feat: Improve performance of atob and btoa

CHANGELOG.md

@@ -1,5 +1,6 @@
 # Changelog
 ### Unreleased
+- Improved performance of `atob` and `btoa`, [#1503](https://github.com/zloirock/core-js/issues/1503), [#1464](https://github.com/zloirock/core-js/issues/1464)
 - Forced replacement of `Iterator.{ concat, zip, zipKeyed }` in the pure version for ensuring proper wrapped `Iterator` instances as the result
 - Compat data improvements:
   - Added [Deno 2.6.7](https://github.com/denoland/deno/releases/tag/v2.6.7) compat data mapping

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

@@ -14,9 +14,11 @@ var whitespaces = /[\t\n\f\r ]+/g;
 var finalEq = /[=]{1,2}$/;
 
 var $atob = getBuiltIn('atob');
+var $Array = Array;
 var fromCharCode = String.fromCharCode;
 var charAt = uncurryThis(''.charAt);
 var replace = uncurryThis(''.replace);
+var join = uncurryThis([].join);
 var exec = uncurryThis(disallowed.exec);
 
 var BASIC = !!$atob && !fails(function () {
@@ -47,21 +49,28 @@ $({ global: true, bind: true, enumerable: true, forced: FORCED }, {
     // `webpack` dev server bug on IE global methods - use call(fn, global, ...)
     if (BASIC && !NO_SPACES_IGNORE && !NO_ENCODING_CHECK) return call($atob, globalThis, data);
     var string = replace(toString(data), whitespaces, '');
-    var output = '';
     var position = 0;
     var bc = 0;
     var length, chr, bs;
-    if (string.length % 4 === 0) {
+    if (!(string.length & 3)) {
       string = replace(string, finalEq, '');
     }
     length = string.length;
-    if (length % 4 === 1 || exec(disallowed, string)) {
+    var lenmod = length & 3;
+    if (lenmod === 1 || exec(disallowed, string)) {
       throw new (getBuiltIn('DOMException'))('The string is not correctly encoded', 'InvalidCharacterError');
     }
+    // (length >> 2) is equivalent for length / 4 floored; * 3 then multiplies the
+    // number of bytes for full quanta
+    // lenmod is length % 4; if there's 2 or 3 bytes it's 1 or 2 bytes of extra output
+    // respectively, so -1, however use a ternary to ensure 0 does not get -1 onto length
+    var output = new $Array((length >> 2) * 3 + (lenmod ? lenmod - 1 : 0));
+    var outputIndex = 0;
     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;
+      bs = bc & 3 ? (bs << 6) + c2i[chr] : c2i[chr];
+      if (bc++ & 3) output[outputIndex++] = fromCharCode(255 & bs >> (-2 * bc & 6));
+    }
+    return join(output, '');
   }
 });

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

@@ -10,6 +10,8 @@ var validateArgumentsLength = require('../internals/validate-arguments-length');
 var i2c = require('../internals/base64-map').i2c;
 
 var $btoa = getBuiltIn('btoa');
+var $Array = Array;
+var join = uncurryThis([].join);
 var charAt = uncurryThis(''.charAt);
 var charCodeAt = uncurryThis(''.charCodeAt);
 
@@ -35,7 +37,12 @@ $({ global: true, bind: true, enumerable: true, forced: !BASIC || NO_ARG_RECEIVI
     // `webpack` dev server bug on IE global methods - use call(fn, global, ...)
     if (BASIC) return call($btoa, globalThis, toString(data));
     var string = toString(data);
-    var output = '';
+    // (string.length + 2) / 3) and then truncating to integer
+    // does the ceil automatically.  << 2 will truncate the integer
+    // while also doing *4.  ceil(length / 3) quanta, 4 bytes output
+    // per quanta for base64.
+    var output = new $Array((string.length + 2) / 3 << 2);
+    var outputIndex = 0;
     var position = 0;
     var map = i2c;
     var block, charCode;
@@ -45,7 +52,7 @@ $({ 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[outputIndex++] = charAt(map, 63 & block >> 8 - position % 1 * 8);
+    } return join(output, '');
   }
 });