How to add custom function to Winston logger in Typescript?

Question

This question is related to Winston Logger but could apply to any other situation. I have the following code:

import winston, {Logger} from 'winston';

interface CustomLogger extends Logger {
  setLoggerContext(userId: string): void;
}

const logger: CustomLogger = winston.createLogger({
  transports: [new winston.transports.Console()],
});

logger.setLoggerContext = (userId: string) => {
  logger.defaultMeta = {userId: userId}
}

export default logger;

However, const logger is red with an error message saying:

Property 'setLoggerContext' is missing in type 'Logger' but required in type 'CustomLogger'.ts(2741)

I want to protect people from putting defaultMeta what they want, and I really need custom function for this, because later we might change winston with something else, so I want a custom context setter.

Answer 1

Simply construct the intersection of the two interfaces at once (TypeScript can't see that you eventually construct something that conforms to the CustomLogger interface, it only considers the const logger: CustomLogger line):

const CustomLoggerMixin = {
  setLoggerContext(userId: string) {
    logger.defaultMeta = {userId: userId};
  }
};

const logger: CustomLogger = Object.assign(
  winston.createLogger({
    transports: [new winston.transports.Console()],
  }),
  CustomLoggerMixin
);