diff --git a/src/browser/js/Caller.zig b/src/browser/js/Caller.zig
index 8a1636f28a..5978897269 100644
--- a/src/browser/js/Caller.zig
+++ b/src/browser/js/Caller.zig
@@ -585,6 +585,9 @@ fn handleError(comptime T: type, comptime F: type, local: *const Local, err: any
     }
 
     const err_local = errorLocal(T, local, info);
+    const env = local.ctx.env;
+    const message = env.error_message orelse "";
+    env.error_message = null;
 
     const js_err: *const v8.Value = blk: {
         // Error constructors use the isolate's current context: enter the
@@ -595,7 +598,7 @@ fn handleError(comptime T: type, comptime F: type, local: *const Local, err: any
 
         break :blk switch (err) {
             error.InvalidArgument => isolate.createTypeError("invalid argument"),
-            error.TypeError => isolate.createTypeError(""),
+            error.TypeError => isolate.createTypeError(message),
             error.RangeError => isolate.createRangeError(""),
             error.OutOfMemory => isolate.createError("out of memory"),
             error.IllegalConstructor => isolate.createError("Illegal Constructor"),
diff --git a/src/browser/js/Env.zig b/src/browser/js/Env.zig
index fae6096f22..8dec568549 100644
--- a/src/browser/js/Env.zig
+++ b/src/browser/js/Env.zig
@@ -109,6 +109,10 @@ tearing_down: bool = false,
 
 heap_limit_protected: bool = false,
 
+// Message for the next TypeError the bridge builds. Set by local.typeError.
+// Think of it as our own little global errno. How cute.
+error_message: ?[]const u8 = null,
+
 pub const InitOpts = struct {
     with_inspector: bool = false,
 };
diff --git a/src/browser/js/Local.zig b/src/browser/js/Local.zig
index 39ee9f44aa..db78a211ca 100644
--- a/src/browser/js/Local.zig
+++ b/src/browser/js/Local.zig
@@ -1512,6 +1512,15 @@ pub fn stackTrace(self: *const Local) !?[]const u8 {
     return buf.written();
 }
 
+// We sometimes need to reject with a specific TypeError message. We can't
+// attach an anything to `error.TypeError`, but we can use a pseudo-global.
+// When caller catches the error.TypeError, it'll look into env.error_message
+// for the message.
+pub fn typeError(self: *const Local, message: []const u8) error{TypeError} {
+    self.ctx.env.error_message = message;
+    return error.TypeError;
+}
+
 // == Promise Helpers ==
 pub fn rejectPromise(self: *const Local, err: js.PromiseResolver.RejectError) js.Promise {
     var resolver = js.PromiseResolver.init(self);
diff --git a/src/browser/tests/net/response.html b/src/browser/tests/net/response.html
index 6fd33d006b..fc01dccd15 100644
--- a/src/browser/tests/net/response.html
+++ b/src/browser/tests/net/response.html
@@ -291,9 +291,10 @@
   await response1.text();
   const usedAfter1 = response1.bodyUsed;
 
-  // Re-consuming a used body rejects.
-  let rejected1 = false;
-  try { await response1.text(); } catch (e) { rejected1 = true; }
+  // Re-consuming a used body rejects with a TypeError (a rejection, not a
+  // synchronous throw), carrying the message the bridge was handed.
+  let rejected1 = null;
+  try { await response1.text(); } catch (e) { rejected1 = e; }
 
   // A bodyless response is never "used".
   const response2 = new Response();
@@ -309,7 +310,8 @@
   await state.done(() => {
     testing.expectFalse(usedBefore1);
     testing.expectTrue(usedAfter1);
-    testing.expectTrue(rejected1);
+    testing.expectEqual('TypeError', rejected1.constructor.name);
+    testing.expectEqual('Body has already been read', rejected1.message);
 
     testing.expectFalse(usedBefore2);
     testing.expectFalse(usedAfter2);
diff --git a/src/browser/webapi/CustomElementRegistry.zig b/src/browser/webapi/CustomElementRegistry.zig
index b963cf04a6..25f8ca25be 100644
--- a/src/browser/webapi/CustomElementRegistry.zig
+++ b/src/browser/webapi/CustomElementRegistry.zig
@@ -157,9 +157,7 @@ pub fn whenDefined(self: *CustomElementRegistry, name: []const u8, frame: *Frame
         return local.resolvePromise(definition.constructor);
     }
 
