Correct way to define an empty dom element in React

Question

I send an optional parameter checkbox inside a prop to a component:

var checkBox = this.props.checkbox ? <span className='checkbox'></span> : null;

Then I put it like this:

<div>
    ...
    {checkBox}
    ...
</div>

As seen from above, I assign null to the variable. But I can instead assign empty string '' which seems to give the same result.

What's the proper one?

Answer 1

You need to use null. If you use an empty string like '' then react will create an empty span dom element, so it's not the same.

var label1 = <label>My Label</label>; // react generates a label element
var label2 = null; // react doesn't generate any dom element
var label3 = ''; // react generates and empty span like <span></span>

Answer 2

Use false, null or undefined.

This snippet shows the behaviour of rendering falsy values in React:

<meta charset="UTF-8">
<script src="https://fb.me/react-0.13.2.js"></script>
<script src="https://fb.me/JSXTransformer-0.13.2.js"></script>
<div id="app"></div>
<script type="text/jsx;harmony=true">void function() { "use strict";

var falsyValues = [false, 0, "" , null, undefined, NaN]

var App = React.createClass({
  componentDidMount() {
    [].slice.call(React.findDOMNode(this).childNodes)
      .forEach(child => console.log(child.outerHTML))
  },
  render() {
    return <div>
      {falsyValues.map(falsy => <div id={'test-' + falsy}>
        Before
        {falsy}
        After
      </div>)}
    </div>
  }
})

React.render(<App/>, document.getElementById('app'))

}()</script>

false, null and undefined don't generate anything:

<div id="test-false" data-reactid=".0.0"><span data-reactid=".0.0.0">Before</span><span data-reactid=".0.0.2">After</span></div>
<div id="test-null" data-reactid=".0.3"><span data-reactid=".0.3.0">Before</span><span data-reactid=".0.3.2">After</span></div>
<div id="test-undefined" data-reactid=".0.4"><span data-reactid=".0.4.0">Before</span><span data-reactid=".0.4.2">After</span></div>

Whereas '', 0 and NaN do:

<div id="test-0" data-reactid=".0.1"><span data-reactid=".0.1.0">Before</span><span data-reactid=".0.1.1">0</span><span data-reactid=".0.1.2">After</span></div>
<div id="test-" data-reactid=".0.2"><span data-reactid=".0.2.0">Before</span><span data-reactid=".0.2.1"></span><span data-reactid=".0.2.2">After</span></div>
<div id="test-NaN" data-reactid=".0.5"><span data-reactid=".0.5.0">Before</span><span data-reactid=".0.5.1">NaN</span><span data-reactid=".0.5.2">After</span></div>

As a convenience. you can conditionally generate content inline if the value you're testing against will be false, null or undefined when the conditional content shouldn't be displayed:

<div>
    ...
    {this.props.checkbox && <span className='checkbox'></span>}
    ...
</div>