How to Get and Set CSS Variable Values with JavaScript

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CSS variables are a very welcome addition to the language, despite them being incredibly basic.  Sure we could use SASS or stylus but languages should never count on developers relying on frameworks and toolkits to accomplish what we know we need.  And just like every other part of a webpage, you can get and manipulate CSS variable values -- let's check out how!

Setting and Using a CSS Variables

The traditional method of using native CSS variables is adding it to root:

:root {
    --my-variable-name: #999999;
}

Simple.  Also remember that CSS variables are nowhere near as powerful as variables within SASS, stylus, etc.

Getting a CSS Variable's Value

To retrieve the value of a CSS variable within the window, you use getComputedStyle and getPropertyValue:

getComputedStyle(document.documentElement)
    .getPropertyValue('--my-variable-name'); // #999999

The computed variable value comes back as a string.

Setting a CSS Variable's Value

To set the value of a CSS variable using JavaScript, you use setProperty on documentElement's style property:

document.documentElement.style
    .setProperty('--my-variable-name', 'pink');

You'll immediately see the new value applied everywhere the variable is used.

I had anticipated the need for disgusting hacks to accomplish CSS variable manipulation with JavaScript so I'm happy it's as easy as described above!

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Discussion

  1. Ben C

    This is great! I hadn’t heard about this feature before now.

    With regard to comparisons with variables in CSS pre-processor languages like SASS, correct me if I’m missing something, but that seems to overlook what is so useful about this new feature, which is that variable values can be manipulated at run-time in the browser, meaning you can take into account contextual data like viewport size or user input; this is of course not possible with SASS variables, whose values are fixed at compile time.

  2. Josep V.

    I’ve been for a few days using this API and its greats. The only thing that I don’t like is that if you set the vars in a Child, and then you want to get the variables in a SubChild, you won’t be able to fetch anything.

    On the other hand if you use it in your css or styles property, it will work… it’s a bit odd that cascading of vars works for the css, but not for the prototype of these elements. Thoughts?

    • Josep V.

      Oh the frameworks backfire sometimes! answering myself on the previous entry… custom properties won’t cascade to the element until they are added to the DOM which *completely* makes sense.

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