Does React block users from finding components behind login?

I'm new to React and am working on an app that requires user authentication. I have my authentication working fine, basically with redux keeping track of an isSignedIn piece of state, which defaults to false. If false, the user is shown a login form, and if true, the user is shown whatever else they need to see. The login form uses axios to send a post to a server-side authentication script, which responds with a JSON object that includes whether the user is valid or not, and the reducer sets isSignedIn accordingly.

I'm curious though, since the entire app is client-side javascript, can't a nefarious user somehow un-pack the script and modify it locally so they can see the components that normally don't render unless logged in?

Some relevant code snippets from the main App component...

render:

render(){
    return (
        <div className="ui container">
            {
                this.props.isSignedIn &&
                <Navigation pathname={this.props.pathname} />
            }
            {
                !this.props.isSignedIn &&
                <Login />
            }
        </div>
    );
}

mapStateToProps:

const mapStateToProps = (state) => {
    return {
        isSignedIn: state.isSignedIn
    };
};