CSSFunctionRule

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.

This interface also inherits methods from CSSGroupingRule.

CSSFunctionRule.getParameters()

Returns an array of objects representing the custom function's parameters.

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.

@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 Document.styleSheets, 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 sheet = document.styleSheets[0];
const cssFunc = 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:
  js

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

• @function
• CSSFunctionDescriptors
• CSSFunctionDeclarations