I'm in wix and I'm trying to pass a function as a call back to a function that came from a backend and call it in my b

import {testCB} from "backend_filename.jsw"

testCB(d=>{
    console.log("inside testCB");
    console.log(d);
},123);

function in backend

export function testCB(cb, arg){
  console.log("inside testCB:");
  console.log(cb); console.log(arg);
  cb(1,arg);
}

The output of the log shows that cb is null but the arg parameter's value passes through as 123. How do I pass a function as an argument through a function I imported from the backend?