/log/mod.ts | std@0.210.0

import * as mod from "https://deno.land/std@0.210.0/log/mod.ts";

Logging library with the support for terminal and file outputs. Also provides interfaces for building custom loggers.

Loggers

Loggers are objects that you interact with. When you use a logger method it constructs a LogRecord and passes it down to its handlers for output. To create custom loggers, specify them in loggers when calling log.setup.

Custom message format

If you want to override default format of message you can define formatter option for handler. It can be either simple string-based format that uses LogRecord fields or more complicated function-based one that takes LogRecord as argument and outputs string.

The default log format is {levelName} {msg}.

Logging Structured JSON Lines

To output logs in a structured JSON format you can configure most handlers with a formatter that produces a JSON string. Either use the premade log.formatters.jsonFormatter or write your own function that takes a LogRecord and returns a JSON.stringify'd object. If you want the log to go to stdout then use ConsoleHandler with the configuration useColors: false to turn off the ANSI terminal colours.

import * as log from "https://deno.land/std@0.210.0/log/mod.ts";

log.setup({
  handlers: {
    default: new log.handlers.ConsoleHandler("DEBUG", {
      formatter: log.formatters.jsonFormatter,
      useColors: false,
    }),
  },
});

The first argument passed to a log function is always treated as the message and will be stringified differently. To have arguments JSON.stringify'd you must pass them after the first.

import * as log from "https://deno.land/std@0.210.0/log/mod.ts";

log.info("This is the message", { thisWillBe: "JSON.stringify'd"});
// {"level":"INFO","datetime":1702501580505,"message":"This is the message","args":{"thisWillBe":"JSON.stringify'd"}}

log.info({ thisWontBe: "JSON.stringify'd"}, "This is an argument");
// {"level":"INFO","datetime":1702501580505,"message":"{\"thisWontBe\":\"JSON.stringify'd\"}","args":"This is an argument"}

Inline Logging

Log functions return the data passed in the msg parameter. Data is returned regardless if the logger actually logs it.

Lazy Log Evaluation

Some log statements are expensive to compute. In these cases, you can use lazy log evaluation to prevent the computation taking place if the logger won't log the message.

NOTE: When using lazy log evaluation, undefined will be returned if the resolver function is not called because the logger won't log it. It is an antipattern use lazy evaluation with inline logging because the return value depends on the current log level.

For module authors

The authors of public modules can let the users display the internal logs of the module by using a custom logger:

import { getLogger } from "https://deno.land/std@0.210.0/log/mod.ts";

function logger() {
  return getLogger("my-awesome-module");
}

export function sum(a: number, b: number) {
  logger().debug(`running ${a} + ${b}`);
  return a + b;
}

export function mult(a: number, b: number) {
  logger().debug(`running ${a} * ${b}`);
  return a * b;
}

The user of the module can then display the internal logs with:

import * as log from "https://deno.land/std@0.210.0/log/mod.ts";
import { sum } from "<the-awesome-module>/mod.ts";

log.setup({
  handlers: {
    console: new log.handlers.ConsoleHandler("DEBUG"),
  },

  loggers: {
    "my-awesome-module": {
      level: "DEBUG",
      handlers: ["console"],
    },
  },
});

sum(1, 2); // prints "running 1 + 2" to the console

Please note that, due to the order of initialization of the loggers, the following won't work:

import { getLogger } from "https://deno.land/std@0.210.0/log/mod.ts";

const logger = getLogger("my-awesome-module");

export function sum(a: number, b: number) {
  logger.debug(`running ${a} + ${b}`); // no message will be logged, because getLogger() was called before log.setup()
  return a + b;
}

Examples

Example 1

import * as log from "https://deno.land/std@0.210.0/log/mod.ts";

// Simple default logger out of the box. You can customize it
// by overriding logger and handler named "default", or providing
// additional logger configurations. You can log any data type.
log.debug("Hello world");
log.info(123456);
log.warning(true);
log.error({ foo: "bar", fizz: "bazz" });
log.critical("500 Internal server error");

// custom configuration with 2 loggers (the default and `tasks` loggers).
log.setup({
  handlers: {
    console: new log.handlers.ConsoleHandler("DEBUG"),

    file: new log.handlers.FileHandler("WARNING", {
      filename: "./log.txt",
      // you can change format of output message using any keys in `LogRecord`.
      formatter: "{levelName} {msg}",
    }),
  },

  loggers: {
    // configure default logger available via short-hand methods above.
    default: {
      level: "DEBUG",
      handlers: ["console", "file"],
    },

    tasks: {
      level: "ERROR",
      handlers: ["console"],
    },
  },
});

let logger;

// get default logger.
logger = log.getLogger();
logger.debug("fizz"); // logs to `console`, because `file` handler requires "WARNING" level.
logger.warning(41256); // logs to both `console` and `file` handlers.

// get custom logger
logger = log.getLogger("tasks");
logger.debug("fizz"); // won't get output because this logger has "ERROR" level.
logger.error({ productType: "book", value: "126.11" }); // log to `console`.

