Fix Seeing “0” in Your JSX Code

By  on  

The early days of the web felt like the wild west when it came to coding practices -- just make it work. Then we became enlightened to better practices, separating HTML from CSS and JavaScript. Then came React and JSX, where we combine JavaScript, HTML, and even CSS with Styled Components -- what an elegant mess we've made!

Every once in a while part of that mess is me seeing 0 displaying in the output of my JSX code, and I'm reminded why: improper handling of variable typing, combined with using &&. Let me explain!

One of the popular patterns in JSX is:

<div>Some header</div>
{someValue && <div>Some header</div>}

The pattern makes sense but check out the difference in outputs between string and number types:

"0" && "Thing"
> "Thing"
0 && "Thing"
> 0

Note that a string value of 0 allows the second value to be returned, but a number typed 0 simply returns the 0. The best practice is always to cast the value to a Boolean in your JSX:

{Boolean(value) && ....}

Typescript and even PropTypes can help to catch these issues but even seasoned veterans sometimes hit these pain points.

Recent Features

  • By
    9 Mind-Blowing WebGL Demos

    As much as developers now loathe Flash, we're still playing a bit of catch up to natively duplicate the animation capabilities that Adobe's old technology provided us.  Of course we have canvas, an awesome technology, one which I highlighted 9 mind-blowing demos.  Another technology available...

  • By
    Regular Expressions for the Rest of Us

    Sooner or later you'll run across a regular expression. With their cryptic syntax, confusing documentation and massive learning curve, most developers settle for copying and pasting them from StackOverflow and hoping they work. But what if you could decode regular expressions and harness their power? In...

Incredible Demos

  • By
    9 Incredible CodePen Demos

    CodePen is a treasure trove of incredible demos harnessing the power of client side languages.   The client side is always limited by what browsers provide us but the creativity and cleverness of developers always pushes the boundaries of what we think the front end can do.  Thanks to CSS...

  • By
    Add Styles to Console Statements

    I was recently checking out Google Plus because they implement some awesome effects.  I opened the console and same the following message: WARNING! Using this console may allow attackers to impersonate you and steal your information using an attack called Self-XSS. Do not enter or paste code that you...

Discussion

  1. Cuong

    You also can use {!!value && .... }

  2. Marko

    I usually like to be more explicit with these checks to make them more clear, so in this case I would maybe go for this:

        {value == 0 && ...}
    

    Even though === strict equality usually is better and I prefer it, but for this case I would say it’s ok.

    But that !!value mentioned by Cuong is also really good approach. It can just trip up less experienced people.
    One pattern that I also avoid is using myArray.length && and I like to be explicit like myArray.length > 0 && since it makes it more obvious what is going on here. It also can avoid these subtle pitfalls.

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!