-    validateName(name) catch |err| switch (err) {
-        error.SyntaxError => return local.rejectPromise(.{ .dom_exception = .{ .err = error.SyntaxError } }),
-    };
+    try validateName(name);
 
     const gop = try self._when_defined.getOrPut(frame.arena, name);
     if (gop.found_existing) {
diff --git a/src/browser/webapi/SubtleCrypto.zig b/src/browser/webapi/SubtleCrypto.zig
index f6b030a9e9..dbf84dd89e 100644
--- a/src/browser/webapi/SubtleCrypto.zig
+++ b/src/browser/webapi/SubtleCrypto.zig
@@ -59,17 +59,15 @@ pub fn generateKey(
         .aes_key_gen => |params| return AES.generate(params, extractable, key_usages, exec),
         .ec_key_gen => |params| return EC.generate(params, extractable, key_usages, exec),
         .rsa_hashed_key_gen => |params| {
-            RSA.validate(params, key_usages) catch |err| {
-                return local.rejectPromise(.{ .dom_exception = .{ .err = err } });
-            };
+            try RSA.validate(params, key_usages);
             log.warn(.not_implemented, "generateKey", .{ .name = params.name });
         },
         .name => |js_name| return generateKeyFromName(try js_name.toSSO(false), extractable, key_usages, exec),
         .object => |object| return generateKeyFromName(try object.name.toSSO(false), extractable, key_usages, exec),
-        .invalid => return local.rejectPromise(.{ .type_error = "invalid algorithm" }),
+        .invalid => return local.typeError("invalid algorithm"),
     }
 
-    return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+    return error.NotSupported;
 }
 
 fn generateKeyFromName(
@@ -77,17 +75,6 @@ fn generateKeyFromName(
     extractable: bool,
     key_usages: []const []const u8,
     exec: *const Execution,
-) !js.Promise {
-    return _generateKeyFromName(name, extractable, key_usages, exec) catch |err| {
-        return exec.js.local.?.rejectPromise(.{ .dom_exception = .{ .err = err } });
-    };
-}
-
-fn _generateKeyFromName(
-    name: String,
-    extractable: bool,
-    key_usages: []const []const u8,
-    exec: *const Execution,
 ) !js.Promise {
     if (name.eql(comptime .wrap("X25519"))) {
         return X25519.init(extractable, key_usages, exec);
@@ -150,18 +137,16 @@ pub fn importKey(
     const is_private = importKind(format, key_data);
     if (asymmetricAllowedUsages(name, is_private)) |allowed| {
         // Public keys may have empty usages; secret/private keys may not.
-        const mask = common.usageMaskInner(allowed, key_usages, is_private) catch |err| {
-            return local.rejectPromise(.{ .dom_exception = .{ .err = err } });
-        };
+        const mask = try common.usageMaskInner(allowed, key_usages, is_private);
         if (EC.canonicalName(name) != null) {
             const der = switch (key_data) {
                 .bytes => |b| b.values,
-                .jwk => return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } }),
+                .jwk => return error.NotSupported,
             };
             return EC.import(name, algo.namedCurve(), format, der, is_private, extractable, mask, exec);
         }
         log.warn(.not_implemented, "SubtleCrypto.importKey", .{ .name = name });
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+        return error.NotSupported;
     }
 
     // Resolve the raw key bytes from the requested format. Symmetric keys
@@ -171,27 +156,22 @@ pub fn importKey(
             break :blk switch (key_data) {
                 .bytes => |b| b.values,
                 // A JWK object passed where a BufferSource is expected.
-                .jwk => return local.rejectPromise(.{ .type_error = "raw format expects a BufferSource" }),
+                .jwk => return local.typeError("raw format expects a BufferSource"),
             };
         }
         if (std.mem.eql(u8, format, "jwk")) {
             const jwk = switch (key_data) {
                 .jwk => |j| j,
-                .bytes => return local.rejectPromise(.{ .type_error = "jwk format expects an object" }),
+                .bytes => return local.typeError("jwk format expects an object"),
             };
             if (!std.mem.eql(u8, jwk.kty, "oct")) {
-                return local.rejectPromise(.{ .dom_exception = .{ .err = error.DataError } });
+                return error.DataError;
             }
-            const k = jwk.k orelse {
-                return local.rejectPromise(.{ .dom_exception = .{ .err = error.DataError } });
-            };
-            break :blk common.base64Decode(exec.local_arena, k) catch |err| switch (err) {
-                error.DataError => return local.rejectPromise(.{ .dom_exception = .{ .err = error.DataError } }),
-                else => |e| return e,
-            };
+            const k = jwk.k orelse return error.DataError;
+            break :blk try common.base64Decode(exec.local_arena, k);
         }
         // spki / pkcs8 (asymmetric formats) are not supported for these algorithms.
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+        return error.NotSupported;
     };
 
