Fix Flow Node Issue “property querySelector of unknown”

By  on  
JavaScript Flow React

Flow, the static type checker used in many React projects, feels like a gift and a curse at times; a gift in that it identifies weaknesses in your code, and a curse that sometimes you feel like you're needlessly adjusting your code to satisfy Flow.  I've grown to appreciate Flow but that doesn't mean I end up spending extra hours finding new ways to code.

I recently ran into an issue where I was querying for a node in a React element and then using querySelector on that node to find a child; surprisingly Flow took issue:

Cannot call node.querySelector because property querySelector of unknown
type [1] is not a function.

71│ const { maxHeight } = this.state;
72│ const node = ReactDOM.findDOMNode(this);
[1] 73│ if (node && node.querySelector) {
74│ const popupNode = node.querySelector(".preview-popup");
75│ if (popupNode) {
76│ popupNode.style.maxHeight = `${maxHeight}px`;
77│ }

/private/tmp/flow/flowlib_34a4c903/react-dom.js
[1] 14│ ): null | Element | Text;

Found 1 error

It turns out that findDOMNode can return type Text, and thus querySelectorAll would be undefined;  Flow doesn't like undefined.  The solution is to use instanceOf  with HTMLElement:

if (node instanceOf HTMLElement) {
    // ...
}

The solution makes sense but a part of me silently rages that && node.querySelector doesn't qualify.  In the end, Flow is so helpful that little changes like this don't get me too wound up.

Recent Features

  • By
    CSS 3D Folding Animation

    Google Plus provides loads of inspiration for front-end developers, especially when it comes to the CSS and JavaScript wonders they create. Last year I duplicated their incredible PhotoStack effect with both MooTools and pure CSS; this time I'm going to duplicate...

  • By
    5 HTML5 APIs You Didn’t Know Existed

    When you say or read "HTML5", you half expect exotic dancers and unicorns to walk into the room to the tune of "I'm Sexy and I Know It."  Can you blame us though?  We watched the fundamental APIs stagnate for so long that a basic feature...

Incredible Demos

  • By
    Create a CSS Flipping Animation

    CSS animations are a lot of fun; the beauty of them is that through many simple properties, you can create anything from an elegant fade in to a WTF-Pixar-would-be-proud effect. One CSS effect somewhere in between is the CSS flip effect, whereby there's...

  • By
    Retrieve Your Gmail Emails Using PHP and IMAP

    Grabbing emails from your Gmail account using PHP is probably easier than you think. Armed with PHP and its IMAP extension, you can retrieve emails from your Gmail account in no time! Just for fun, I'll be using the MooTools Fx.Accordion plugin...

Discussion

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