std/log/README.md

Deno standard library
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Log

Usage

import * as log from "https://deno.land/std@$STD_VERSION/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).
await 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

Advanced usage

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.

LogRecord

LogRecord is an object that encapsulates provided message and arguments as well some meta data that can be later used when formatting a message.

interface LogRecord {
  readonly msg: string;
  readonly args: any[];
  readonly datetime: Date;
  readonly level: number;
  readonly levelName: string;
  readonly loggerName: string;
}

Log Levels

The different log levels are exported in the LogLevels enum type. It defaults to INFO if none is specified.

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.

log module comes with three built-in handlers:

ConsoleHandler

This is the default logger. It will output color coded log messages to the console via console.log(). This logger takes HandlerOptions:

import type { LogRecord } from "https://deno.land/std@$STD_VERSION/log/mod.ts";

type FormatterFunction = (logRecord: LogRecord) => string;

interface HandlerOptions {
  formatter?: string | FormatterFunction; //see `Custom message format` below
}

FileHandler

This handler will output to a file using an optional mode (default is a, e.g. append). The file will grow indefinitely. It uses a buffer for writing to file. Logs can be manually flushed with fileHandler.flush(). Log messages with a log level greater than error are immediately flushed. Logs are also flushed on process completion. This logger takes FileOptions:

import {
  FormatterFunction,
  LogMode,
} from "https://deno.land/std@$STD_VERSION/log/mod.ts";

interface FileHandlerOptions {
  formatter?: string | FormatterFunction; //see `Custom message format` below
  filename: string;
  mode?: LogMode; // 'a', 'w', 'x'
}

Behavior of the log modes is as follows:

  • 'a' - Default mode. Appends new log messages to the end of an existing log file, or create a new log file if none exists.
  • 'w' - Upon creation of the handler, any existing log file will be removed and a new one created.
  • 'x' - This will create a new log file and throw an error if one already exists.

This handler requires --allow-write permission on the log file.

RotatingFileHandler

This handler extends the functionality of the FileHandler by "rotating" the log file when it reaches a certain size. maxBytes specifies the maximum size in bytes that the log file can grow to before rolling over to a new one. If the size of the new log message plus the current log file size exceeds maxBytes then a roll over is triggered. When a roll over occurs, before the log message is written, the log file is renamed and appended with .1. If a .1 version already existed, it would have been renamed .2 first and so on. The maximum number of log files to keep is specified by maxBackupCount. After the renames are complete the log message is written to the original, now blank, file.

Example: Given log.txt, log.txt.1, log.txt.2 and log.txt.3, a maxBackupCount of 3 and a new log message which would cause log.txt to exceed maxBytes, then log.txt.2 would be renamed to log.txt.3 (thereby discarding the original contents of log.txt.3 since 3 is the maximum number of backups to keep), log.txt.1 would be renamed to log.txt.2, log.txt would be renamed to log.txt.1 and finally log.txt would be created from scratch where the new log message would be written.

This handler uses a buffer for writing log messages to file. Logs can be manually flushed with fileHandler.flush(). Log messages with a log level greater than ERROR are immediately flushed. Logs are also flushed on process completion.

Options for this handler are:

import type {
  FormatterFunction,
  LogMode,
} from "https://deno.land/std@$STD_VERSION/log/mod.ts";

interface RotatingFileHandlerOptions {
  maxBytes: number;
  maxBackupCount: number;
  formatter?: string | FormatterFunction; //see `Custom message format` below
  filename: string;
  mode?: LogMode; // 'a', 'w', 'x'
}

Additional notes on mode as described above:

  • 'a' Default mode. As above, this will pick up where the logs left off in rotation, or create a new log file if it doesn't exist.
  • 'w' in addition to starting with a clean filename, this mode will also cause any existing backups (up to maxBackupCount) to be deleted on setup giving a fully clean slate.
  • 'x' requires that neither filename, nor any backups (up to maxBackupCount), exist before setup.

This handler requires both --allow-read and --allow-write permissions on the log files.

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}.

Eg.

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

await 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.

Custom handlers

Custom handlers can be implemented by subclassing BaseHandler or WriterHandler.

BaseHandler is bare-bones handler that has no output logic at all,

WriterHandler is an abstract class that supports any target with Writer interface.

During setup async hooks setup and destroy are called, you can use them to open and close file/HTTP connection or any other action you might need.

For examples check source code of FileHandler and TestHandler.

Inline Logging

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

import * as logger from "https://deno.land/std@$STD_VERSION/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

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.

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

function expensiveFn(num: number) {
  // do some expensive computation
}

// `expensiveFn(5)` is only evaluated if this logger is configured for debug logging.
logger.debug(() => `this is expensive: ${expensiveFn(5)}`);

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.

Example:

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

await 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