NodeJS/Javascript: how to detect an error object return value?

I have a function that might return some object or might return a custom error object. I am unable to detect error object type.

I have tried constructor.match(/Error/i) or tried to work on Object.keys of prototype but nothing worked. Following is the code?

function x() {
    try {
        ...
    } catch {e} {
       let err = new Error('caught error')
       return err
    }
    return someObject
}

//the following gives: TypeError: err.constructor.match is not a function
if (x().constructor.match(/Error/i)) {
   //log error
}
//do something

Any ideas how to detect the output error type?

via NodejsStarter