The read-only name property of the CSSPropertyRule interface represents the property name, this being the serialization of the name given to the custom property in the @property rule's prelude.

A string.

This stylesheet contains a single @property rule. The first CSSRule returned will be a CSSPropertyRule representing this rule. The name property returns the string "--property-name", which is the name given to the custom property in CSS.

@property --property-name {
  syntax: "<color>";
  inherits: false;
  initial-value: #c0ffee;
}

let myRules = document.styleSheets[0].cssRules;
console.log(myRules[0].name); //the string "--property-name"