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An example repo showing how to write a complex log extension for LoopBack 4


This repository shows you how to use @loopback/cli to write a complex logging extension that requires a Component, Decorator, and a Mixin.

To use this extension you can add the LogMixin to your Application which will provide you a function to set the Application wide log level as well as automatically load the LogComponent. Only Controller methods configured at or above the logLevel will be logged.

You may alternatively load LogComponent yourself and set the log level using the appropriate binding keys manually if you don’t wish to use the LogMixin.

Possible levels are: DEBUG < INFO < WARN < ERROR < OFF

Possible levels are represented as numbers but users can use LOG_LEVEL.${level} to specify the value instead of using numbers.

A decorator enables you to set the log level for Controller methods, at or above which it should be logged.

Example Usage

import {LogMixin, LOG_LEVEL, log} from 'loopback4-example-log-extension';
// Other imports ...

class LogApp extends LogMixin(BootMixin(RestApplication)) {
  constructor(options?: ApplicationConfig) {

    this.projectRoot = __dirname;

class MyController {
  hello() {
    return 'Hello LoopBack';

  helloName() {
    return 'Hello Name';

Cloning the example project locally

You can obtain a local clone of this project (without the rest of our monorepo) using the following command:

lb4 example log-extension


Install @loopback/cli by running npm i -g @loopback/cli.

Initialize your new extension project as follows: lb4 extension

  • Project name: loopback4-example-log-extension
  • Project description: An example extension project for LoopBack 4
  • Project root directory: (loopback4-example-log-extension)
  • Component class name: LogComponent
  • Select features to enable in the project’: tslint, prettier, mocha, loopbackBuild

Now you can write the extension as follows:


Define Binding keys here for the component as well as any constants for the user (for this extension that’ll be the logLevel enum).

 * Binding keys used by this component.
export namespace EXAMPLE_LOG_BINDINGS {
  export const APP_LOG_LEVEL = BindingKey.create<LOG_LEVEL>(
  export const TIMER = BindingKey.create<TimerFn>('example.log.timer');
  export const LOGGER = BindingKey.create<LogWriterFn>('example.log.logger');
  export const LOG_ACTION = BindingKey.create<LogFn>('example.log.action');

 * Enum to define the supported log levels
export enum LOG_LEVEL {


Before we continue, we will need to install a new dependecy as follows:

npm i @loopback/rest

Now we define TypeScript type definitions / interfaces for complex types and functions here.

import {Request, OperationArgs} from '@loopback/rest';

 * A function to perform REST req/res logging action
export interface LogFn {
    req: Request,
    args: OperationArgs,
    // tslint:disable-next-line:no-any
    result: any,
    startTime?: HighResTime,
  ): Promise<void>;

  startTimer(): HighResTime;

 * Log level metadata
export type LevelMetadata = {level: number};

 * High resolution time as [seconds, nanoseconds]. Used by process.hrtime().
export type HighResTime = [number, number]; // [seconds, nanoseconds]

 * Log writing function
export type LogWriterFn = (msg: string, level: number) => void;

 * Timer function for logging
export type TimerFn = (start?: HighResTime) => HighResTime;


Extension developers can create decorators to provide “hints” (or metadata) to user artifacts such as controllers and their methods. These “hints” allow the extension to add extra processing accordingly.

For this extension, the decorator marks which controller methods should be logged (and optionally at which level they should be logged). We leverage @loopback/metadata module to implement the decorator and inspection function.

import {LOG_LEVEL, EXAMPLE_LOG_BINDINGS} from '../keys';
import {
} from '@loopback/context';
import {LevelMetadata} from '../types';

 * Mark a controller method as requiring logging (input, output & timing)
 * if it is set at or greater than Application LogLevel.
 * @param level The Log Level at or above it should log
export function log(level?: number) {
  if (level === undefined) level = LOG_LEVEL.WARN;
  return MethodDecoratorFactory.createDecorator<LevelMetadata>(

 * Fetch log level stored by `@log` decorator.
 * @param controllerClass Target controller
 * @param methodName Target method
export function getLogMetadata(
  controllerClass: Constructor<{}>,
  methodName: string,
): LevelMetadata {
  return (
    ) || {level: LOG_LEVEL.OFF}


Extension users must set an app wide log level at or above which the decorated controller methods will be logged. A user can do so by binding the level to example.log.level but this can be a hassle.

A mixin makes it easier for the user to set the application wide log level by providing it via ApplicationOptions or using a helper method app.logLevel(level: number).

import {Constructor} from '@loopback/context';
import {EXAMPLE_LOG_BINDINGS} from '../keys';
import {LogComponent} from '../component';

// tslint:disable-next-line:no-any
export function LogMixin<T extends Constructor<any>>(superClass: T) {
  return class extends superClass {
    // tslint:disable-next-line:no-any
    constructor(...args: any[]) {
      if (this.options && this.options.logLevel) {

    logLevel(level: LOG_LEVEL) {


A Providers is a class that returns a value() function that can be invoked by LoopBack 4.


