3 files changed, 9 insertions, 6 deletions
diff --git a/ext/fetch/22_body.js b/ext/fetch/22_body.js
index 49da149c2..acbef4ec4 100644
--- a/ext/fetch/22_body.js
+++ b/ext/fetch/22_body.js
@@ -372,7 +372,7 @@
     return { body, contentType };
   }
 
-  webidl.converters["BodyInit"] = (V, opts) => {
+  webidl.converters["BodyInit_DOMString"] = (V, opts) => {
     // Union for (ReadableStream or Blob or ArrayBufferView or ArrayBuffer or FormData or URLSearchParams or USVString)
     if (V instanceof ReadableStream) {
       // TODO(lucacasonato): ReadableStream is not branded
@@ -393,10 +393,13 @@
         return webidl.converters["ArrayBufferView"](V, opts);
       }
     }
-    return webidl.converters["USVString"](V, opts);
+    // BodyInit conversion is passed to extractBody(), which calls core.encode().
+    // core.encode() will UTF-8 encode strings with replacement, being equivalent to the USV normalization.
+    // Therefore we can convert to DOMString instead of USVString and avoid a costly redundant conversion.
+    return webidl.converters["DOMString"](V, opts);
   };
-  webidl.converters["BodyInit?"] = webidl.createNullableConverter(
-    webidl.converters["BodyInit"],
+  webidl.converters["BodyInit_DOMString?"] = webidl.createNullableConverter(
+    webidl.converters["BodyInit_DOMString"],
   );
 
   window.__bootstrap.fetchBody = { mixinBody, InnerBody, extractBody };
diff --git a/ext/fetch/23_request.js b/ext/fetch/23_request.js
index 06843d60c..dfb7e2127 100644
--- a/ext/fetch/23_request.js
+++ b/ext/fetch/23_request.js
@@ -445,7 +445,7 @@
       {
         key: "body",
         converter: webidl.createNullableConverter(
-          webidl.converters["BodyInit"],
+          webidl.converters["BodyInit_DOMString"],
         ),
       },
       { key: "redirect", converter: webidl.converters["RequestRedirect"] },
diff --git a/ext/fetch/23_response.js b/ext/fetch/23_response.js
index 32ce88d42..61e1de151 100644
--- a/ext/fetch/23_response.js
+++ b/ext/fetch/23_response.js
@@ -253,7 +253,7 @@
      */
     constructor(body = null, init = {}) {
       const prefix = "Failed to construct 'Response'";
-      body = webidl.converters["BodyInit?"](body, {
+      body = webidl.converters["BodyInit_DOMString?"](body, {
         prefix,
         context: "Argument 1",
       });