fix(logger): decorated class methods cannot access this (aws-powertools#1060)

Author: dreamorosi

docs/core/logger.md

@@ -155,9 +155,12 @@ Key | Example
 
     }
 
-    export const myFunction = new Lambda();
-    export const handler = myFunction.handler;
+    const myFunction = new Lambda();
+    export const handler = myFunction.handler.bind(myFunction); // (1)
     ```
+
+    1. Binding your handler method allows your handler to access `this` within the class methods.
+
 === "Manual"
 
     ```typescript hl_lines="7"
@@ -233,10 +236,12 @@ This is disabled by default to prevent sensitive info being logged
 
     }
 
-    export const myFunction = new Lambda();
-    export const handler = myFunction.handler;
+    const myFunction = new Lambda();
+    export const handler = myFunction.handler.bind(myFunction); // (1)
     ```
 
+    1. Binding your handler method allows your handler to access `this` within the class methods.
+
 ### Appending persistent additional log keys and values
 
 You can append additional persistent keys and values in the logs generated during a Lambda invocation using either mechanism:
@@ -398,10 +403,12 @@ If you want to make sure that persistent attributes added **inside the handler f
 
     }
 
-    export const myFunction = new Lambda();
-    export const handler = myFunction.handler;
+    const myFunction = new Lambda();
+    export const handler = myFunction.handler.bind(myFunction); // (1)
     ```
 
+    1. Binding your handler method allows your handler to access `this` within the class methods.
+
 In each case, the printed log will look like this:
 
 === "First invocation"

