
    return (
      <div>
        <h1>Hello, world!</h1>
        <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
      </div>
    );
  }
}

ReactDOM.render(
  <Clock />,
  document.getElementById('root')
);

Now the clock ticks every second.

Let's quickly recap what's going on and the order in which the methods are called:

  1. When <Clock /> is passed to ReactDOM.render(), React calls the constructor of the Clock component. Since Clock needs to display the current time, it initializes this.state with an object including the current time. We will later update this state.
  2. React then calls the Clock component's render() method. This is how React learns what should be displayed on the screen. React then updates the DOM to match the Clock's render output.
  3. When the Clock output is inserted in the DOM, React calls the componentDidMount() lifecycle method. Inside it, the Clock component asks the browser to set up a timer to call the component's tick() method once a second.
  4. Every second the browser calls the tick() method. Inside it, the Clock component schedules a UI update by calling setState() with an object containing the current time. Thanks to the setState() call, React knows the state has changed, and calls the render() method again to learn what should be on the screen. This time, this.state.date in the render() method will be different, and so the render output will include the updated time. React updates the DOM accordingly.
  5. If the Clock component is ever removed from the DOM, React calls the componentWillUnmount() lifecycle method so the timer is stopped.

Using State Correctly

There are three things you should know about setState().

Do Not Modify State Directly

For example, this will not re-render a component:

// Wrong
this.state.comment = 'Hello';
Instead, use setState():
// Correct
this.setState({comment: 'Hello'});

The only place where you can assign this.state is the constructor.

State Updates May Be Asynchronous

React may batch multiple setState() calls into a single update for performance.

Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state.

For example, this code may fail to update the counter:

// Wrong
this.setState({
  counter: this.state.counter + this.props.increment,
});

To fix it, use a second form of setState() that accepts a function rather than an object. That function will receive the previous state as the first argument, and the props at the time the update is applied as the second argument:

// Correct
this.setState((state, props) => ({
  counter: state.counter + props.increment
}));
We used an arrow function above, but it also works with regular functions:
// Correct
this.setState(function(state, props) {
  return {
    counter: state.counter + props.increment
  };
});