diff --git a/tslib.d.ts b/tslib.d.ts
index a6aea8b..76e89d1 100644
--- a/tslib.d.ts
+++ b/tslib.d.ts
@@ -27,7 +27,7 @@ export declare function __read(o: any, n?: number): any[];
 export declare function __spread(...args: any[][]): any[];
 /** @deprecated since TypeScript 4.2 */
 export declare function __spreadArrays(...args: any[][]): any[];
-export declare function __spreadArray(to: any[], from: any[]): any[];
+export declare function __spreadArray(to: any[], from: any[], pack?: boolean): any[];
 export declare function __await(v: any): any;
 export declare function __asyncGenerator(thisArg: any, _arguments: any, generator: Function): any;
 export declare function __asyncDelegator(o: any): any;
diff --git a/tslib.es6.js b/tslib.es6.js
index 5de1e31..93311d3 100644
--- a/tslib.es6.js
+++ b/tslib.es6.js
@@ -162,10 +162,14 @@ export function __spreadArrays() {
     return r;
 }
 
-export function __spreadArray(to, from) {
-    for (var i = 0, il = from.length, j = to.length; i < il; i++, j++)
-        to[j] = from[i];
-    return to;
+export function __spreadArray(to, from, pack) {
+    if (pack || arguments.length === 2) for (var i = 0, l = from.length, ar; i < l; i++) {
+        if (ar || !(i in from)) {
+            if (!ar) ar = Array.prototype.slice.call(from, 0, i);
+            ar[i] = from[i];
+        }
+    }
+    return to.concat(ar || from);
 }
 
 export function __await(v) {
diff --git a/tslib.js b/tslib.js
index e462859..62639f0 100644
--- a/tslib.js
+++ b/tslib.js
@@ -203,10 +203,14 @@ var __createBinding;
         return r;
     };
 
-    __spreadArray = function (to, from) {
-        for (var i = 0, il = from.length, j = to.length; i < il; i++, j++)
-            to[j] = from[i];
-        return to;
+    __spreadArray = function (to, from, pack) {
+        if (pack || arguments.length === 2) for (var i = 0, l = from.length, ar; i < l; i++) {
+            if (ar || !(i in from)) {
+                if (!ar) ar = Array.prototype.slice.call(from, 0, i);
+                ar[i] = from[i];
+            }
+        }
+        return to.concat(ar || from);
     };
 
     __await = function (v) {