diff --git a/src/browser/js/Context.zig b/src/browser/js/Context.zig
index da7362aa..cd775332 100644
--- a/src/browser/js/Context.zig
+++ b/src/browser/js/Context.zig
@@ -545,13 +545,13 @@ pub fn dynamicModuleCallback(
 
         break :blk js.String.toSliceZ(.{ .local = &local, .handle = resource_name.? }) catch |err| {
             log.err(.app, "OOM", .{ .err = err, .src = "dynamicModuleCallback1" });
-            return @constCast((local.rejectPromise("Out of memory") catch return null).handle);
+            return @constCast(local.rejectPromise(.{ .generic_error = "Out of memory" }).handle);
         };
     };
 
     const specifier = js.String.toSliceZ(.{ .local = &local, .handle = v8_specifier.? }) catch |err| {
         log.err(.app, "OOM", .{ .err = err, .src = "dynamicModuleCallback2" });
-        return @constCast((local.rejectPromise("Out of memory") catch return null).handle);
+        return @constCast(local.rejectPromise(.{ .generic_error = "Out of memory" }).handle);
     };
 
     const normalized_specifier = self.script_manager.?.resolveSpecifier(
@@ -560,14 +560,14 @@ pub fn dynamicModuleCallback(
         specifier,
     ) catch |err| {
         log.err(.app, "OOM", .{ .err = err, .src = "dynamicModuleCallback3" });
-        return @constCast((local.rejectPromise("Out of memory") catch return null).handle);
+        return @constCast(local.rejectPromise(.{ .generic_error = "Out of memory" }).handle);
     };
 
     const promise = self._dynamicModuleCallback(normalized_specifier, resource, &local) catch |err| blk: {
         log.err(.js, "dynamic module callback", .{
             .err = err,
         });
-        break :blk local.rejectPromise("Failed to load module") catch return null;
+        break :blk local.rejectPromise(.{ .generic_error = "Out of memory" });
     };
     return @constCast(promise.handle);
 }
diff --git a/src/browser/js/Local.zig b/src/browser/js/Local.zig
index f913dafd..af7d1116 100644
--- a/src/browser/js/Local.zig
+++ b/src/browser/js/Local.zig
@@ -1206,9 +1206,9 @@ pub fn stackTrace(self: *const Local) !?[]const u8 {
 }
 
 // == Promise Helpers ==
-pub fn rejectPromise(self: *const Local, value: anytype) !js.Promise {
+pub fn rejectPromise(self: *const Local, comptime kind: js.PromiseResolver.RejectError) js.Promise {
     var resolver = js.PromiseResolver.init(self);
-    resolver.reject("Local.rejectPromise", value);
+    resolver.rejectError("Local.rejectPromise", kind);
     return resolver.promise();
 }
 
diff --git a/src/browser/js/PromiseResolver.zig b/src/browser/js/PromiseResolver.zig
index 6386569a..7743fa48 100644
--- a/src/browser/js/PromiseResolver.zig
+++ b/src/browser/js/PromiseResolver.zig
@@ -22,6 +22,8 @@ const v8 = js.v8;
 const log = @import("../../log.zig");
 const DOMException = @import("../webapi/DOMException.zig");
 
+const DOMException = @import("../webapi/DOMException.zig");
+
 const PromiseResolver = @This();
 
 local: *const js.Local,
@@ -65,20 +67,43 @@ pub fn reject(self: PromiseResolver, comptime source: []const u8, value: anytype
     };
 }
 