     if (AES.canonicalName(name) != null) {
@@ -202,9 +182,7 @@ pub fn importKey(
     // no length constraint and these keys are non-extractable.
     inline for ([_][]const u8{ "HKDF", "PBKDF2" }) |derive_name| {
         if (eqlIgnoreCase(name, derive_name)) {
-            const mask = common.usageMask(&.{ "deriveKey", "deriveBits" }, key_usages) catch |err| {
-                return local.rejectPromise(.{ .dom_exception = .{ .err = err } });
-            };
+            const mask = try common.usageMask(&.{ "deriveKey", "deriveBits" }, key_usages);
             const crypto_key = try CryptoKey.init(exec, .{
                 ._type = .derive,
                 ._kind = .secret,
@@ -222,13 +200,13 @@ pub fn importKey(
                 .string => |s| s,
                 .object => |o| o.name,
             },
-            else => return local.rejectPromise(.{ .type_error = "HMAC import requires a hash" }),
+            else => return local.typeError("HMAC import requires a hash"),
         };
         return HMAC.import(hash_name, raw, extractable, key_usages, exec);
     }
 
     log.warn(.not_implemented, "SubtleCrypto.importKey", .{ .name = name });
-    return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+    return error.NotSupported;
 }
 
 /// Whether the requested format/key-data describe a private key. The format
@@ -274,7 +252,7 @@ pub fn exportKey(
 ) !js.Promise {
     const local = exec.js.local.?;
     if (!key.canExportKey()) {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.InvalidAccessError } });
+        return error.InvalidAccessError;
     }
 
     if (std.mem.eql(u8, format, "raw")) {
@@ -288,10 +266,10 @@ pub fn exportKey(
     const is_unsupported = std.mem.eql(u8, format, "pkcs8") or std.mem.eql(u8, format, "spki");
     if (is_unsupported) {
         log.warn(.not_implemented, "SubtleCrypto.exportKey", .{ .format = format });
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+        return error.NotSupported;
     }
 
-    return local.rejectPromise(.{ .type_error = "invalid format" });
+    return local.typeError("invalid format");
 }
 
 /// The JSON Web Key returned for symmetric ("oct") keys.
@@ -318,7 +296,7 @@ fn exportJwk(key: *CryptoKey, exec: *const Execution) !js.Promise {
         },
         else => {
             log.warn(.not_implemented, "SubtleCrypto.exportKey", .{ .format = "jwk", .type = key._type });
-            return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+            return error.NotSupported;
         },
     };
 
@@ -340,9 +318,7 @@ pub fn deriveBits(
     exec: *const Execution,
 ) !js.Promise {
     const local = exec.js.local.?;
-    const bits = deriveRaw(algo, base_key, length, base_key.canDeriveBits(), exec) catch |err| {
-        return rejectDerive(local, err);
-    };
+    const bits = try deriveRaw(algo, base_key, length, base_key.canDeriveBits(), exec);
     return local.resolvePromise(js.ArrayBuffer{ .values = bits });
 }
 
@@ -357,35 +333,28 @@ pub fn deriveKey(
     key_usages: []const []const u8,
     exec: *const Execution,
 ) !js.Promise {
-    const local = exec.js.local.?;
     // The base key's deriveKey usage (not deriveBits) gates this operation.
     const usage_ok = base_key.canDeriveKey();
 
     switch (derived) {
         .keyed => |k| {
             if (AES.canonicalName(k.name) == null) {
-                return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+                return error.NotSupported;
             }
-            const bits = deriveRaw(algo, base_key, k.length, usage_ok, exec) catch |err| {
-                return rejectDerive(local, err);
-            };
+            const bits = try deriveRaw(algo, base_key, k.length, usage_ok, exec);
             return AES.import(k.name, bits, extractable, key_usages, exec);
         },
         .hmac => |h| {
             const hash_name = h.hash.name();
-            const hash_md = crypto.findDigest(hash_name) catch {
-                return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
-            };
+            const hash_md = crypto.findDigest(hash_name) catch return error.NotSupported;
             // Default length, per spec, is the hash's block size (in bits).
             const length: u32 = h.length orelse @intCast(crypto.EVP_MD_block_size(hash_md) * 8);
-            const bits = deriveRaw(algo, base_key, length, usage_ok, exec) catch |err| {
-                return rejectDerive(local, err);
-            };
+            const bits = try deriveRaw(algo, base_key, length, usage_ok, exec);
             return HMAC.import(hash_name, bits, extractable, key_usages, exec);
         },
         .object, .name => {
             log.warn(.not_implemented, "SubtleCrypto.deriveKey", .{});
-            return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+            return error.NotSupported;
         },
     }
 }
@@ -426,16 +395,6 @@ fn deriveRaw(
     }
 }
 
