Formats

mime-logger lets you control how each log line looks: use one of the four built-in presets, write a format string with tokens, or provide a fully custom FormatFn.

Presets

Import Formats and pass a preset to opts.format:

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

const logger = new Logger("app", { format: Formats.pretty });

Field	Type	Required	Description
Formats.pretty	FormatFn	optional	Default. Dimmed timestamp, colored level badge, yellow name in parens, colored message.
Formats.minimal	FormatFn	optional	Level padded to 5 chars, dimmed name, plain message. Compact single-process output.
Formats.detailed	FormatFn	optional	Caller file:line prepended, ISO timestamp. Best for debugging stack locations.
Formats.json	FormatFn	optional	Structured JSON: { timestamp, level, name, message }. Pipe to log aggregators.

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

for (const preset of ["pretty", "minimal", "json"] as const) {
  console.log(`-- ${preset} --`);
  const logger = new Logger("app", { format: Formats[preset] });
  logger.info("Server started on port 3000");
}

Format string tokens

Pass a string instead of a FormatFn to use token substitution. Useful for simple custom layouts without writing a function.

const logger = new Logger("api", {
  format: "{time} [{level}] {name} — {message}",
});

Field	Type	Required	Description
{time}	string	optional	Localized time with milliseconds: [HH:MM:SS.mmm]
{date}	string	optional	Localized date string.
{iso}	string	optional	ISO 8601 timestamp.
{level}	string	optional	Level name: info | warn | error.
{name}	string	optional	Logger name (empty string if unnamed).
{message}	string	optional	The log message.
{pid}	string	optional	Current process ID.
{env}	string	optional	NODE_ENV or "production" as fallback.
{file}	string	optional	Caller source filename.
{line}	string	optional	Caller line number.
{function}	string	optional	Caller function name.
{caller}	string	optional	One frame above the caller function.
%s	string	optional	Interpolation placeholder for extra args.

Performance

{file}, {line}, {function}, and {caller} trigger a stack trace capture on every log call. Avoid them in hot paths.

Custom format function

For full control, provide a FormatFn. It receives a FormatObject and a ColorHelpers instance.

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

const myFormat: FormatFn = (obj, c) => {
  const ts = c.dim(obj.timestamp.toISOString());
  const level = obj.level.toUpperCase().padEnd(5);
  const name = obj.name ? ` ${c.cyan(obj.name)}` : "";
  return `${ts} ${level}${name} ${obj.message}`;
};

const logger = new Logger("app", { format: myFormat });

FormatObject

interface FormatObject {
  message: string;
  name?: string;
  timestamp: Date;
  level: LogLevel;
  args: any[];
}

ColorHelpers

interface ColorHelpers {
  red: (s: string) => string;
  yellow: (s: string) => string;
  blue: (s: string) => string;
  green: (s: string) => string;
  cyan: (s: string) => string;
  magenta: (s: string) => string;
  gray: (s: string) => string;
  bold: (s: string) => string;
  dim: (s: string) => string;
}

Color functions are powered by ansis and automatically strip ANSI codes when writing to non-TTY outputs (e.g., files).