Create a Controlled Input works, but why?

General
1

My solution works, I’m wondering why though.
on the handleChange function, where is the ‘event’ argument coming from exactly? How is this working?
SPOILER SOLUTION AHEAD
Thanks!


class ControlledInput extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      input: ''
    };
    // change code below this line
    this.handleChange = this.handleChange.bind(this);
    // change code above this line
  }
  // change code below this line
  handleChange(event) {
    this.setState({
      input: event.target.value
    });
  }
  // change code above this line
  render() {
    return (
      <div>
        { /* change code below this line */}
        <input value={this.state.input} onChange={this.handleChange}/>
        { /* change code above this line */}
        <h4>Controlled Input:</h4>
        <p>{this.state.input}</p>
      </div>
    );
  }
};

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User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/73.0.3683.103 Safari/537.36.

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