diff --git a/src/vs/workbench/api/browser/mainThreadDocumentsAndEditors.ts b/src/vs/workbench/api/browser/mainThreadDocumentsAndEditors.ts
index a14b9720272..16424601c26 100644
--- a/src/vs/workbench/api/browser/mainThreadDocumentsAndEditors.ts
+++ b/src/vs/workbench/api/browser/mainThreadDocumentsAndEditors.ts
@@ -36,32 +36,32 @@ namespace delta {
 	export function ofSets<T>(before: Set<T>, after: Set<T>): { removed: T[], added: T[] } {
 		const removed: T[] = [];
 		const added: T[] = [];
-		before.forEach(element => {
+		for (let element of before) {
 			if (!after.has(element)) {
 				removed.push(element);
 			}
-		});
-		after.forEach(element => {
+		}
+		for (let element of after) {
 			if (!before.has(element)) {
 				added.push(element);
 			}
-		});
+		}
 		return { removed, added };
 	}
 
 	export function ofMaps<K, V>(before: Map<K, V>, after: Map<K, V>): { removed: V[], added: V[] } {
 		const removed: V[] = [];
 		const added: V[] = [];
-		before.forEach((value, index) => {
+		for (let [index, value] of before) {
 			if (!after.has(index)) {
 				removed.push(value);
 			}
-		});
-		after.forEach((value, index) => {
+		}
+		for (let [index, value] of after) {
 			if (!before.has(index)) {
 				added.push(value);
 			}
-		});
+		}
 		return { removed, added };
 	}
 }