Take a look at the Button component class.

Notice that on line 8, Button expects to receive a prop named text. The received text will be displayed inside of a <button></button> element.

What if nobody passes any text to Button?

If nobody passes any text to Button, then Button‘s display will be blank. It would be better if Button could display a default message instead.

You can make this happen by giving your component class a property named defaultProps:

class Example extends React.Component {
  render() {
    return <h1>{this.props.text}</h1>;
  }
}

Example.defaultProps;

The defaultProps property should be equal to an object:

class Example extends React.Component {
  render() {
    return <h1>{this.props.text}</h1>;
  }
}

// Set defaultProps equal to an object:
Example.defaultProps = {};

Inside of this object, write properties for any default props that you’d like to set:

class Example extends React.Component {
  render() {
    return <h1>{this.props.text}</h1>;
  }
}

Example.defaultProps = { text: 'yo' }; 

If an <Example /> doesn’t get passed any text, then it will display “yo.”

If an <Example /> does get passed some text, then it will display that passed-in text.