React

HTML

<div id="app"></div>

CSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

#app {
  background: #fff;
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}

li {
  margin: 8px 0;
}

h2 {
  font-weight: bold;
  margin-bottom: 15px;
}

.done {
  color: rgba(0, 0, 0, 0.3);
  text-decoration: line-through;
}

input {
  margin-right: 5px;
}

React

class Child extends React.Component {
    constructor(props) {
        super(props);
        this.state = {
            isLoggingIn: true
        }
        this.toggleLoggingIn = this.toggleLoggingIn.bind(this)
    }

    toggleLoggingIn() {
        const isLogin = !this.state.isLoggingIn
        this.setState({
            isLoggingIn: isLogin
        });
        console.log(this.props.loginSwitch);
        this.props.loginSwitch(isLogin ? "Log in" : "Register");
    };

    render() {
        return (
            <button onClick={this.toggleLoggingIn} type="link" style={{paddingLeft: 0}}>{this.props.name}!</button>
        );
    }
}

class Parent extends React.Component {
    constructor(props) {
        super(props);
        this.state = {
            loginModalName: "Log in"
        }
        this.updateModalName = this.updateModalName.bind(this)
    }

    updateModalName(name) {
        console.log(name);
        this.setState({
            loginModalName: name
        });
    };

    render() {
        return (
            <Child loginSwitch={this.updateModalName} name={this.state.loginModalName}/>
        );
    }
}

ReactDOM.render(<Parent />, document.querySelector("#app"))