I use OAuth library from Google in order to set connection with Spotify. There is a problem. When createService() and authCallback() is part of auth object, raised error:

Couldn't find script function: authCallback()

Why is the callback function not visible inside the auth object?

The code for this case:

function doGet() {
    if (auth.hasAccess()) {
        main();
    } else {
        return auth.createFlow();
    }
}

const auth = (function () {
    const CLIENT_ID = '...';
    const CLIENT_SECRET = '...';

    const _service = createService();

    function createService() {
        return OAuth2.createService('spotify')
            .setAuthorizationBaseUrl('https://accounts.spotify.com/authorize')
            .setTokenUrl('https://accounts.spotify.com/api/token')
            .setClientId(CLIENT_ID)
            .setClientSecret(CLIENT_SECRET)
            .setCallbackFunction('authCallback') // set callback
            .setPropertyStore(PropertiesService.getUserProperties())
            .setScope('playlist-read-private playlist-modify-private playlist-modify-public user-library-read')
            .setParam('response_type', 'code')
            .setParam('redirect_uri', getRedirectUri());
    }

    function authCallback(request) {
        let isAuthorized = _service.handleCallback(request);
        if (isAuthorized) {
            return HtmlService.createHtmlOutput('Success! You can close this tab.');
        } else {
            return HtmlService.createHtmlOutput('Denied. You can close this tab');
        }
    }

    ...

    return {
        hasAccess: hasAccess,
        getAccessToken: getAccessToken,
        createFlow: createFlow,
    };
})();

But if do this without auth object, no error and success callback:

function createService() {
    return OAuth2.createService('spotify')
        .setCallbackFunction('authCallback')
        // ...
}

function authCallback(request) {
    // ...
}

I can do this, but then it makes no sense to hide implementation details in the auth object:

const auth = (function () {
    function createService() {
        return OAuth2.createService('spotify')
            .setCallbackFunction('authCallback')
            // ...
    }

    function authCallback(request) {
        // ...
    }

    return {
        // ...
        authCallback: authCallback,
    };
})();

function authCallback(request) {
    return auth.authCallback(request);
}

function doGet() {
    if (auth.hasAccess()) {
        main();
    } else {
        return auth.createFlow();
    }
}

Full code with error

function doGet() {
    if (auth.hasAccess()) {
        main();
    } else {
        return auth.createFlow();
    }
}

const auth = (function () {
    const CLIENT_ID = '...';
    const CLIENT_SECRET = '...';

    const _service = createService();

    function createService() {
        return OAuth2.createService('spotify')
            .setAuthorizationBaseUrl('https://accounts.spotify.com/authorize')
            .setTokenUrl('https://accounts.spotify.com/api/token')
            .setClientId(CLIENT_ID)
            .setClientSecret(CLIENT_SECRET)
            .setCallbackFunction('authCallback')
            .setPropertyStore(PropertiesService.getUserProperties())
            .setScope('playlist-read-private playlist-modify-private playlist-modify-public user-library-read')
            .setParam('response_type', 'code')
            .setParam('redirect_uri', getRedirectUri());
    }

    function authCallback(request) {
        let isAuthorized = _service.handleCallback(request);
        if (isAuthorized) {
            return HtmlService.createHtmlOutput('Success! You can close this tab.');
        } else {
            return HtmlService.createHtmlOutput('Denied. You can close this tab');
        }
    }

    function getRedirectUri() {
        let scriptId = encodeURIComponent(ScriptApp.getScriptId());
        let template = 'https://script.google.com/macros/d/%s/usercallback';
        return Utilities.formatString(template, scriptId);
    }

    function hasAccess() {
        return _service.hasAccess();
    }

    function getAccessToken() {
        return _service.getAccessToken();
    }

    function createFlow() {
        let template = '<a href="%s" target="_blank">Authorize</a>';
        let html = Utilities.formatString(template, _service.getAuthorizationUrl());
        return HtmlService.createHtmlOutput(html);
    }

    return {
        hasAccess: hasAccess,
        getAccessToken: getAccessToken,
        createFlow: createFlow,
    };
})();