-/// Maps a KDF error to the spec-mandated DOMException (OutOfMemory propagates).
-fn rejectDerive(local: *const js.Local, err: KDF.Error) !js.Promise {
-    return switch (err) {
-        error.InvalidAccessError => local.rejectPromise(.{ .dom_exception = .{ .err = error.InvalidAccessError } }),
-        error.NotSupported => local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } }),
-        error.OperationError => local.rejectPromise(.{ .dom_exception = .{ .err = error.OperationError } }),
-        error.OutOfMemory => error.OutOfMemory,
-    };
-}
-
 /// Encrypts data with the given key and algorithm.
 pub fn encrypt(
     _: *const SubtleCrypto,
@@ -469,15 +428,10 @@ fn cryptOp(
     const params = switch (algo) {
         .params => |p| p,
         // A bare string identifier carries no iv/counter, so it can't drive AES.
-        .name => return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } }),
+        .name => return error.NotSupported,
     };
 
-    const out = AES.crypt(params, key, data, encrypting, exec) catch |err| switch (err) {
-        error.InvalidAccessError => return local.rejectPromise(.{ .dom_exception = .{ .err = error.InvalidAccessError } }),
-        error.OperationError => return local.rejectPromise(.{ .dom_exception = .{ .err = error.OperationError } }),
-        error.NotSupported => return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } }),
-        error.OutOfMemory => return error.OutOfMemory,
-    };
+    const out = try AES.crypt(params, key, data, encrypting, exec);
     return local.resolvePromise(js.ArrayBuffer{ .values = out });
 }
 
@@ -495,7 +449,7 @@ pub fn sign(
         .hmac => return HMAC.sign(algo, key, data, exec),
         else => {
             log.warn(.not_implemented, "SubtleCrypto.sign", .{ .key_type = key._type });
-            return exec.js.local.?.rejectPromise(.{ .dom_exception = .{ .err = error.InvalidAccessError } });
+            return error.InvalidAccessError;
         },
     };
 }
@@ -509,14 +463,13 @@ pub fn verify(
     data: []const u8, // ArrayBuffer.
     exec: *const Execution,
 ) !js.Promise {
-    const local = exec.js.local.?;
     if (!algo.isHMAC()) {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.InvalidAccessError } });
+        return error.InvalidAccessError;
     }
 
     return switch (key._type) {
         .hmac => HMAC.verify(key, signature, data, exec),
-        else => local.rejectPromise(.{ .dom_exception = .{ .err = error.InvalidAccessError } }),
+        else => error.InvalidAccessError,
     };
 }
 
@@ -540,16 +493,14 @@ pub fn digest(_: *const SubtleCrypto, algo: DigestInput, data: js.TypedArray(u8)
     const local = exec.js.local.?;
 
     const algo_name = algo.name() orelse {
-        return local.rejectPromise(.{ .type_error = "required member name is undefined" });
+        return local.typeError("required member name is undefined");
     };
     if (algo_name.len > 10) {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+        return error.NotSupported;
     }
 
     const normalized = std.ascii.upperString(exec.buf, algo_name);
-    const digest_type = crypto.findDigest(normalized) catch {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
-    };
+    const digest_type = crypto.findDigest(normalized) catch return error.NotSupported;
 
     const bytes = data.values;
     const out = exec.buf[0..crypto.EVP_MAX_MD_SIZE];
diff --git a/src/browser/webapi/crypto/AES.zig b/src/browser/webapi/crypto/AES.zig
index 63005f0c0f..3cb68142ec 100644
--- a/src/browser/webapi/crypto/AES.zig
+++ b/src/browser/webapi/crypto/AES.zig
@@ -85,9 +85,7 @@ pub fn generate(
     exec: *const Execution,
 ) !js.Promise {
     const local = exec.js.local.?;
-    validate(params, key_usages) catch |err| {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = err } });
-    };
+    try validate(params, key_usages);
 
     // validate() already confirmed the usages and length are well-formed.
     const allowed = allowedUsages(params.name).?;
@@ -122,15 +120,13 @@ pub fn import(
     const local = exec.js.local.?;
 
     const canonical = canonicalName(name) orelse {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+        return error.NotSupported;
     };
 
