MapKit JS mapkit.init authorizationCallback

I have created a script to return a valid jwt token for mapkitjs. I have attempted to use both the fetch and xhr methods to call the script, and both methods receive the token, which I can display using a javascript alert call. I know the token is a good token because I can copy the token from the alert and brute force it into the code and the map is displayed. However, I cannot figure out what is wrong with the token in terms of the "expected pattern." I receive this error in the javascript console: "Unhandled Promise Rejection: SyntaxError: The string did not match the expected pattern." I return a simple string that is only the token itself. I have tried returning a json object like {"jwt":"the token"} or {"JWT":"thetoken"}. I tried changing res.test() to res.json(), although probably not at the some time as when I returned the json object. But then, I don't think that is the real problem, because Apple's description on using the token would not be ... then(res => res.text()). Naturally, I assumed that a text string exactly like is quoted using ... 'const jwt = "thetoken"' was what I was supposed to return to the fetch call. Can someone please tell me what I don't understand about the "expected pattern?"