react.js

<Username
  handler={this.handler} // <---- 'handler' will be available from `props.handler` within the component
  username={this.state.username}
/>


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class Username extends React.Component {
	render() {
		return (
			<input
				name="username"
				value={this.props.username}
				onChange={this.props.handler} // <-- this is shorthand for `onChange={event => this.props.handler(event)}`
			/>
		)
	}
}