Logging in Lambda functions

Applicable when

• You want to enable or disable logging for your lambda based on the configuration (e.g. environment variable)

Non-applicable when

• None

Implementation

A simple drop-in replacement for the Typescript console logger (created as a separate file and then imported):

// tslint:disable:no-any
// tslint:disable:no-console

/**
 * Set lambda env variable LOG_LEVEL to one of the values below
 */
enum LogLevel {
  DEBUG = 'DEBUG',
  INFO = 'INFO',
  WARN = 'WARN',
  ERROR = 'ERROR',
}

const DefaultLogLevel = LogLevel.DEBUG;

const LogLevelPriority = {
  [LogLevel.DEBUG]: 0,
  [LogLevel.INFO]: 1,
  [LogLevel.WARN]: 2,
  [LogLevel.ERROR]: 3,
};

export class Log {
  public static debug(message?: any, ...optionalParams: any[]): void {
    if (Log.isLogEnabled(LogLevel.DEBUG)) {
      console.debug(message, optionalParams);
    }
  }

  public static info(message?: any, ...optionalParams: any[]): void {
    if (Log.isLogEnabled(LogLevel.INFO)) {
      console.info(message, optionalParams);
    }
  }

  public static warn(message?: any, ...optionalParams: any[]): void {
    if (Log.isLogEnabled(LogLevel.WARN)) {
      console.warn(message, optionalParams);
    }
  }

  public static error(message?: any, ...optionalParams: any[]): void {
    if (Log.isLogEnabled(LogLevel.ERROR)) {
      console.error(message, optionalParams);
    }
  }

  private static currentLogLevel(): LogLevel {
    return (process.env.LOG_LEVEL as LogLevel) ?? DefaultLogLevel;
  }

  private static isLogEnabled(level: LogLevel): boolean {
    return LogLevelPriority[Log.currentLogLevel()] <= LogLevelPriority[level];
  }
}

 If you need to log to another destination, you can easily add it to this logger.

Usage:

import { APIGatewayProxyEvent, APIGatewayProxyResult } from 'aws-lambda';
import { Log } from '../util/logging';

// Proxy integration handler should conform this type: Handler<APIGatewayProxyEvent, APIGatewayProxyResult>
export async function handler(event: APIGatewayProxyEvent): Promise {
  Log.info(`Method called for api ${event.requestContext.apiId}`);
  if (event.body) {
    try {
      Log.debug('Payload before parsing', event.body);
      const payload = JSON.parse(event.body);

      Log.info('Method invoked with payload', payload);
    } catch (e) {
      Log.warn('Cannot process payload', event.body, e);
    }
  }

  return {
    statusCode: 200,
    body: JSON.stringify({
      message: 'OK',
    }),
  };
}