Get another component input value React js

You can try lifting the state up.

Create a new component that will contain your two components. In that new component, create a function that you will pass as props inside the sidebar component.

class ContainerComponent extends React.Component {
    constructor() {
        super();
        this.state = {
            valueThatNeedsToBeShared: '' 
        }
    }

    handleChange(e) {
        this.setState({valueThatNeedsToBeShared: e.target.value})
    }

    render() {
        return (
            <div>
                <AppComponent value={this.state.valueThatNeedsToBeShared} />
                <SidebarComponent handleChange={this.handleClick.bind(this)} value={this.state.valueThatNeedsToBeShared} />
            </div>
        )
    }
}

const SidebarComponent = ({handleChange, value}) => <aside>
    <input value={value} onChange={handleChange} />
</aside>

const AppComponent = ({value}) => <div>
    value from sidebar: {value}
</div>

In pure react it is possible by adding callback from one component and use it into parent component to change their state and then send input value from parent's state to props of your second component