hellog

Your new logger !

hellog is a general-purpose logging library.

It offers a console.log-like API and formatting, extensible type-safety colored lines and timestamps (or not if desired), all that with 0 dependencies.

1. Installation
2. Get started
3. Transports and options
   1. Console
   2. File
4. Formatters
   1. Available formatters
   2. Add custom formatters
5. Extensibility
   1. Context elaboration
      1. Presentation
      2. Message context elaboration
   2. Transport
      1. Preparation
      2. Formatting
      3. Transportation

Install

npm install --save @steffthestunt/hellog

Get started

const { Hellog } = require('hellog');

const logger = new Hellog();

const obj = {
  foo: {
    foo: {
      foo: {
        foo: {
          foo: 'bar',
        },
      },
    },
  },
};

logger.log(obj);
logger.success('Hello world !');
logger.debug(obj);
logger.error(new Error('test'));
logger.warn(new Error('test'));

Transport and options

Console

This is the transport used by default. It outputs messages withing the console, enriched with some information and colors.

const { Hellog, transports } = require('hellog');

const transport = new transports.Console();

const logger = new Hellog({
  transports: [transport],
});

File

A transport for outputting messages within a file.

const { Hellog, transports } = require('hellog');

const transport = new transports.FileTransport();

const logger = new Hellog({
  transports: [transport],
});

It ouptus JSON objects by default:

{"timestamp":"2021-12-26T15:14:13.668Z","message":"{\n  some: 'bar',\n  foo: { nested: { bar: 'test', test: 'bar', hello: 2 } }\n}","level":"info"}
{"timestamp":"2021-12-26T15:14:13.669Z","message":"{\n  some: 'bar',\n  foo: { nested: { bar: 'test', test: 'bar', hello: 2 } }\n}","level":"success"}
{"timestamp":"2021-12-26T15:14:13.669Z","message":"{\n  some: 'bar',\n  foo: { nested: { bar: 'test', test: 'bar', hello: 2 } }\n}","level":"debug"}
{"timestamp":"2021-12-26T15:14:13.669Z","message":"Error: test\n    at Object.<anonymous> (/Users/escape/Code/hellog/tester.js:19:14)\n    at Module._compile (node:internal/modules/cjs/loader:1101:14)\n    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10)\n    at Module.load (node:internal/modules/cjs/loader:981:32)\n    at Function.Module._load (node:internal/modules/cjs/loader:822:12)\n    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)\n    at node:internal/main/run_main_module:17:47","level":"error"}
{"timestamp":"2021-12-26T15:14:13.669Z","message":"Error: test\n    at Object.<anonymous> (/Users/escape/Code/hellog/tester.js:20:13)\n    at Module._compile (node:internal/modules/cjs/loader:1101:14)\n    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10)\n    at Module.load (node:internal/modules/cjs/loader:981:32)\n    at Function.Module._load (node:internal/modules/cjs/loader:822:12)\n    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)\n    at node:internal/main/run_main_module:17:47","level":"warn"}

It can also output strings:

const { Hellog, transports } = require('hellog');

const fileTransport = new transports.FileTransport({
  outType: 'string',
  filename: '/path/to/your/log/file', // defaults to "output.log"
});

const logger = new Hellog({
  transports: [fileTransport],
});

2021-12-26 15:20:42 | [info]    | {
2021-12-26 15:20:42 | [info]    |   some: 'bar',
2021-12-26 15:20:42 | [info]    |   foo: { nested: { bar: 'test', test: 'bar', hello: 2 } }
2021-12-26 15:20:42 | [info]    | }
2021-12-26 15:20:42 | [success] | {
2021-12-26 15:20:42 | [success] |   some: 'bar',
2021-12-26 15:20:42 | [success] |   foo: { nested: { bar: 'test', test: 'bar', hello: 2 } }
2021-12-26 15:20:42 | [success] | }
2021-12-26 15:20:42 | [debug]   | {
2021-12-26 15:20:42 | [debug]   |   some: 'bar',
2021-12-26 15:20:42 | [debug]   |   foo: { nested: { bar: 'test', test: 'bar', hello: 2 } }
2021-12-26 15:20:42 | [debug]   | }
2021-12-26 15:20:42 | [error]   | Error: test
2021-12-26 15:20:42 | [error]   |     at Object.<anonymous> (/Users/escape/Code/hellog/tester.js:21:14)
2021-12-26 15:20:42 | [error]   |     at Module._compile (node:internal/modules/cjs/loader:1101:14)
2021-12-26 15:20:42 | [error]   |     at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10)
2021-12-26 15:20:42 | [error]   |     at Module.load (node:internal/modules/cjs/loader:981:32)
2021-12-26 15:20:42 | [error]   |     at Function.Module._load (node:internal/modules/cjs/loader:822:12)
2021-12-26 15:20:42 | [error]   |     at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
2021-12-26 15:20:42 | [error]   |     at node:internal/main/run_main_module:17:47
2021-12-26 15:20:42 | [warn] | Error: test
2021-12-26 15:20:42 | [warn] |     at Object.<anonymous> (/Users/escape/Code/hellog/tester.js:22:13)
2021-12-26 15:20:42 | [warn] |     at Module._compile (node:internal/modules/cjs/loader:1101:14)
2021-12-26 15:20:42 | [warn] |     at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10)
2021-12-26 15:20:42 | [warn] |     at Module.load (node:internal/modules/cjs/loader:981:32)
2021-12-26 15:20:42 | [warn] |     at Function.Module._load (node:internal/modules/cjs/loader:822:12)
2021-12-26 15:20:42 | [warn] |     at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
2021-12-26 15:20:42 | [warn] |     at node:internal/main/run_main_module:17:47

Options

outType: 'json' | 'string'

Defaults to "json", the type of output of the file transport

filename: string

Defaults to "output.log", the path to the file where logs are written. Can absolute or relative. However, we recommend using absolute paths.