-    const mask = common.usageMask(allowedUsages(name).?, key_usages) catch |err| {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = err } });
-    };
+    const mask = try common.usageMask(allowedUsages(name).?, key_usages);
 
     if (raw.len != 16 and raw.len != 24 and raw.len != 32) {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.DataError } });
+        return error.DataError;
     }
 
     const crypto_key = try CryptoKey.init(exec, .{
diff --git a/src/browser/webapi/crypto/EC.zig b/src/browser/webapi/crypto/EC.zig
index 6e249e2229..73bc5d392c 100644
--- a/src/browser/webapi/crypto/EC.zig
+++ b/src/browser/webapi/crypto/EC.zig
@@ -94,9 +94,7 @@ pub fn generate(
     exec: *const Execution,
 ) !js.Promise {
     const local = exec.js.local.?;
-    validate(params, key_usages) catch |err| {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = err } });
-    };
+    try validate(params, key_usages);
 
     const name = canonicalName(params.name).?;
     const curve = curveCanonical(params.namedCurve).?;
@@ -111,7 +109,7 @@ pub fn generate(
     const ec = crypto.EC_KEY_new_by_curve_name(nid) orelse return error.OutOfMemory;
     defer crypto.EC_KEY_free(ec);
     if (crypto.EC_KEY_generate_key(ec) != 1) {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.OperationError } });
+        return error.OperationError;
     }
 
     const private_pkey = crypto.EVP_PKEY_new() orelse return error.OutOfMemory;
@@ -167,28 +165,28 @@ pub fn import(
 
     const canonical = canonicalName(name).?;
     const curve = curveCanonical(named_curve) orelse {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+        return error.NotSupported;
     };
 
     var ptr: [*c]const u8 = der.ptr;
     const pkey: *crypto.EVP_PKEY = blk: {
         if (std.mem.eql(u8, format, "spki")) {
             break :blk crypto.d2i_PUBKEY(null, &ptr, @intCast(der.len)) orelse {
-                return local.rejectPromise(.{ .dom_exception = .{ .err = error.DataError } });
+                return error.DataError;
             };
         }
         if (std.mem.eql(u8, format, "pkcs8")) {
             break :blk crypto.d2i_AutoPrivateKey(null, &ptr, @intCast(der.len)) orelse {
-                return local.rejectPromise(.{ .dom_exception = .{ .err = error.DataError } });
+                return error.DataError;
             };
         }
         // jwk / raw not implemented yet.
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+        return error.NotSupported;
     };
     errdefer crypto.EVP_PKEY_free(pkey);
 
     if (crypto.EVP_PKEY_id(pkey) != crypto.EVP_PKEY_EC) {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.DataError } });
+        return error.DataError;
     }
 
     const crypto_key = try CryptoKey.init(exec, .{
diff --git a/src/browser/webapi/crypto/HMAC.zig b/src/browser/webapi/crypto/HMAC.zig
index 19f6d51c6b..882a977adb 100644
--- a/src/browser/webapi/crypto/HMAC.zig
+++ b/src/browser/webapi/crypto/HMAC.zig
@@ -40,7 +40,7 @@ pub fn init(
     // The union probe match to get here is pretty simple, so we can end up here
     // for an unknown/invalid algo.
     if (!std.ascii.eqlIgnoreCase(params.name, "HMAC")) {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
+        return error.NotSupported;
     }
     const hash_name = switch (params.hash) {
         .string => |str| str,
@@ -48,9 +48,7 @@ pub fn init(
     };
     // Per spec, an unrecognized hash is caught during algorithm normalization
     // and surfaces as NotSupportedError.
-    const digest = crypto.findDigest(hash_name) catch return local.rejectPromise(.{
-        .dom_exception = .{ .err = error.NotSupported },
-    });
+    const digest = crypto.findDigest(hash_name) catch return error.NotSupported;
 
     // HMAC only accepts sign / verify; any other usage is a SyntaxError per
     // the spec, even when the entry exists elsewhere in CryptoKey.Usages.
@@ -61,15 +59,11 @@ pub fn init(
         } else if (std.mem.eql(u8, usage, "verify")) {
             mask |= CryptoKey.Usages.verify;
         } else {
-            return local.rejectPromise(.{
-                .dom_exception = .{ .err = error.SyntaxError },
-            });
+            return error.SyntaxError;
         }
     }
     if (key_usages.len == 0) {
-        return local.rejectPromise(.{
-            .dom_exception = .{ .err = error.SyntaxError },
-        });
+        return error.SyntaxError;
     }
 
     const block_size: usize = blk: {
@@ -115,16 +109,12 @@ pub fn import(
 ) !js.Promise {
     const local = exec.js.local.?;
 
