feat: pre-allocating array

Author: Chanran

packages/core-js/modules/web.atob.js

@@ -5,6 +5,7 @@ var getBuiltIn = require('../internals/get-built-in');
 var uncurryThis = require('../internals/function-uncurry-this');
 var call = require('../internals/function-call');
 var fails = require('../internals/fails');
+var from = require('../internals/array-from');
 var toString = require('../internals/to-string');
 var validateArgumentsLength = require('../internals/validate-arguments-length');
 var c2i = require('../internals/base64-map').c2i;
@@ -47,7 +48,8 @@ $({ global: true, bind: true, enumerable: true, forced: FORCED }, {
     // `webpack` dev server bug on IE global methods - use call(fn, global, ...)
     if (BASIC && !NO_SPACES_IGNORE && !NO_ENCODING_CHECK) return call($atob, globalThis, data);
     var string = replace(toString(data), whitespaces, '');
-    var output = [];
+    // every 4 characters produce 3 bytes, so ceil(length * 3 / 4) is the number of bytes
+    var output = Array.from({ length: Math.ceil(string.length * 3 / 4) });
     var position = 0;
     var bc = 0;
     var length, chr, bs;

packages/core-js/modules/web.btoa.js

@@ -35,7 +35,8 @@ $({ global: true, bind: true, enumerable: true, forced: !BASIC || NO_ARG_RECEIVI
     // `webpack` dev server bug on IE global methods - use call(fn, global, ...)
     if (BASIC) return call($btoa, globalThis, toString(data));
     var string = toString(data);
-    var output = [];
+    // every 3 characters produce 4 bytes, so ceil(length * 4 / 3) is the number of bytes
+    var output = Array.from({ length: Math.ceil(string.length * 4 / 3) });
     var position = 0;
     var map = i2c;
     var block, charCode;