A timer than can be used to time the function that is being logged.

import {Provider} from '@loopback/context';
import {TimerFn, HighResTime} from '../types';

export class TimerProvider implements Provider<TimerFn> {
  constructor() {}
  value(): TimerFn {
    return (start?: HighResTime): HighResTime => {
      if (!start) return process.hrtime();
      return process.hrtime(start);


This will be the most important provider for the extension as it is responsible for actually logging the request. The extension will retrieve the metadata stored by the @log() decorator using the controller and method name. Since bindings are resolved at runtime and these values change with each request, inject.getter() must be used to get a function capable of resolving the value when called. The action provider will look as follows:

import {inject, Provider, Constructor, Getter} from '@loopback/context';
import {CoreBindings} from '@loopback/core';
import {OperationArgs, Request} from '@loopback/rest';
import {getLogMetadata} from '../decorators/log.decorator';
import {EXAMPLE_LOG_BINDINGS, LOG_LEVEL} from '../keys';
import {
} from '../types';
import chalk from 'chalk';

export class LogActionProvider implements Provider<LogFn> {
  // LogWriteFn is an optional dependency and it falls back to `logToConsole`
  @inject(EXAMPLE_LOG_BINDINGS.LOGGER, {optional: true})
  private logWriter: LogWriterFn = logToConsole;

  @inject(EXAMPLE_LOG_BINDINGS.APP_LOG_LEVEL, {optional: true})
  private logLevel: number = LOG_LEVEL.WARN;

    private readonly getController: Getter<Constructor<{}>>,
    private readonly getMethod: Getter<string>,
    @inject(EXAMPLE_LOG_BINDINGS.TIMER) public timer: TimerFn,
  ) {}

  value(): LogFn {
    const fn = <LogFn>((
      req: Request,
      args: OperationArgs,
      // tslint:disable-next-line:no-any
      result: any,
      start?: HighResTime,
    ) => {
      return this.action(req, args, result, start);

    fn.startTimer = () => {
      return this.timer();

    return fn;

  private async action(
    req: Request,
    args: OperationArgs,
    // tslint:disable-next-line:no-any
    result: any,
    start?: HighResTime,
  ): Promise<void> {
    const controllerClass = await this.getController();
    const methodName: string = await this.getMethod();

    const metadata: LevelMetadata = getLogMetadata(controllerClass, methodName);
    const level: number | undefined = metadata ? metadata.level : undefined;

    if (
      level !== undefined &&
      this.logLevel !== LOG_LEVEL.OFF &&
      level >= this.logLevel &&
      level !== LOG_LEVEL.OFF
    ) {
      if (!args) args = [];
      let msg = `${req.url} :: ${}.`;
      msg += `${methodName}(${args.join(', ')}) => `;

      if (typeof result === 'object') msg += JSON.stringify(result);
      else msg += result;

      if (start) {
        const timeDiff: HighResTime = this.timer(start);
        const time: number =
          timeDiff[0] * 1000 + Math.round(timeDiff[1] * 1e-4) / 100;
        msg = `${time}ms: ${msg}`;

      this.logWriter(msg, level);

function logToConsole(msg: string, level: number) {
  let output;
  switch (level) {
      output = chalk.white(`DEBUG: ${msg}`);
    case LOG_LEVEL.INFO:
      output =`INFO: ${msg}`);
    case LOG_LEVEL.WARN:
      output = chalk.yellow(`WARN: ${msg}`);
      output =`ERROR: ${msg}`);
  if (output) console.log(output);


Export all the files to ensure a user can import the necessary components.

export * from './decorators/log.decorator';
export * from './mixins/log-level.mixin';
export * from './providers/log-action.provider';
export * from './providers/timer.provider';
export * from './component';
export * from './types';
export * from './keys';


Package the providers in the component to their appropriate Binding keys so they are automatically bound when a user adds the component to their application.

import {Component, ProviderMap} from '@loopback/core';
import {EXAMPLE_LOG_BINDINGS} from './keys';
import {LogActionProvider} from './providers/log-action.provider';
import {TimerProvider} from './providers/timer.provider';

export class LogComponent implements Component {
  providers?: ProviderMap = {
    [EXAMPLE_LOG_BINDINGS.TIMER.key]: TimerProvider,
    [EXAMPLE_LOG_BINDINGS.LOG_ACTION.key]: LogActionProvider,


Tests should be written to ensure the behaviour implemented is correct and future modifications don’t break this expected behavior (unless it’s intentional in which case the tests should be updated as well).

Take a look at the test folder to see the variety of tests written for this extension. There are unit tests to test functionality of individual functions as well as an extension acceptance test which tests the entire extension as a whole (everything working together).



Run npm test from the root folder.


See all contributors.