22.event-handlers.md

Event Handlers

Binding event handlers in the constructor.

Most of the times we handle DOM events in the component that contains the elements dispatching the events. Like in the example below, we have a click handler and we want to run a function or method of the same component:

class Switcher extends React.Component {
  render() {
    return (
      <button onClick={ this._handleButtonClick }>
        click me
      </button>
    );
  }
  _handleButtonClick() {
    console.log('Button is clicked');
  }
}

That's all fine because _handleButtonClick is a function and we indeed pass a function to the onClick attribute.

The problem is that as it is the code doesn't keep the scope. So, if we have to use this inside _handleButtonClick we'll get an error.

class Switcher extends React.Component {
  constructor(props) {
    super(props);
    this.state = { name: 'React in patterns' };
  }
  render() {
    return (
      <button onClick={ this._handleButtonClick }>
        click me
      </button>
    );
  }
  _handleButtonClick() {
    console.log(`Button is clicked inside ${ this.state.name }`);
    // leads to
    // Uncaught TypeError: Cannot read property 'state' of null
  }
}

What we normally do is to use bind like so:

<button onClick={ this._handleButtonClick.bind(this) }>
  click me
</button>

However, this means that the bind function is called again and again because we may render the button many times. A better approach would be to create the bindings in the constructor of the component:

class Switcher extends React.Component {
  constructor(props) {
    super(props);
    this.state = { name: 'React in patterns' };
    this._buttonClick = this._handleButtonClick.bind(this);
  }
  render() {
    return (
      <button onClick={ this._buttonClick }>
        click me
      </button>
    );
  }
  _handleButtonClick() {
    console.log(`Button is clicked inside ${ this.state.name }`);
  }
}

The other alternative is to use arrow functions for the onClick prop function assignment. Arrow functions don't affect the context at invocation time (this value from the surrounding scope is used).

Facebook by the way recommend the same technique while dealing with functions that need the context of the same component. The binding in the constructor may be also useful if we pass callbacks down the tree.

A short hand example using arrow functions and avoid having to use the constructor:

class Switcher extends React.Component {
  state = { name: 'React in patterns' };

  render() {
    return (
      <button onClick={ this._handleButtonClick }>
        click me
      </button>
    );
  }
  _handleButtonClick = () => {
    console.log(`Button is clicked inside ${ this.state.name }`);
  }
}