npm package discovery and stats viewer.

Discover Tips

  • General search

    [free text search, go nuts!]

  • Package details

    pkg:[package-name]

  • User packages

    @[username]

Sponsor

Optimize Toolset

I’ve always been into building performant and accessible sites, but lately I’ve been taking it extremely seriously. So much so that I’ve been building a tool to help me optimize and monitor the sites that I build to make sure that I’m making an attempt to offer the best experience to those who visit them. If you’re into performant, accessible and SEO friendly sites, you might like it too! You can check it out at Optimize Toolset.

About

Hi, 👋, I’m Ryan Hefner  and I built this site for me, and you! The goal of this site was to provide an easy way for me to check the stats on my npm packages, both for prioritizing issues and updates, and to give me a little kick in the pants to keep up on stuff.

As I was building it, I realized that I was actually using the tool to build the tool, and figured I might as well put this out there and hopefully others will find it to be a fast and useful way to search and browse npm packages as I have.

If you’re interested in other things I’m working on, follow me on Twitter or check out the open source projects I’ve been publishing on GitHub.

I am also working on a Twitter bot for this site to tweet the most popular, newest, random packages from npm. Please follow that account now and it will start sending out packages soon–ish.

Open Software & Tools

This site wouldn’t be possible without the immense generosity and tireless efforts from the people who make contributions to the world and share their work via open source initiatives. Thank you 🙏

© 2024 – Pkg Stats / Ryan Hefner

css-var-listener

v0.0.3

Published

Give meaning to your Custom Properties in CSS with a javascript callback. Every time CSS applies your css var to an element (or removes it), a callback is called.

Downloads

11

Readme

James0x57

cssVarListener

doesn't rely on getComputedStyle() or other functions that cause reflow

npm install css-var-listener

Usage

import cssVarListener from "css-var-listener"

cssVarListener.add("--yourvar", callback, options)

Where:

  • "--yourvar" is any custom property you want to add functionality to
  • callback is a function that takes 1 argument, wich is an object containing:
    • target - the element your CSS is applying "--yourvar" to
    • value - the value of "--yourvar" as determined by the document's stylesheets (see Value determination and specificity at the bottom of this readme)
    • oldValue - the previous value of "--yourvar" if a change in the DOM, :hover states, or stylesheet adding/removal caused it to change
    • compiledValue - the value of "--yourvar" with any var(--dep) replaced with its value as determined by the document's stylesheets
    • oldCompiledValue - the previous compiled value
    • prop - the css variable name "--yourvar"
  • options is an object with optional properties:
    • ignoreAttr: "data-optional-ignore-attr" causes DOM changes within to be ignored (see Ignoring Observer Changes below)

Lifecycle

  • when cssVarListener is called with a prop, it searches the style tags and link'd stylesheets for that prop and caches selectors that set it (plus the specificity and the var value of that rule).
  • The callback is called for every element it applies to initially - just once with the final value.
  • Rules in the CSS with :hover selectors (that set your var) will result in mouseenter and mouseleave events being attached so the var value will be recalculated as needed (and callbacks will be called if/when the var's value changes for an element)
    • CSS selectors with unnecessary/extra :hover at multiple levels are not currently supported
    • for example: div:hover > a:hover is not supported
    • but div:hover > a, div > a:hover is fine
  • an internal observer watches for elements or attributes to be added/changed/removed from the DOM. On any change, it runs the selector cache to determine any callbacks that need to happen
  • if an element is removed or otherwise has your var no longer applied, the callback will be called with the element and undefined for the newValue
  • When a style tag or link'd stylesheet is added or removed, the selector cache is rebuilt and callbacks are called only for elements with a changed value
  • ::pseudo-elements aren't captured by document.querySelectorAll so no callbacks happen for these rules

Ignoring Observer Changes

You can use the built in data-css-var-listener-ignore to ignore all element changes within for all vars enhanced with cssVarListener. This prevents the cascade from checking if the cached selectors apply (or stop applying) to new/different elements.

The ignore attribute is especially important to add to javascript animation containers since every DOM change is observed.

Adding ignoreAttr to cssVarListener's options param (ignoreAttr: "data-optional-ignore-attr") allows specific callbacks (or depentant packages) to individually opt-out of cascade checks from dom changes if needed.

Note: Initial callbacks and initially applied :hover checks for your CSS var still happen if they apply to elements within. Only further changes are ignored. Stylesheets added within are still tracked.

Ignoring styleSheets

Putting data-css-var-listener-ignore-stylesheets on a stylesheet's element or ancestor will cause the stylesheet to be ignored. This prevents newly added or removed stylesheets from rebuilding the selector cache (and subsequent cascade check).

Value determination and specificity

  • If your style has a space ( between: these; ), the value will start with the space. All values are strings (or undefined)
  • !important flag is ignored in the specificity calculation
  • Inline styles (style attribute on the element) are not considered (your callback could override the var inline without blocking potential future callbacks)
  • Any other deviation from CSS in specificity is unintentional, please file an issue if you encounter one