A simple logger for Node.js.

Proudly made with ♥ in Valencia, Spain, EU.

Usage

npm install justo-logger

By convention, the package should be imported as log:

const log = require("justo-logger");

Table of contents

1. Architecture
2. Log levels
3. Format patterns
4. Console loggers
5. Colored console loggers
6. Loggers

Architecture

We have a unique component, the logger. A logger represents an object to write log entries. The logger is the component to receive the log messages. When the user registers a log message, the logger creates a log entry and writes it if the entry level is greater than or equal to the minimum level configured.

A log entry contains:

• The source logger.
• The level.
• The timestamp, when the entry was created.
• The message.

Here's an illustrative example:

//(1) create logger and writer
var logger = new ConsoleLogger("app", {minLevel: Level.INFO, maxLevel: Level.FATAL});

//(2) write log entries
logger.debug("My debug message");
logger.info("My info message");
logger.warn("My warn message");
logger.error("My error message");
logger.fatal("My fatal message");

The logger receives the entries DEBUG, INFO, WARN, ERROR and FATAL, but it doesn't write DEBUG, because we have configured it to write only the entries with level greater than or equal to INFO.

Levels

The log level indicates the importance of the entry, from minor to major:

• DEBUG. Debug Message. Only used when development.
• INFO. Informational message.
• WARN. Warning message. For example, the use of something deprecated.
• ERROR. Error message. Something has gone wrong, but the app or component can continue working.
• FATAL. Fatal error message. Something has gone wrong and the app or component can't continue working.

By type of level, we must use the following logger methods:

logger.debug(msg)
logger.info(msg)
logger.warn(msg)
logger.error(msg)
logger.fatal(msg)

The methods can accept several arguments. Their concatenation is the message. Example:

logger.debug("This", "is", "the", "message");
//similar to
logger.debug("This is the message");

Format patterns

A format pattern is a string that describe the entry syntax.

Wildcards:

• %s. The source logger name.
• %l. The level. DEBUG, INFO, WARN, etc.
• %t. The timestamp: yyyy-mm-dd hh:mm:ss.
• %m. The message.

The default pattern is %l [%t]: %m.

Console loggers

A console logger writes to the console. It writes the DEBUG and INFO entries using console.log(), while the other levels with console.error().

Constructors:

constructor()
constructor(name : string)
constructor(opts : object)

The logger options are:

• name (string). The logger name. Default: logger.
• enabled (boolean). Must it write the entries? true, yep; false, nope. Default: true.
• minLevel (Level or string). The minimum level to write.
• maxLevel (Level or string). The maximum level to write.
• pattern (string). The format pattern for all entries.
• patterns (object). The format patterns for specified entries: debug (string), info (string), warn (string), error (string) and/or fatal (string).

Example:

const log = require("justo-logger");
const ConsoleLogger = log.logger.ConsoleLogger;
const Level = log.Level;

var logger = new ConsoleLogger({
  name: "app",
  minLevel: Level.INFO,
  maxLevel: Level.FATAL,
  pattern: "%l: %m",
  patterns: {
    info: "%m"
  }
});

The default format pattern is %l: %m, except for INFO that uses %m. The DEBUG entries won't be written, because the minimum level is INFO.

Colored console loggers

A colored console logger is similar to ConsoleLogger, but always uses console.log() and it writes the level name with colors.

Constructors:

constructor()
constructor(name : string)
constructor(opts : object)

The logger options are:

• name (string). The logger name. Default: logger.
• enabled (boolean). Must it write the entries? true, yep; false, nope. Default: true.
• minLevel (Level or string). The minimum level to write.
• maxLevel (Level or string). The maximum level to write.
• pattern (string). The format pattern for all entries.
• patterns (object). The format patterns for specified entries: debug (string), info (string), warn (string), error (string) and/or fatal (string).
• theme (object). The color theme.

When a colored console logger is used, we have to create themes, objects with the colors for each level. Right now, we can use the following colors:

• black
• blue
• cyan
• gray
• green
• grey
• magenta
• red
• white
• yellow

Example:

const ColoredConsoleLogger = require("justo-logger").logger.ColoredConsoleLogger;
const Level = require("justo-logger").Level;

var logger = new ColoredConsoleLogger({
  name: "app",
  minLevel: Level.INFO,
  maxLevel: Level.FATAL,
  pattern: "%l: %m",
  patterns: {
    info: "%m"
  },
  theme: {
    debug: "gray",
    info: "white",
    warn: "yellow",
    error: "red",
    fatal: "red"
  }
});

Loggers

The Loggers class is an array of loggers. This array contains the following methods:

//add a logger
add(logger)

//invoke the *() method of its loggers
debug(msg)
info(msg)
warn(msg)
error(msg)
fatal(msg)

Example:

const log = require("justo-logger");
const Loggers = log.Loggers;
const ConsoleLogger = log.logger.ConsoleLogger;
const Level = log.Level;

var loggers = new Loggers();
logger.add(new ConsoleLogger({minLogger: Level: INFO}));
logger.add(new ColoredConsoleLogger({minLogger: Level: DEBUG}));

//entry written by the 2nd logger
loggers.debug("the message");

//entry written by all loggers
loggers.info("the message");