Returning multiple elements in JSX

In the lastest react version you can wrap them in an empty fragment:

<>
  <NavItem onClick={this.login}>Zaloguj się</NavItem>
  <NavItem onClick={this.login}>Zaloguj się</NavItem>
</>

If you are using React v16.2.0 and above you can use the shorter version of Fragments

<>
 <NavItem onClick={this.login}>Zaloguj się</NavItem>
 <NavItem onClick={this.login}>Zaloguj się</NavItem>
</>

If you are using a version below React v16 you can only return one element in jsx, so you have to wrap your elements inside one:

<div>
 <NavItem onClick={this.login}>Zaloguj się</NavItem>
 <NavItem onClick={this.login}>Zaloguj się</NavItem>
</div>

If you are using React v16 and abose you can use Fragments

import React, { Fragment } from 'react';

...
...

    <Fragment>
     <NavItem onClick={this.login}>Zaloguj się</NavItem>
     <NavItem onClick={this.login}>Zaloguj się</NavItem>
    <Fragment/>

You could also return an Array of Elements as announced here:

 return [
    <NavItem key="1" onClick={this.login}>Zaloguj się</NavItem>,
    <NavItem key="2" onClick={this.login}>Zaloguj się</NavItem>
  ];

Depending on your environment you might not be able to use all of these solutions: support for fragments