Formatters

Every transport takes a formatters parameter in it's options.

const csle = new transports.Console<ExtendedContext>({
  // These are the default formatters, and the logger will default to this list
  formatters: [
    formatters.addLevel(true),
    formatters.addTimestamp(),
    formatters.colorize(),
  ],
});

const logger = new Hellog<ExtendedContext>({
  transports: [csle],
});

logger.suceess('I hve been formatted !');

Available formatters

addTimestamp

Add the current date at the beginning of the line.

2021-12-26 15:20:42 | {
2021-12-26 15:20:42 |   some: 'bar',
2021-12-26 15:20:42 |   foo: { nested: { bar: 'test', test: 'bar', hello: 2 } }
2021-12-26 15:20:42 | }

addLevel(bold: boolean = true)

Add the log level at the beginning of the line. It can be displayed in bold or not.

[info] | {
[info] |   some: 'bar',
[info] |   foo: { nested: { bar: 'test', test: 'bar', hello: 2 } }
[info] | }

colorize

Appends colorizing escape characters at the beginning and the end of the formatted string, depending on the log level

Custom formatters

You can implement and add custom formatters, as long as they match the Formatter signature.

type Formatter<T extends PreparedMessage = PreparedMessage> = (
  current: string,
  context: T
) => string;

Example: A simple formatter that will insert a hello message at the beginning of the log

const exampleFormatter = (current: string, context: PreparedMessage) {
  return "hello " + current
}

Before extending PreparedMessage in your formatter declaration, you should read the Extensibility section

Extensibility

The logging functionalities of Hellog are designed to be extensible. You can add personalized behaviour to the following elements of the logger:

• Message context elaboration
• Message transportation
  • Preparation
  • Formatting
  • Transportation

Hellog is also designed to guarantee type-safety even with your custom implementations !

To better understand where you can perform custom logic, let's quickly review the lifecycle of a message within the logger.

The logger is called

new Hellog().log({ foo: bar }, 'Hello world');

Context elaboration

Presentation

The very first thing done by Hellog is converting these inputs into a string.

We use util.format to do so, the same function used in the global console. That is the reason why Hellog can offer a console-like API.

Once we have a string, a context object is created. At this point, the type of the context object is PreparedMessage.

{
  timestamp: Date,
  message: string
  level: LogLevel
}

Then, for each transport, it's log method is called. This method is guaranteed to exist, because it is on the BaseTransport abstract class, extended by all transports classes.

Here the default implementation of the log method on the BaseTransport class. Let's break this down.

export abstract class BaseTransport<
  T extends PreparedMessage = PreparedMessage
> {
  //...

  log(log: T) {
    const messages = this.prepareTransport(log);
    const fMessages = messages.map((message) => this.format(message));
    fMessages.forEach((fMessage) => this.transport(fMessage));
  }

  //...
}

Improve context elaboration

If you need to add additional data into the PreparedMessage object being passed along this cycle, you can using the additionalContext when instanciating the Hellog object.

You can even do it with type safety and completion !

import { Hellog } from './logger';

interface ExtendedContext extends PreparedMessage {
  id: number;
}

const displayId: Formatter<ExtendedContext> = (
  current: string,
  context: ExtendedContext
) => {
  return `${context.id} | ${current}`;
};

const csle = new transports.Console<ExtendedContext>({
  formatters: [
    formatters.addLevel(true),
    displayId,
    formatters.addTimestamp(),
    formatters.colorize(),
  ],
});

const logger = new Hellog<ExtendedContext>({
  transports: [csle],
  additionalContext: (message) => ({
    ...message,
    id: Math.floor(Math.random() * 1000),
  }),
});

logger.error(new Error('test'));

Transport

Preparation

In Hellog, messages are formatted and displayed on a per-line basis. Meaning here that if your log spans on mutliple lines, it is breaked into a list of lines on the "\n" character.

Each one of these lines will be formatted during the rest of the execution.

This behaviour can be overriden when writing your own transport class. For example, the File transport is implemented as follows:

export class FileTransport<T extends PreparedMessage = PreparedMessage> {
  //...

  prepareTransport(log: T): T[] {
    if (this.outType === 'json') {
      return [log];
    } else {
      return log.message.split('\n').map((message) => ({ ...log, message }));
    }
  }

  //...
}

Here, we don't want to break line when outputting JSON message, this is left to any system that will monitor this and read the log file.

Each input in the resulting list is passed to the format method of the transport.

Formatting

This section is about extending the formatting logic. See here for Custom Formatters

Formatting is performed on a per-line basis. Each transport is instanciated with a list of formatters, that extends the Formatter type.

export type Formatter<T extends PreparedMessage = PreparedMessage> = (
  current: string,
  context: T
) => string;

For each line, every formatters are called in the order defined by the list provided during class instanciation. By default the base transport uses three standard formats (provided by the library itself).

Here is the default implementation of the format step:

export abstract class BaseTransport<
  T extends PreparedMessage = PreparedMessage
> {
  formatters: Formatter[] = [addLevel(), addTimestamp(), colorize()];

  //...

  format(preparedLine: T): string {
    let formattedMessage = preparedLine.message;
    for (const formatter of this.formatters) {
      formattedMessage = formatter(formattedMessage, preparedLine);
    }
    return formattedMessage;
  }

  //...
}

You can override this behaviour when doing your own transport class.

Transportation

Once the log is nothing more than a list of string, the transport method of the transport is called for each resulting string.

Very various thing happen at this point. by default, a simple console.log is performed here. The FileTransport writes into a fs.WriteStream. You can implement a transport where the lines are sent over HTTP. Here are some examples.