CSSFunctionRule

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The CSSFunctionRule interface of the CSS Object Model represents CSS @function (custom function) at-rules.

Instance properties

This interface also inherits properties from CSSGroupingRule.

CSSFunctionRule.name Read only: Returns a string representing the custom function's name.
CSSFunctionRule.returnType Read only: Returns a string representing the custom function's return type.

Instance methods

This interface also inherits methods from CSSGroupingRule.

CSSFunctionRule.getParameters(): Returns an array of objects representing the custom function's parameters.

Examples

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Basic `CSSFunctionRule` usage

In this example, we define a CSS custom function and then access it using the CSSOM.

CSS

Our CSS defines a custom function using the @function at-rule. The function is called --lighter(), and outputs a lightened version of an input color. --lighter() accepts two parameters, a <color> and a <number>. It returns an oklch() color created using relative color syntax; the input color is transformed into an oklch() color and its lightness channel is increased by the input number.

css

@function --lighter(--color <color>, --lightness-adjust <number>: 0.2) returns
  <color> {
  result: oklch(from var(--color) calc(l + var(--lightness-adjust)) c h);
}

JavaScript

Our script starts by getting a reference to the stylesheet attached to our document using HTMLStyleElement.sheet, then getting a reference to the only rule in the stylesheet, the CSSFunctionRule — via CSSStylesheet.cssRules. We then log each of the CSSFunctionRule members to the console.

// Get a CSSFunctionRule
const cssFunc = document.getElementById("css-output").sheet.cssRules[0];

// Accessing CSSFunctionRule members
console.log(cssFunc.name);
console.log(cssFunc.returnType);
console.log(cssFunc.getParameters());

The name property is equal to --lighter.
The returnType property is equal to <color>.

The getParameters() method returns an array that looks like so:

[
  { name: "--color", type: "<color>" },
  { defaultValue: "0.2", name: "--lightness-adjust", type: "<number>" },
];

Specifications

CSS Functions and Mixins Module