-    const digest = crypto.findDigest(hash_name) catch return local.rejectPromise(.{
-        .dom_exception = .{ .err = error.NotSupported },
-    });
+    const digest = crypto.findDigest(hash_name) catch return error.NotSupported;
 
-    const mask = common.usageMask(&.{ "sign", "verify" }, key_usages) catch |err| {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = err } });
-    };
+    const mask = try common.usageMask(&.{ "sign", "verify" }, key_usages);
 
     if (raw.len == 0) {
-        return local.rejectPromise(.{ .dom_exception = .{ .err = error.DataError } });
+        return error.DataError;
     }
 
     const crypto_key = try CryptoKey.init(exec, .{
diff --git a/src/browser/webapi/crypto/X25519.zig b/src/browser/webapi/crypto/X25519.zig
index adaa09bf70..8ada529395 100644
--- a/src/browser/webapi/crypto/X25519.zig
+++ b/src/browser/webapi/crypto/X25519.zig
@@ -42,9 +42,7 @@ pub fn init(
     // Calculate usages; only matters for private key.
     // Only deriveKey() and deriveBits() be used for X25519.
     if (key_usages.len == 0) {
-        return local.rejectPromise(.{
-            .dom_exception = .{ .err = error.SyntaxError },
-        });
+        return error.SyntaxError;
     }
     var mask: u8 = 0;
     iter_usages: for (key_usages) |usage| {
@@ -55,9 +53,7 @@ pub fn init(
             }
         }
         // Unknown usage if got here.
-        return local.rejectPromise(.{
-            .dom_exception = .{ .err = error.SyntaxError },
-        });
+        return error.SyntaxError;
     }
 
     const public_value = try exec.local_arena.alloc(u8, crypto.X25519_PUBLIC_VALUE_LEN);
diff --git a/src/browser/webapi/net/Request.zig b/src/browser/webapi/net/Request.zig
index 18e8d1ff36..58b44d56dd 100644
--- a/src/browser/webapi/net/Request.zig
+++ b/src/browser/webapi/net/Request.zig
@@ -237,24 +237,21 @@ pub fn getBodyUsed(self: *const Request) bool {
     return self._body_used;
 }
 
-// Marks a present body consumed; returns a rejected promise if it already was.
-fn consume(self: *Request, local: *const js.Local) ?js.Promise {
+// Marks a present body consumed; a TypeError if it already was.
+fn consume(self: *Request, local: *const js.Local) !void {
     if (self._body == null) {
-        return null;
+        return;
     }
 
     if (self._body_used) {
-        return local.rejectPromise(.{ .type_error = "Body has already been read" });
+        return local.typeError("Body has already been read");
     }
     self._body_used = true;
-    return null;
 }
 
 pub fn blob(self: *Request, exec: *const Execution) !js.Promise {
     const local = exec.js.local.?;
-    if (self.consume(local)) |rejected| {
-        return rejected;
-    }
+    try self.consume(local);
 
     const body = self._body orelse "";
     const headers = try self.getHeaders(exec);
@@ -266,17 +263,13 @@ pub fn blob(self: *Request, exec: *const Execution) !js.Promise {
 
 pub fn text(self: *Request, exec: *const Execution) !js.Promise {
     const local = exec.js.local.?;
-    if (self.consume(local)) |rejected| {
-        return rejected;
-    }
+    try self.consume(local);
     return local.resolvePromise(body_init.stripUtf8Bom(self._body orelse ""));
 }
 
 pub fn json(self: *Request, exec: *const Execution) !js.Promise {
     const local = exec.js.local.?;
-    if (self.consume(local)) |rejected| {
-        return rejected;
-    }
+    try self.consume(local);
 
     const value = local.parseJSON(body_init.stripUtf8Bom(self._body orelse "")) catch {
         return local.rejectPromise(.{ .syntax_error = "failed to parse" });
@@ -286,32 +279,26 @@ pub fn json(self: *Request, exec: *const Execution) !js.Promise {
 
 pub fn arrayBuffer(self: *Request, exec: *const Execution) !js.Promise {
     const local = exec.js.local.?;
-    if (self.consume(local)) |rejected| {
-        return rejected;
-    }
+    try self.consume(local);
     return local.resolvePromise(js.ArrayBuffer{ .values = self._body orelse "" });
 }
 
 pub fn bytes(self: *Request, exec: *const Execution) !js.Promise {
     const local = exec.js.local.?;
-    if (self.consume(local)) |rejected| {
-        return rejected;
-    }
+    try self.consume(local);
     return local.resolvePromise(js.TypedArray(u8){ .values = self._body orelse "" });
 }
 
 pub fn formData(self: *Request, exec: *const Execution) !js.Promise {
     const local = exec.js.local.?;
-    if (self.consume(local)) |rejected| {
-        return rejected;
-    }
+    try self.consume(local);
 