-pub const RejectError = union(enum) {
-    generic: []const u8,
+pub const RejectError = union(enum(u4)) {
+    /// Not to be confused with `DOMException`; this is bare `Error`.
+    generic_error: []const u8,
+    range_error: []const u8,
+    reference_error: []const u8,
+    syntax_error: []const u8,
     type_error: []const u8,
-    dom_exception: anyerror,
+    wasm_compile_error: void, // TODO.
+    wasm_link_error: void, // TODO.
+    wasm_runtime_error: void, // TODO.
+    wasm_suspend_error: void, // TODO.
+    /// DOM exceptions are unknown to V8, belongs to web standards.
+    dom_exception: struct { err: anyerror },
 };
-pub fn rejectError(self: PromiseResolver, comptime source: []const u8, err: RejectError) void {
-    const handle = switch (err) {
-        .type_error => |str| self.local.isolate.createTypeError(str),
-        .generic => |str| self.local.isolate.createError(str),
+
+/// Rejects the promise w/ an error object.
+pub fn rejectError(
+    self: PromiseResolver,
+    comptime source: []const u8,
+    comptime kind: RejectError,
+) void {
+    const handle = switch (kind) {
+        .generic_error => |msg| self.local.isolate.createError(msg),
+        .range_error => |msg| self.local.isolate.createRangeError(msg),
+        .reference_error => |msg| self.local.isolate.createReferenceError(msg),
+        .syntax_error => |msg| self.local.isolate.createSyntaxError(msg),
+        .type_error => |msg| self.local.isolate.createTypeError(msg),
+        // "Exceptional".
         .dom_exception => |exception| {
-            self.reject(source, DOMException.fromError(exception));
+            self._reject(DOMException.fromError(exception.err) orelse unreachable) catch |reject_err| {
+                log.err(.bug, "rejectDomException", .{ .source = source, .err = reject_err, .persistent = false });
+            };
             return;
         },
+        inline else => unreachable,
     };
+
     self._reject(js.Value{ .handle = handle, .local = self.local }) catch |reject_err| {
         log.err(.bug, "rejectError", .{ .source = source, .err = reject_err, .persistent = false });
     };
diff --git a/src/browser/webapi/CustomElementRegistry.zig b/src/browser/webapi/CustomElementRegistry.zig
index 6702a52e..25ed8857 100644
--- a/src/browser/webapi/CustomElementRegistry.zig
+++ b/src/browser/webapi/CustomElementRegistry.zig
@@ -125,8 +125,8 @@ pub fn whenDefined(self: *CustomElementRegistry, name: []const u8, page: *Page)
         return local.resolvePromise(definition.constructor);
     }
 
-    validateName(name) catch |err| {
-        return local.rejectPromise(DOMException.fromError(err) orelse unreachable);
+    validateName(name) catch |err| switch (err) {
+        error.SyntaxError => return local.rejectPromise(.{ .dom_exception = .{ .err = error.SyntaxError } }),
     };
 
     const gop = try self._when_defined.getOrPut(page.arena, name);
diff --git a/src/browser/webapi/SubtleCrypto.zig b/src/browser/webapi/SubtleCrypto.zig
index 4839a69a..7d6df2e3 100644
--- a/src/browser/webapi/SubtleCrypto.zig
+++ b/src/browser/webapi/SubtleCrypto.zig
@@ -96,8 +96,8 @@ pub fn generateKey(
     key_usages: []const []const u8,
     page: *Page,
 ) !js.Promise {
-    const key_or_pair = CryptoKey.init(algorithm, extractable, key_usages, page) catch |err| {
-        return page.js.local.?.rejectPromise(@errorName(err));
+    const key_or_pair = CryptoKey.init(algorithm, extractable, key_usages, page) catch {
+        return page.js.local.?.rejectPromise(.{ .dom_exception = .{ .err = error.SyntaxError } });
     };
 
     return page.js.local.?.resolvePromise(key_or_pair);
@@ -112,7 +112,7 @@ pub fn exportKey(
     page: *Page,
 ) !js.Promise {
     if (!key.canExportKey()) {
-        return error.InvalidAccessError;
+        return page.js.local.?.rejectPromise(.{ .dom_exception = .{ .err = error.InvalidAccessError } });
     }
 
     if (std.mem.eql(u8, format, "raw")) {
@@ -124,9 +124,10 @@ pub fn exportKey(
 
     if (is_unsupported) {
         log.warn(.not_implemented, "SubtleCrypto.exportKey", .{ .format = format });
+        return page.js.local.?.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
     }
 
-    return page.js.local.?.rejectPromise(@errorName(error.NotSupported));
+    return page.js.local.?.rejectPromise(.{ .type_error = "invalid format" });
 }
 
 /// Derive a secret key from a master key.
@@ -148,7 +149,7 @@ pub fn deriveBits(
                 log.warn(.not_implemented, "SubtleCrypto.deriveBits", .{ .name = name });
             }
 
-            return page.js.local.?.rejectPromise(@errorName(error.NotSupported));
+            return page.js.local.?.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
         },
     };
 }
@@ -185,19 +186,19 @@ pub fn sign(
         .hmac => {
             // Verify algorithm.
             if (!algorithm.isHMAC()) {
-                return page.js.local.?.rejectPromise(@errorName(error.InvalidAccessError));
+                return page.js.local.?.rejectPromise(.{ .dom_exception = .{ .err = error.InvalidAccessError } });
             }
 