packages/logger/src/Logger.ts

@@ -1,5 +1,5 @@
 import { Console } from 'console';
-import type { Context } from 'aws-lambda';
+import type { Context, Handler } from 'aws-lambda';
 import { Utility } from '@aws-lambda-powertools/commons';
 import { LogFormatterInterface, PowertoolLogFormatter } from './formatter';
 import { LogItem } from './log';
@@ -99,8 +99,8 @@ import type {
  *     }
  * }
  *
- * export const myFunction = new Lambda();
- * export const handler = myFunction.handler;
+ * const handlerClass = new Lambda();
+ * export const handler = handlerClass.handler.bind(handlerClass);
  * ```
  *
  * @class
@@ -279,30 +279,33 @@ class Logger extends Utility implements ClassThatLogs {
       /**
        * The descriptor.value is the method this decorator decorates, it cannot be undefined.
        */
-      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
       const originalMethod = descriptor.value;
 
-      descriptor.value = (event, context, callback) => {
+      // eslint-disable-next-line @typescript-eslint/no-this-alias
+      const loggerRef = this;
+      // Use a function() {} instead of an () => {} arrow function so that we can
+      // access `myClass` as `this` in a decorated `myClass.myMethod()`.
+      descriptor.value = (function (this: Handler, event, context, callback) {
 
         let initialPersistentAttributes = {};
         if (options && options.clearState === true) {
-          initialPersistentAttributes = { ...this.getPersistentLogAttributes() };
+          initialPersistentAttributes = { ...loggerRef.getPersistentLogAttributes() };
         }
 
-        Logger.injectLambdaContextBefore(this, event, context, options);
+        Logger.injectLambdaContextBefore(loggerRef, event, context, options);
 
         /* eslint-disable  @typescript-eslint/no-non-null-assertion */
         let result: unknown;
         try {
-          result = originalMethod!.apply(target, [ event, context, callback ]);
+          result = originalMethod!.apply(this, [ event, context, callback ]);
         } catch (error) {
           throw error;
         } finally {
-          Logger.injectLambdaContextAfterOrOnError(this, initialPersistentAttributes, options);
+          Logger.injectLambdaContextAfterOrOnError(loggerRef, initialPersistentAttributes, options);
         }
 
         return result;
-      };
+      });
     };
   }
 

packages/logger/tests/e2e/sampleRate.decorator.test.FunctionCode.ts

@@ -2,27 +2,39 @@ import { Logger } from '../../src';
 import { APIGatewayProxyEvent, Context } from 'aws-lambda';
 import { LambdaInterface } from '@aws-lambda-powertools/commons';
 
-const SAMPLE_RATE = parseFloat(process.env.SAMPLE_RATE);
-const LOG_MSG = process.env.LOG_MSG;
+const SAMPLE_RATE = parseFloat(process.env.SAMPLE_RATE || '0.1');
+const LOG_MSG = process.env.LOG_MSG || 'Hello World';
 
 const logger = new Logger({
   sampleRateValue: SAMPLE_RATE,
 });
 
 class Lambda implements LambdaInterface {
+  private readonly logMsg: string;
+
+  public constructor() {
+    this.logMsg = LOG_MSG;
+  }
+
   // Decorate your handler class method
   @logger.injectLambdaContext()
+  // eslint-disable-next-line @typescript-eslint/ban-ts-comment
+  // @ts-ignore
   public async handler(event: APIGatewayProxyEvent, context: Context): Promise<{requestId: string}> {
-    logger.debug(LOG_MSG);
-    logger.info(LOG_MSG);
-    logger.warn(LOG_MSG);
-    logger.error(LOG_MSG);
-
+    this.printLogInAllLevels();
+    
     return {
       requestId: context.awsRequestId,
     };
   }
+
+  private printLogInAllLevels() : void {
+    logger.debug(this.logMsg);
+    logger.info(this.logMsg);
+    logger.warn(this.logMsg);
+    logger.error(this.logMsg);
+  }
 }
 
-export const myFunction = new Lambda();
-export const handler = myFunction.handler;
+const myFunction = new Lambda();
+export const handler = myFunction.handler.bind(myFunction);

packages/logger/tests/unit/Logger.test.ts

@@ -1115,6 +1115,57 @@ describe('Class: Logger', () => {
 
     });
 
+    test('when used as decorator the value of `this` is preserved on the decorated method/class', async () => {
+
+      // Prepare
+      const logger = new Logger({
+        logLevel: 'DEBUG',
+      });
+      const consoleSpy = jest.spyOn(logger['console'], 'info').mockImplementation();
+
+      class LambdaFunction implements LambdaInterface {
+        private readonly memberVariable: string;
+
+        public constructor(memberVariable: string) {
+          this.memberVariable = memberVariable;
+        }
+
+        @logger.injectLambdaContext()
+        // eslint-disable-next-line @typescript-eslint/ban-ts-comment
+        // @ts-ignore
+        public handler<TResult>(_event: unknown, _context: Context, _callback: Callback<TResult>): void | Promise<TResult> {
+          this.dummyMethod();
+
+          return;
+        }
+
+        private dummyMethod(): void {
+          logger.info({ message: `memberVariable:${this.memberVariable}` });
+        }
+      }
+
+      // Act
+      const lambda = new LambdaFunction('someValue');
+      const handler = lambda.handler.bind(lambda);
+      await handler({}, dummyContext, () => console.log('Lambda invoked!'));
+
+      // Assess
+      expect(consoleSpy).toBeCalledTimes(1);
+      expect(consoleSpy).toHaveBeenNthCalledWith(1, JSON.stringify({
+        cold_start: true,
+        function_arn: 'arn:aws:lambda:eu-west-1:123456789012:function:foo-bar-function',
+        function_memory_size: 128,
+        function_name: 'foo-bar-function',
+        function_request_id: 'c6af9ac6-7b61-11e6-9a41-93e812345678',
+        level: 'INFO',
+        message: 'memberVariable:someValue',
+        service: 'hello-world',
+        timestamp: '2016-06-20T12:08:10.000Z',
+        xray_trace_id: '1-5759e988-bd862e3fe1be46a994272793',
+      }));
+
+    });
+
   });
 
   describe('Method: refreshSampleRateCalculation', () => {