fix(ext): use `String#toWellFormed` in ext/webidl and ext/node (#21054)

Fixes #18802 

This PR adds `util.toUSVString` to node:util:

```js
import util from "node:util";
util.toUSVString("string\ud801"); // => "string\ufffd"
```

4 files changed, 21 insertions, 26 deletions

diff --git a/cli/tests/node_compat/test/parallel/test-util.js b/cli/tests/node_compat/test/parallel/test-util.js
index 5d1ed9b30..eaffc7f6d 100644
--- a/cli/tests/node_compat/test/parallel/test-util.js
+++ b/cli/tests/node_compat/test/parallel/test-util.js
@@ -161,8 +161,7 @@ assert.strictEqual(util.isFunction(function() {}), true);
 assert.strictEqual(util.isFunction(), false);
 assert.strictEqual(util.isFunction('string'), false);
 
-// TODO(wafuwafu13): Enable this when `toUSVString` is ready.
-// assert.strictEqual(util.toUSVString('string\ud801'), 'string\ufffd');
+assert.strictEqual(util.toUSVString('string\ud801'), 'string\ufffd');
 
 {
   assert.strictEqual(util.types.isNativeError(new Error()), true);
diff --git a/cli/tests/unit_node/util_test.ts b/cli/tests/unit_node/util_test.ts
index 64020141a..e90eed831 100644
--- a/cli/tests/unit_node/util_test.ts
+++ b/cli/tests/unit_node/util_test.ts
@@ -209,6 +209,14 @@ Deno.test({
 });
 
 Deno.test({
+  name: "[util] toUSVString",
+  fn() {
+    assertEquals(util.toUSVString("foo"), "foo");
+    assertEquals(util.toUSVString("bar\ud801"), "bar\ufffd");
+  },
+});
+
+Deno.test({
   name: "[util] isDate",
   fn() {
     // Test verifies the method is exposed. See _util/_util_types_test for details
diff --git a/ext/node/polyfills/util.ts b/ext/node/polyfills/util.ts
index 1b611b1fd..2e25746ef 100644
--- a/ext/node/polyfills/util.ts
+++ b/ext/node/polyfills/util.ts
@@ -37,7 +37,9 @@ const {
   SafeSet,
   SetPrototypeAdd,
   SetPrototypeHas,
+  StringPrototypeIsWellFormed,
   StringPrototypePadStart,
+  StringPrototypeToWellFormed,
 } = primordials;
 
 export {
@@ -187,6 +189,13 @@ export const TextDecoder = _TextDecoder;
 export type TextEncoder = import("./_utils.ts")._TextEncoder;
 export const TextEncoder = _TextEncoder;
 
+export function toUSVString(str: string): string {
+  if (StringPrototypeIsWellFormed(str)) {
+    return str;
+  }
+  return StringPrototypeToWellFormed(str);
+}
+
 function pad(n: number) {
   return StringPrototypePadStart(NumberPrototypeToString(n), 2, "0");
 }
@@ -309,6 +318,7 @@ export default {
   stripVTControlCharacters,
   TextDecoder,
   TextEncoder,
+  toUSVString,
   log,
   debuglog,
   isDeepStrictEqual,
diff --git a/ext/webidl/00_webidl.js b/ext/webidl/00_webidl.js
index c3b00286f..956def267 100644
--- a/ext/webidl/00_webidl.js
+++ b/ext/webidl/00_webidl.js
@@ -70,8 +70,8 @@ const {
   // TODO(lucacasonato): add SharedArrayBuffer to primordials
   // SharedArrayBufferPrototype
   String,
-  StringFromCodePoint,
   StringPrototypeCharCodeAt,
+  StringPrototypeToWellFormed,
   Symbol,
   SymbolIterator,
   SymbolToStringTag,
@@ -425,29 +425,7 @@ converters.ByteString = (V, prefix, context, opts) => {
 
 converters.USVString = (V, prefix, context, opts) => {
   const S = converters.DOMString(V, prefix, context, opts);
-  const n = S.length;
-  let U = "";
-  for (let i = 0; i < n; ++i) {
-    const c = StringPrototypeCharCodeAt(S, i);
-    if (c < 0xd800 || c > 0xdfff) {
-      U += StringFromCodePoint(c);
-    } else if (0xdc00 <= c && c <= 0xdfff) {
-      U += StringFromCodePoint(0xfffd);
-    } else if (i === n - 1) {
-      U += StringFromCodePoint(0xfffd);
-    } else {
-      const d = StringPrototypeCharCodeAt(S, i + 1);
-      if (0xdc00 <= d && d <= 0xdfff) {
-        const a = c & 0x3ff;
-        const b = d & 0x3ff;
-        U += StringFromCodePoint((2 << 15) + (2 << 9) * a + b);
-        ++i;
-      } else {
-        U += StringFromCodePoint(0xfffd);
-      }
-    }
-  }
-  return U;
+  return StringPrototypeToWellFormed(S);
 };
 
 converters.object = (V, prefix, context, _opts) => {