The spec for this action:
https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf#G11.2111850
An example of a form using this action:
https://www2.illinois.gov/rev/forms/reg/Documents/reg-1-mr.pdf
if (!(jsDict instanceof Dict)) {
        return;
      }
      if (!isName(jsDict.get("S"), "JavaScript")) {
        return;
      }
      let js = jsDict.get("JS");
      if (isStream(js)) {
        js = js.getString();
      } else if (typeof js !== "string") {
        return;
      }
      if (javaScript === null) {
        javaScript = new Map();
      }
      javaScript.set(name, stringToPDFString(js));
    }
    if (obj instanceof Dict && obj.has("JavaScript")) {
      const nameTree = new NameTree(obj.getRaw("JavaScript"), this.xref);
      for (const [key, value] of nameTree.getAll()) {
        appendIfJavaScriptDict(key, value);
      }
    }
    
    const openAction = this._catDict.get("OpenAction");
    if (openAction) {
      appendIfJavaScriptDict("OpenAction", openAction);
    }
    return javaScript;
  }