Logging service interface

logging.ts

const len = <T, TIterable extends Array<T>>(iterable: TIterable): number => iterable.length;
const hasMember = <T>(param: T, fieldName: string): boolean => fieldName in param;

interface CategoryInfo {
	id: number;
	name: string;
}

interface LogMessage {
	logText: string;
	catInfo: CategoryInfo | CategoryInfo[] | string | string[];
	logDate: Date;
	logData: any | any[];
	errInfo?: string | object | any[] | null;
}

interface LogPacket {
	magicId: number;
	payload: null | LogMessage | CategoryInfo;
}

class LoggingService /* extends AbstractCategoryLogger */ {
	private defMsg: LogMessage = {
		logText: '',
		catInfo: [],
		logData: {},
		logDate: new Date
	};
	
	// This constructor comes from interfacing with the package, "typescript-logging".
	// It's provided to demonstrate how this class is intended to be used.
	constructor( /*category: Category, runtimeSettings: RuntimeSettings */) {
		/*
		super(category, runtimeSettings);
		
		if (len(category.children) > 0) {
			category.children.forEach( instance => {
				const packet: LogPacket = this.create(instance);
			});
		}
		else {
			const packet: LogPacket = this.create(category);
		}
		*/
	}
	
	private create(payload: CategoryLogMessage | Category): LogPacket {
		const packet: LogPacket = { magicId: 0x4C4F47474552, payload: null } as LogPacket;
		
		if (hasMember(payload, '_message')) {
			const { message, categories, error, logFormat, date: logDate } = payload as CategoryLogMessage;
			
			if (typeof message === typeof {}) {
				const { msg: logText, data: logData } = message as LogData;
				packet.payload = { logText, logData } as LogMessage;
			}
			// String or null?
			else {
				packet.payload = { logText: message } as LogMessage;
			}
			
			packet.payload.catInfo = [];
			if (len(categories) > 1) {
				for (const { id, name } of categories) {
					const category: CategoryInfo = { name, id };
/*
ERROR in logging.ts
./logging.ts
[tsl] ERROR in logging.ts(80,34)
      TS2345: Argument of type 'CategoryInfo' is not assignable to parameter of type 'CategoryInfo & string'.
  Type 'CategoryInfo' is not assignable to type 'string'.
*/
					packet.payload.catInfo.push(category);
				}
			}
			else {
				const [ category ] = categories;
				packet.payload.catInfo = category;
			}
			
			if (error != undefined) {
				console.log({ message, logDate, error, logFormat });
			}
		}
		else {
			const { id, name } = payload as Category;
			packet.payload = { id, name } as CategoryInfo;
		}
		
		return packet;
	}
	
	// This is the only thing you really need to implement.
	// Here, we'll just write the completed message to the 
	// array.
	protected doLog(logEvent: CategoryLogMessage): void {
		this.create(logEvent);
	}
}