diff --git a/js/src/jsnum.cpp b/js/src/jsnum.cpp
index c64a1149f0..b778c8d739 100644
--- a/js/src/jsnum.cpp
+++ b/js/src/jsnum.cpp
@@ -1614,6 +1614,27 @@ js::ToInt32Slow(JSContext* cx, const HandleValue v, int32_t* out)
     return true;
 }
 
+bool
+js::ToInt32OrBigIntSlow(JSContext* cx, MutableHandleValue vp)
+{
+    MOZ_ASSERT(!vp.isInt32());
+    if (vp.isDouble()) {
+        vp.setInt32(ToInt32(vp.toDouble()));
+        return true;
+    }
+
+    if (!ToNumeric(cx, vp)) {
+        return false;
+    }
+
+    if (vp.isBigInt()) {
+        return true;
+    }
+
+    vp.setInt32(ToInt32(vp.toNumber()));
+    return true;
+}
+
 JS_PUBLIC_API(bool)
 js::ToUint32Slow(JSContext* cx, const HandleValue v, uint32_t* out)
 {
diff --git a/js/src/jsnum.h b/js/src/jsnum.h
index 295d237c22..9866a91eeb 100644
--- a/js/src/jsnum.h
+++ b/js/src/jsnum.h
@@ -376,6 +376,18 @@ ToNumeric(ExclusiveContext* cx, JS::MutableHandleValue vp)
     return ToNumericSlow(cx, vp);
 }
 
+bool
+ToInt32OrBigIntSlow(JSContext* cx, JS::MutableHandleValue vp);
+
+MOZ_ALWAYS_INLINE MOZ_MUST_USE bool
+ToInt32OrBigInt(JSContext* cx, JS::MutableHandleValue vp)
+{
+    if (vp.isInt32()) {
+        return true;
+    }
+    return ToInt32OrBigIntSlow(cx, vp);
+}
+
 void FIX_FPU();
 
 } /* namespace js */