Nodejs Winston Logging Format

Winston logger is one of the popular logging libraries in Node.js. Setting it up is as simple as this

const logger = winston.createLogger({
  level: "info",
  format: winston.format.combine(
    winston.format.splat(),
    winston.format.json(),
    winston.format.errors({ stack: true }),
  ),
  transports: [new winston.transports.Console({ level: "info" })],
});

winston uses logform a library for managing different kinds of formats. errors - This will be helpful if you need to print the stack traces of errors. You can simply pass the instance of Error to the log function.

simple - As name says, it simply logs the message like printf in C.

splat - This is very useful if you want to print objects, arrays. It uses Node.js built-in util.format. %s, %d are common known formats.

%j does JSON.stringify of your object. But, if your object has circular references, it prints [Circular]. What is circular reference? Consider the following snippet. obj.xyz attribute has the reference to the whole object.

var obj = { abc: 123 };
obj.xyz = obj;

%o String representation of object with all the properties including non-enumerable properties and proxies. %O Similar to %o but without non-enumerable properties and proxies.

Most of the times, in your code, you need to use %s, %O to log the message. In very rare case, you need to use %o.