buffer: implement btoa and atob

Signed-off-by: James M Snell <jasnell@gmail.com>
PR-URL: #37529
Fixes: #3462
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: jasnell

doc/api/buffer.md

@@ -3112,6 +3112,44 @@ While, the `Buffer` object is available as a global, there are additional
 `Buffer`-related APIs that are available only via the `buffer` module
 accessed using `require('buffer')`.
 
+### `buffer.atob(data)`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `data` {any} The Base64-encoded input string.
+
+Decodes a string of Base64-encoded data into bytes, and encodes those bytes
+into a string using Latin-1 (ISO-8859-1).
+
+The `data` may be any JavaScript-value that can be coerced into a string.
+
+**This function is only provided for compatibility with legacy web platform APIs
+and should never be used in new code, because they use strings to represent
+binary data and predate the introduction of typed arrays in JavaScript.
+For code running using Node.js APIs, converting between base64-encoded strings
+and binary data should be performed using `Buffer.from(str, 'base64')` and
+`buf.toString('base64')`.**
+
+### `buffer.btoa(data)`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `data` {any} An ASCII (Latin1) string.
+
+Decodes a string into bytes using Latin-1 (ISO-8859), and encodes those bytes
+into a string using Base64.
+
+The `data` may be any JavaScript-value that can be coerced into a string.
+
+**This function is only provided for compatibility with legacy web platform APIs
+and should never be used in new code, because they use strings to represent
+binary data and predate the introduction of typed arrays in JavaScript.
+For code running using Node.js APIs, converting between base64-encoded strings
+and binary data should be performed using `Buffer.from(str, 'base64')` and
+`buf.toString('base64')`.**
+
 ### `buffer.INSPECT_MAX_BYTES`
 <!-- YAML
 added: v0.5.4

lib/buffer.js

@@ -1202,13 +1202,49 @@ if (internalBinding('config').hasIntl) {
   };
 }
 
+let DOMException;
+
+const lazyInvalidCharError = hideStackFrames((message, name) => {
+  if (DOMException === undefined)
+    DOMException = internalBinding('messaging').DOMException;
+  throw new DOMException('Invalid character', 'InvalidCharacterError');
+});
+
+function btoa(input) {
+  // TODO(@jasnell): The implementation here has not been performance
+  // optimized in any way.
+  input = `${input}`;
+  for (let n = 0; n < input.length; n++) {
+    if (input[n].charCodeAt(0) > 0xff)
+      lazyInvalidCharError();
+  }
+  const buf = Buffer.from(input, 'latin1');
+  return buf.toString('base64');
+}
+
+const kBase64Digits =
+  'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
+
+function atob(input) {
+  // TODO(@jasnell): The implementation here has not been performance
+  // optimized in any way.
+  input = `${input}`;
+  for (let n = 0; n < input.length; n++) {
+    if (!kBase64Digits.includes(input[n]))
+      lazyInvalidCharError();
+  }
+  return Buffer.from(input, 'base64').toString('latin1');
+}
+
 module.exports = {
   Buffer,
   SlowBuffer,
   transcode,
   // Legacy
   kMaxLength,
-  kStringMaxLength
+  kStringMaxLength,
+  btoa,
+  atob,
 };
 
 ObjectDefineProperties(module.exports, {