diff --git a/src/core/formatter.ts b/src/core/formatter.ts
index cf06a84..afecc1b 100644
--- a/src/core/formatter.ts
+++ b/src/core/formatter.ts
@@ -5,10 +5,10 @@ import { LogMessage } from './type/log-message-type';
 export class Formatter implements FormatterInterface {
   constructor(
     private readonly serviceName: string,
-    private readonly isDevelopmentEnv: boolean,
+    private readonly isDevelopmentEnv?: boolean,
   ) {}
 
-  format(message: string, level: string, args: LogMessageOptions): string | void {
+  format(message: string, level: string, args: LogMessageOptions): string {
     const formattedMessage = {
       message,
       level,
@@ -17,7 +17,10 @@ export class Formatter implements FormatterInterface {
       ...args,
     } as LogMessage;
 
-    if (this.isDevelopmentEnv) console.log(formattedMessage); return null;
+    if (this.isDevelopmentEnv) {
+      console.log(formattedMessage);
+      return '';
+    }
 
     return JSON.stringify(formattedMessage);
   }
diff --git a/src/core/type/config.ts b/src/core/type/config.ts
index f391ae5..b86b3ea 100644
--- a/src/core/type/config.ts
+++ b/src/core/type/config.ts
@@ -3,4 +3,5 @@ import { LogLevel } from "./log-level";
 export type Config = {
   level: LogLevel;
   serviceName: string;
+  isDevelopmentEnv?: boolean;
 };