diff --git a/packages/core-js/modules/web.atob.js b/packages/core-js/modules/web.atob.js
index c3221c6647c0..1f6b31594e2d 100644
--- a/packages/core-js/modules/web.atob.js
+++ b/packages/core-js/modules/web.atob.js
@@ -47,7 +47,7 @@ $({ global: true, bind: true, enumerable: true, forced: FORCED }, {
     // `webpack` dev server bug on IE global methods - use call(fn, global, ...)
     if (BASIC && !NO_SPACES_IGNORE && !NO_ENCODING_CHECK) return call($atob, globalThis, data);
     var string = replace(toString(data), whitespaces, '');
-    var output = '';
+    var output = [];
     var position = 0;
     var bc = 0;
     var length, chr, bs;
@@ -61,7 +61,7 @@ $({ global: true, bind: true, enumerable: true, forced: FORCED }, {
     while (position < length) {
       chr = charAt(string, position++);
       bs = bc % 4 ? bs * 64 + c2i[chr] : c2i[chr];
-      if (bc++ % 4) output += fromCharCode(255 & bs >> (-2 * bc & 6));
-    } return output;
+      if (bc++ % 4) output.push(fromCharCode(255 & bs >> (-2 * bc & 6)));
+    } return output.join('');
   }
 });
diff --git a/packages/core-js/modules/web.btoa.js b/packages/core-js/modules/web.btoa.js
index b8bda16c5b17..28f317e3f468 100644
--- a/packages/core-js/modules/web.btoa.js
+++ b/packages/core-js/modules/web.btoa.js
@@ -35,7 +35,7 @@ $({ global: true, bind: true, enumerable: true, forced: !BASIC || NO_ARG_RECEIVI
     // `webpack` dev server bug on IE global methods - use call(fn, global, ...)
     if (BASIC) return call($btoa, globalThis, toString(data));
     var string = toString(data);
-    var output = '';
+    var output = [];
     var position = 0;
     var map = i2c;
     var block, charCode;
@@ -45,7 +45,7 @@ $({ global: true, bind: true, enumerable: true, forced: !BASIC || NO_ARG_RECEIVI
         throw new (getBuiltIn('DOMException'))('The string contains characters outside of the Latin1 range', 'InvalidCharacterError');
       }
       block = block << 8 | charCode;
-      output += charAt(map, 63 & block >> 8 - position % 1 * 8);
-    } return output;
+      output.push(charAt(map, 63 & block >> 8 - position % 1 * 8));
+    } return output.join('');
   }
 });