diff --git a/types/json2csv/JSON2CSVBase.d.ts b/types/json2csv/JSON2CSVBase.d.ts
index c5f328db1f..1faf2a5318 100644
--- a/types/json2csv/JSON2CSVBase.d.ts
+++ b/types/json2csv/JSON2CSVBase.d.ts
@@ -92,9 +92,10 @@ declare abstract class JSON2CSVBase<T> {
      * Performs the flattening of a data row recursively
      *
      * @param {object} dataRow Original JSON object
+     * @param {string} separator Separator to be used as the flattened field name
      * @returns {object} Flattened object
      */
-    protected flatten(dataRow: T): object;
+    protected flatten(dataRow: T, separator: string): object;
 
     /**
      * Performs the unwind recursively in specified sequence
diff --git a/types/json2csv/JSON2CSVTransform.d.ts b/types/json2csv/JSON2CSVTransform.d.ts
index 1785e09214..f453ece8b1 100644
--- a/types/json2csv/JSON2CSVTransform.d.ts
+++ b/types/json2csv/JSON2CSVTransform.d.ts
@@ -80,9 +80,10 @@ declare class JSON2CSVTransform<T> extends Transform { // implements JSON2CSVBas
      * Performs the flattening of a data row recursively
      *
      * @param {object} dataRow Original JSON object
+     * @param {string} separator Separator to be used as the flattened field name
      * @returns {object} Flattened object
      */
-    protected flatten(dataRow: T): object;
+    protected flatten(dataRow: T, separator: string): object;
 
     /**
      * Performs the unwind recursively in specified sequence
diff --git a/types/json2csv/index.d.ts b/types/json2csv/index.d.ts
index 55a1829625..55aca2687c 100644
--- a/types/json2csv/index.d.ts
+++ b/types/json2csv/index.d.ts
@@ -1,4 +1,4 @@
-// Type definitions for json2csv 4.1
+// Type definitions for json2csv 4.2
 // Project: https://github.com/zemirco/json2csv
 // Definitions by: Juanjo Diaz <https://github.com/juanjoDiaz>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
diff --git a/types/json2csv/json2csv-tests.ts b/types/json2csv/json2csv-tests.ts
index 67c46cd1c3..42f8a58b70 100644
--- a/types/json2csv/json2csv-tests.ts
+++ b/types/json2csv/json2csv-tests.ts
@@ -76,7 +76,7 @@ class ParserExt extends Parser<ExampleObj> {
         s = this.processValue(new Date(), true);
         s = this.processValue({}, true);
         s = this.processValue([], true);
-        const flattenedData: object = this.flatten({});
+        const flattenedData: object = this.flatten({}, '.');
         const unwindedData: object[] = this.unwindData([], []);
     }
 }
@@ -110,7 +110,7 @@ class TransformExt extends Transform<ExampleObj> {
         s = this.processValue(new Date(), true);
         s = this.processValue({}, true);
         s = this.processValue([], true);
-        const flattenedData: object = this.flatten({});
+        const flattenedData: object = this.flatten({}, '.');
         const unwindedData: object[] = this.unwindData([], []);
     }
 }