     // Per Fetch, a null body acts as an empty byte sequence.
     const body = self._body orelse "";
 
     const headers = try self.getHeaders(exec);
     const content_type = try headers.get("content-type", exec) orelse {
-        return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" });
+        return local.typeError("Failed to parse body as FormData");
     };
     var it = ContentTypeIterator.init(content_type);
     const essence = it.essence;
@@ -322,12 +309,12 @@ pub fn formData(self: *Request, exec: *const Execution) !js.Promise {
     if (std.ascii.eqlIgnoreCase(essence, "multipart/form-data")) {
         const boundary = it.findBoundary();
         if (boundary.len == 0) {
-            return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" });
+            return local.typeError("Failed to parse body as FormData");
         }
 
         const form_data = FormData.initFromMultipart(body, boundary, exec) catch |err| switch (err) {
             error.OutOfMemory => return err,
-            else => return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" }),
+            else => return local.typeError("Failed to parse body as FormData"),
         };
         return local.resolvePromise(form_data);
     }
@@ -335,12 +322,12 @@ pub fn formData(self: *Request, exec: *const Execution) !js.Promise {
     if (std.ascii.eqlIgnoreCase(essence, "application/x-www-form-urlencoded")) {
         const form_data = FormData.initFromUrlEncoded(body, exec) catch |err| switch (err) {
             error.OutOfMemory => return err,
-            else => return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" }),
+            else => return local.typeError("Failed to parse body as FormData"),
         };
         return local.resolvePromise(form_data);
     }
 
-    return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" });
+    return local.typeError("Failed to parse body as FormData");
 }
 
 pub fn clone(self: *const Request, exec: *const Execution) !*Request {
diff --git a/src/browser/webapi/net/Response.zig b/src/browser/webapi/net/Response.zig
index b3343a2a65..50d3f4a09b 100644
--- a/src/browser/webapi/net/Response.zig
+++ b/src/browser/webapi/net/Response.zig
@@ -292,43 +292,38 @@ pub fn getBodyUsed(self: *const Response) bool {
     };
 }
 
-// Marks a present body consumed; returns a rejected promise if it already was.
-fn consume(self: *Response, local: *const js.Local) ?js.Promise {
+// Marks a present body consumed; a TypeError if it already was.
+fn consume(self: *Response, local: *const js.Local) !void {
     switch (self._body) {
-        .empty => return null,
+        .empty => return,
         else => {},
     }
     if (self._body_used) {
-        return local.rejectPromise(.{ .type_error = "Body has already been read" });
+        return local.typeError("Body has already been read");
     }
     self._body_used = true;
-    return null;
 }
 
 pub fn getText(self: *Response, exec: *const Execution) !js.Promise {
     const local = exec.js.local.?;
-    if (self.consume(local)) |rejected| {
-        return rejected;
-    }
+    try self.consume(local);
 
     const body = switch (self._body) {
         .bytes => |b| body_init.stripUtf8Bom(b),
         .empty => "",
-        .stream => return local.rejectPromise(.{ .type_error = "Cannot read text from stream body" }),
+        .stream => return local.typeError("Cannot read text from stream body"),
     };
     return local.resolvePromise(body);
 }
 
 pub fn getJson(self: *Response, exec: *const Execution) !js.Promise {
     const local = exec.js.local.?;
-    if (self.consume(local)) |rejected| {
-        return rejected;
-    }
+    try self.consume(local);
 
     const body = switch (self._body) {
         .bytes => |b| body_init.stripUtf8Bom(b),
         .empty => "",
-        .stream => return local.rejectPromise(.{ .type_error = "Cannot read JSON from stream body" }),
+        .stream => return local.typeError("Cannot read JSON from stream body"),
     };
     const value = local.parseJSON(body) catch {
         return local.rejectPromise(.{ .syntax_error = "failed to parse" });
@@ -338,9 +333,7 @@ pub fn getJson(self: *Response, exec: *const Execution) !js.Promise {
 
