Regular function that return the result of an immediately invoked nested arrow function

index.js

function average() {
  return (() => {
    const length = arguments.length;
    
    if (length == 0) return 0;
    
    const numbers = Array.prototype.slice.call(arguments);
    const sumReduceFn = function (a, b) { return a + Number(b) };
    
    return numbers.reduce(sumReduceFn, 0) / length;
  })();
}