Optic's streams allows you to format your logs however you wish, either through your own custom formatting or several out of the box formatters. Formatters are typically set directly on the stream via withFormat().

Three out of the box formatters are available.

This formatter allows you to transform a log record to string using tokens as placeholders for the various log record fields. When formatting a log record the tokens are replaced by values of the log record fields. Any unrecognized tokens (as well as other text) are left unmodified.

Available tokens are:

• {msg} - The log record message
• {metadata} - An array of metadata associated with the log record
• {dateTime} - The date/time the log record was created
• {level} - The log level of the log record
• {logger} - The name of this logger

The constructed token template is then passed to the withFormat function.

Complete example using TokenReplacer:

import { ConsoleStream, Logger } from "https://deno.land/x/optic/mod.ts";
import { TokenReplacer } from "https://deno.land/x/optic/formatters/mod.ts";

const logger = new Logger().addStream(
  new ConsoleStream()
    .withFormat(
      new TokenReplacer()
        .withFormat("{dateTime} Level: [{level}] Msg: {msg}")
        .withDateTimeFormat("hh:mm:ss YYYY-MM-DD")
        .withLevelPadding(10)
        .withColor(),
    ),
);

logger.info("hello world");
// Outputs in color to console: 22:09:54 2020-07-10 Level: [Info      ] Msg: hello world

• withDateTimeFormat allows for custom date/time formats and is described in more detail below.
• withLevelPadding allows you to pad the level name to allow the message to start at the same column in the console (as different levels have different lengths).
• withColor allows you to specify if the formatter output should be wrapped in color based on the log level

The defaults for this formatter are:

• Format is "{dateTime} {level} {msg} {metadata}"
• No color
• Optimal padding
• UTC date/time format. Example: 2020-07-11T20:16:34.477Z

This formatter will take a log record and construct a structured JSON formatted string with the specified log record fields. You may additionally specify the JSON string to be pretty printed and the format of the date/time field.

Available fields are:

• msg - The log record message
• metadata - An array of metadata associated with the log record
• dateTime - The date/time the log record was created
• level - The log level of the log record
• logger - The name of this logger

Complete example using JsonFormatter:

import { ConsoleStream, Logger } from "https://deno.land/x/optic/mod.ts";
import { JsonFormatter } from "https://deno.land/x/optic/formatters/mod.ts";

const logger = new Logger().addStream(
  new ConsoleStream()
    .withFormat(
      new JsonFormatter()
        .withFields(["dateTime", "level", "logger", "msg"])
        .withDateTimeFormat("hh:mm:ss YYYY-MM-DD")
        .withPrettyPrintIndentation(2)
    ),
);

logger.info("Hello world");

// Outputs:
{
  "dateTime": "22:49:09 2020-07-10",
  "level": "Info",
  "logger": "default",
  "msg": "Hello world"
}

The defaults for this formatter are:

• Fields of ["dateTime", "level", "msg", "metadata"]
• UTC date/time format. Example: 2020-07-11T20:16:34.477Z
• No pretty printing

This is a formatter to be used by other formatters to output the date and time with a custom specification. An input string defines the formatting of the timestamp using pre-defined tokens. The output date/time string is constructed from the local date/time.

E.g. to use as a standalone class

import { SimpleDateTimeFormatter } from "https://deno.land/x/optic/formatters/mod.ts";

const dtf = new SimpleDateTimeFormatter("hh:mm:ss:SSS YYYY-MM-DD");
const dateTime = dtf.formatDateTime(new Date());

The formatting tokens are as per below. Any characters not formatted are left as is. Tokens are case sensitive.

Optic's formatters allow you to add this formatter as follows, typically using the shorthand of just the formatting string:

import { ConsoleStream, Logger } from "https://deno.land/x/optic/mod.ts";
import { TokenReplacer } from "https://deno.land/x/optic/formatters/mod.ts";

const logger = new Logger().addStream(
  new ConsoleStream()
    .withFormat(
      new TokenReplacer()
        .withDateTimeFormat("hh:mm:ss:SSS YYYY-MM-DD"),
      // equivalent to:
      // .withDateTimeFormat(new SimpleDateTimeFormatter("hh:mm:ss:SSS YYYY-MM-DD"))
    ),
);

You can easily supply your own formatting capabilities via an implementation of the Formatter interface:

export interface Formatter<T> {
  format(logRecord: LogRecord): T;
}

Example:

import {
  ConsoleStream,
  Formatter,
  Logger,
  LogRecord,
} from "https://deno.land/x/optic/mod.ts";

class MyFormatter implements Formatter<string> {
  format(logRecord: LogRecord): string {
    return "Hello! " + logRecord.msg;
  }
}

const logger = new Logger().addStream(
  new ConsoleStream().withFormat(new MyFormatter()),
);

logger.info("Some info message");

// Outputs to console: "Hello! Some info message"