// if you try to use a logger that hasn't been configured
// you're good to go, it gets created automatically with level set to 0
// so no message is logged.
const unknownLogger = log.getLogger("mystery");
unknownLogger.info("foobar"); // no-op

Custom message format example

import * as log from "https://deno.land/std@0.210.0/log/mod.ts";

log.setup({
  handlers: {
    stringFmt: new log.handlers.ConsoleHandler("DEBUG", {
      formatter: "[{levelName}] {msg}",
    }),

    functionFmt: new log.handlers.ConsoleHandler("DEBUG", {
      formatter: (logRecord) => {
        let msg = `${logRecord.level} ${logRecord.msg}`;

        logRecord.args.forEach((arg, index) => {
          msg += `, arg${index}: ${arg}`;
        });

        return msg;
      },
    }),

    anotherFmt: new log.handlers.ConsoleHandler("DEBUG", {
      formatter: "[{loggerName}] - {levelName} {msg}",
    }),
  },

  loggers: {
    default: {
      level: "DEBUG",
      handlers: ["stringFmt", "functionFmt"],
    },
    dataLogger: {
      level: "INFO",
      handlers: ["anotherFmt"],
    },
  },
});

// calling:
log.debug("Hello, world!", 1, "two", [3, 4, 5]);
// results in: [DEBUG] Hello, world!
// output from "stringFmt" handler.
// 10 Hello, world!, arg0: 1, arg1: two, arg3: [3, 4, 5] // output from "functionFmt" formatter.

// calling:
log.getLogger("dataLogger").error("oh no!");
// results in:
// [dataLogger] - ERROR oh no! // output from anotherFmt handler.

JSON to stdout with no color example

import * as log from "https://deno.land/std@0.210.0/log/mod.ts";

log.setup({
  handlers: {
    jsonStdout: new log.handlers.ConsoleHandler("DEBUG", {
      formatter: log.formatters.jsonFormatter,
      useColors: false,
    }),
  },

  loggers: {
    default: {
      level: "DEBUG",
      handlers: ["jsonStdout"],
    },
  },
});

// calling:
log.info("Hey");
// results in:
// {"level":"INFO","datetime":1702481922294,"message":"Hey"}

// calling:
log.info("Hey", { product: "nail" });
// results in:
// {"level":"INFO","datetime":1702484111115,"message":"Hey","args":{"product":"nail"}}

// calling:
log.info("Hey", 1, "two", [3, 4, 5]);
// results in:
// {"level":"INFO","datetime":1702481922294,"message":"Hey","args":[1,"two",[3,4,5]]}

Custom JSON example

import * as log from "https://deno.land/std@0.210.0/log/mod.ts";

log.setup({
  handlers: {
    customJsonFmt: new log.handlers.ConsoleHandler("DEBUG", {
      formatter: (record) => JSON.stringify({
        lvl: record.level,
        msg: record.msg,
        time: record.datetime.toISOString(),
        name: record.loggerName,
      }),
      useColors: false,
    }),
  },

  loggers: {
    default: {
      level: "DEBUG",
      handlers: ["customJsonFmt"],
    },
  },
});

// calling:
log.info("complete");
// results in:
// {"lvl":20,"msg":"complete","time":"2023-12-13T16:38:27.328Z","name":"default"}

Inline Logging

import * as logger from "https://deno.land/std@0.210.0/log/mod.ts";

const stringData: string = logger.debug("hello world");
const booleanData: boolean = logger.debug(true, 1, "abc");
const fn = (): number => {
  return 123;
};
const resolvedFunctionData: number = logger.debug(fn());
console.log(stringData); // 'hello world'
console.log(booleanData); // true
console.log(resolvedFunctionData); // 123

Lazy Log Evaluation

import * as log from "https://deno.land/std@0.210.0/log/mod.ts";

log.setup({
  handlers: {
    console: new log.handlers.ConsoleHandler("DEBUG"),
  },

  loggers: {
    tasks: {
      level: "ERROR",
      handlers: ["console"],
    },
  },
});

function someExpensiveFn(num: number, bool: boolean) {
  // do some expensive computation
}

// not logged, as debug < error.
const data = log.debug(() => someExpensiveFn(5, true));
console.log(data); // undefined

Classes

c Logger
c LoggerConfig
c LogRecord	An object that encapsulates provided message and arguments as well some metadata that can be later used when formatting a message.

Variables

v formatters
v handlers	Handlers are responsible for actual output of log messages. When a handler is called by a logger, it firstly checks that `LogRecord`'s level is not lower than level of the handler. If level check passes, handlers formats log record into string and outputs it to target.
v LogLevels	Use this to retrieve the numeric log level by it's associated name. Defaults to INFO.

Functions

f critical	Log with critical level, using default logger.
f debug	Log with debug level, using default logger.
f error	Log with error level, using default logger.
f getLogger	Get a logger instance. If not specified `name`, get the default logger.
f info	Log with info level, using default logger.
f setup	Setup logger config.
f warning	Log with warning level, using default logger.

Interfaces

I HandlerOptions
I LogConfig

Type Aliases

T FormatterFunction
T LevelName	Union of valid log level names
T LogLevel	Union of valid log levels
T LogMode