The prefix property of the CSSCounterStyleRule interface gets and sets the value of the prefix descriptor. If the descriptor does not have a value set, this attribute returns an empty string.

A string

The following example shows a @counter-style rule. In JavaScript, myRules[0] is this @counter-style rule, returning prefix gives us the value "Chapter ".

@counter-style chapters {
  system: numeric;
  symbols: "0" "1" "2" "3" "4" "5" "6" "7" "8" "9";
  prefix: "Chapter ";
}

let myRules = document.styleSheets[0].cssRules;
console.log(myRules[0].prefix); // "Chapter "