 pub fn arrayBuffer(self: *Response, exec: *const Execution) !js.Promise {
     const local = exec.js.local.?;
-    if (self.consume(local)) |rejected| {
-        return rejected;
-    }
+    try self.consume(local);
 
     return switch (self._body) {
         .bytes => |body| local.resolvePromise(js.ArrayBuffer{ .values = body }),
@@ -464,11 +457,11 @@ const StreamConsumer = struct {
 
 pub fn blob(self: *Response, exec: *const Execution) !js.Promise {
     const local = exec.js.local.?;
-    if (self.consume(local)) |rejected| return rejected;
+    try self.consume(local);
     const body = switch (self._body) {
         .bytes => |b| b,
         .empty => "",
-        .stream => return local.rejectPromise(.{ .type_error = "Cannot read blob from stream body" }),
+        .stream => return local.typeError("Cannot read blob from stream body"),
     };
     const content_type = try self._headers.get("content-type", exec) orelse "";
     const b = try Blob.initFromBytes(body, content_type, exec);
@@ -477,26 +470,26 @@ pub fn blob(self: *Response, exec: *const Execution) !js.Promise {
 
 pub fn bytes(self: *Response, exec: *const Execution) !js.Promise {
     const local = exec.js.local.?;
-    if (self.consume(local)) |rejected| return rejected;
+    try self.consume(local);
     const body = switch (self._body) {
         .bytes => |b| b,
         .empty => "",
-        .stream => return local.rejectPromise(.{ .type_error = "Cannot read bytes from stream body" }),
+        .stream => return local.typeError("Cannot read bytes from stream body"),
     };
     return local.resolvePromise(js.TypedArray(u8){ .values = body });
 }
 
 pub fn formData(self: *Response, exec: *const Execution) !js.Promise {
     const local = exec.js.local.?;
-    if (self.consume(local)) |rejected| return rejected;
+    try self.consume(local);
     const body = switch (self._body) {
         .bytes => |b| b,
         .empty => "",
-        .stream => return local.rejectPromise(.{ .type_error = "Cannot read FormData from stream body" }),
+        .stream => return local.typeError("Cannot read FormData from stream body"),
     };
 
     const content_type = try self._headers.get("content-type", exec) orelse {
-        return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" });
+        return local.typeError("Failed to parse body as FormData");
     };
     var it = ContentTypeIterator.init(content_type);
     const essence = it.essence;
@@ -507,12 +500,12 @@ pub fn formData(self: *Response, exec: *const Execution) !js.Promise {
     if (std.ascii.eqlIgnoreCase(essence, "multipart/form-data")) {
         const boundary = it.findBoundary();
         if (boundary.len == 0) {
-            return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" });
+            return local.typeError("Failed to parse body as FormData");
         }
 
         const form_data = FormData.initFromMultipart(body, boundary, exec) catch |err| switch (err) {
             error.OutOfMemory => return err,
-            else => return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" }),
+            else => return local.typeError("Failed to parse body as FormData"),
         };
         return local.resolvePromise(form_data);
     }
@@ -520,12 +513,12 @@ pub fn formData(self: *Response, exec: *const Execution) !js.Promise {
     if (std.ascii.eqlIgnoreCase(essence, "application/x-www-form-urlencoded")) {
         const form_data = FormData.initFromUrlEncoded(body, exec) catch |err| switch (err) {
             error.OutOfMemory => return err,
-            else => return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" }),
+            else => return local.typeError("Failed to parse body as FormData"),
         };
         return local.resolvePromise(form_data);
     }
 
-    return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" });
+    return local.typeError("Failed to parse body as FormData");
 }
 
 pub fn clone(self: *const Response, exec: *const Execution) !*Response {
diff --git a/src/browser/webapi/storage/idb/IDBFactory.zig b/src/browser/webapi/storage/idb/IDBFactory.zig
index b520acd53a..531a280a54 100644
--- a/src/browser/webapi/storage/idb/IDBFactory.zig
+++ b/src/browser/webapi/storage/idb/IDBFactory.zig
@@ -389,7 +389,7 @@ const DeleteContext = struct {
 pub fn databases(_: *IDBFactory, exec: *Execution) !js.Promise {
     const local = exec.js.local.?;
     // unavailable for opaque origins, e.g. about:blank
-    const origin = exec.origin() orelse return local.rejectPromise(.{ .dom_exception = .{ .err = error.SecurityError } });
+    const origin = exec.origin() orelse return error.SecurityError;
     const engine = try exec.session.idb.engineForOrigin(origin);
     return local.resolvePromise(try engine.databases(exec.call_arena));
 }