Output Targets

By default mime-logger writes to the console. You can redirect to a file, a custom function, or any combination of the above.

Console output

The default. No configuration needed:

const logger = new Logger("app");
// equivalent to:
const logger = new Logger("app", { output: "console" });

File output

Pass a FileOutputOptions object to write to a file. The path supports time-based tokens so each run (or each hour/day) gets its own file.

import Logger, { type FileOutputOptions } from "mime-logger";

const fileOut: FileOutputOptions = {
  type: "file",
  path: "./logs/app-{year}-{month}-{day}.log",
};

const logger = new Logger("app", { output: fileOut });

Path tokens

Field	Type	Required	Description
{year}	string	optional	Four-digit year, e.g. 2025.
{month}	string	optional	Two-digit month, e.g. 05.
{day}	string	optional	Two-digit day of month.
{date}	string	optional	Full ISO date, e.g. 2025-05-15.
{hour}	string	optional	Two-digit hour (24 h).
{minutes}	string	optional	Two-digit minutes.
{seconds}	string	optional	Two-digit seconds.
{ms}	string	optional	Three-digit milliseconds.

File streams are opened on first write and kept open for the process lifetime.

Custom output function

Provide a function to send log lines anywhere — a remote sink, a queue, an in-memory buffer for tests, etc.

const logs: string[] = [];

const logger = new Logger("app", {
  output: (message, obj) => {
    logs.push(message);
  },
});

The callback signature:

(message: string, obj: FormatObject) => void

message is the fully formatted string (after format runs). obj is the raw FormatObject if you need the structured data.

Multiple outputs

Pass an array to write to several targets at once:

import Logger, { Formats } from "mime-logger";

const logger = new Logger("app", {
  format: Formats.json,
  output: [
    "console",
    { type: "file", path: "./logs/{year}-{month}-{day}.log" },
  ],
});

Every target receives the same formatted message.