feat(webidl): better error message for ByteString converter

lib/fetch/webidl.js

@@ -388,10 +388,6 @@ webidl.converters.DOMString = function (V, opts = {}) {
   return String(V)
 }
 
-// Check for 0 or more characters outside of the latin1 range.
-// eslint-disable-next-line no-control-regex
-const isLatin1 = /^[\u0000-\u00ff]{0,}$/
-
 // https://webidl.spec.whatwg.org/#es-ByteString
 webidl.converters.ByteString = function (V) {
   // 1. Let x be ? ToString(V).
@@ -400,8 +396,15 @@ webidl.converters.ByteString = function (V) {
 
   // 2. If the value of any element of x is greater than
   //    255, then throw a TypeError.
-  if (!isLatin1.test(x)) {
-    throw new TypeError('Argument is not a ByteString')
+  for (let index = 0; index < x.length; index++) {
+    const charCode = x.charCodeAt(index)
+
+    if (charCode > 255) {
+      throw new TypeError(
+        'Cannot convert argument to a ByteString because the character at' +
+        `index ${index} has a value of ${charCode} which is greater than 255.`
+      )
+    }
   }
 
   // 3. Return an IDL ByteString value whose length is the

test/webidl/converters.js

@@ -189,5 +189,14 @@ test('ByteString', (t) => {
     webidl.converters.ByteString('')
   })
 
+  // https://github.com/nodejs/undici/issues/1590
+  t.throws(() => {
+    const char = String.fromCharCode(256)
+    webidl.converters.ByteString(`invalid${char}char`)
+  }, {
+    message: 'Cannot convert argument to a ByteString because the character at' +
+             'index 7 has a value of 256 which is greater than 255.'
+  })
+
   t.end()
 })