             // Call sign for HMAC.
-            const result = key.signHMAC(data, page) catch |err| {
-                return page.js.local.?.rejectPromise(@errorName(err));
+            const result = key.signHMAC(data, page) catch {
+                return page.js.local.?.rejectPromise(.{ .dom_exception = .{ .err = error.InvalidAccessError } });
             };
 
             return page.js.local.?.resolvePromise(result);
         },
         else => {
             log.warn(.not_implemented, "SubtleCrypto.sign", .{ .key_type = key._type });
-            return page.js.local.?.rejectPromise(@errorName(error.InvalidAccessError));
+            return page.js.local.?.rejectPromise(.{ .dom_exception = .{ .err = error.InvalidAccessError } });
         },
     };
 }
@@ -211,18 +212,20 @@ pub fn verify(
     data: []const u8, // ArrayBuffer.
     page: *Page,
 ) !js.Promise {
-    if (!algorithm.isHMAC()) return error.InvalidAccessError;
+    if (!algorithm.isHMAC()) {
+        return page.js.local.?.rejectPromise(.{ .dom_exception = .{ .err = error.InvalidAccessError } });
+    }
 
     return switch (key._type) {
         .hmac => key.verifyHMAC(signature, data, page),
-        else => return error.InvalidAccessError,
+        else => page.js.local.?.rejectPromise(.{ .dom_exception = .{ .err = error.InvalidAccessError } }),
     };
 }
 
 pub fn digest(_: *const SubtleCrypto, algorithm: []const u8, data: js.TypedArray(u8), page: *Page) !js.Promise {
     const local = page.js.local.?;
     if (algorithm.len > 10) {
-        return local.rejectPromise(DOMException.fromError(error.NotSupported));
+        return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
     }
     const normalized = std.ascii.lowerString(&page.buf, algorithm);
     if (std.mem.eql(u8, normalized, "sha-1")) {
@@ -245,7 +248,7 @@ pub fn digest(_: *const SubtleCrypto, algorithm: []const u8, data: js.TypedArray
         Sha512.hash(data.values, page.buf[0..Sha512.digest_length], .{});
         return local.resolvePromise(js.ArrayBuffer{ .values = page.buf[0..Sha512.digest_length] });
     }
-    return local.rejectPromise(DOMException.fromError(error.NotSupported));
+    return local.rejectPromise(.{ .dom_exception = .{ .err = error.NotSupported } });
 }
 
 /// Returns the desired digest by its name.
diff --git a/src/browser/webapi/net/Fetch.zig b/src/browser/webapi/net/Fetch.zig
index 0a44aae2..de1af4ec 100644
--- a/src/browser/webapi/net/Fetch.zig
+++ b/src/browser/webapi/net/Fetch.zig
@@ -243,7 +243,7 @@ fn httpErrorCallback(ctx: *anyopaque, err: anyerror) void {
     defer ls.deinit();
 
     // fetch() must reject with a TypeError on network errors per spec
-    ls.toLocal(self._resolver).rejectError("fetch error", .{ .type_error = @errorName(err) });
+    ls.toLocal(self._resolver).rejectError("fetch error", .{ .type_error = "fetch error" });
 }
 
 fn httpShutdownCallback(ctx: *anyopaque) void {
diff --git a/src/browser/webapi/net/Request.zig b/src/browser/webapi/net/Request.zig
index 6ba8f224..81ef8ba9 100644
--- a/src/browser/webapi/net/Request.zig
+++ b/src/browser/webapi/net/Request.zig
@@ -192,8 +192,8 @@ pub fn text(self: *const Request, page: *Page) !js.Promise {
 pub fn json(self: *const Request, page: *Page) !js.Promise {
     const body = self._body orelse "";
     const local = page.js.local.?;
-    const value = local.parseJSON(body) catch |err| {
-        return local.rejectPromise(.{@errorName(err)});
+    const value = local.parseJSON(body) catch {
+        return local.rejectPromise(.{ .syntax_error = "failed to parse" });
     };
     return local.resolvePromise(try value.persist());
 }
diff --git a/src/browser/webapi/net/Response.zig b/src/browser/webapi/net/Response.zig
index 6a926369..7020ac05 100644
--- a/src/browser/webapi/net/Response.zig
+++ b/src/browser/webapi/net/Response.zig
@@ -139,8 +139,8 @@ pub fn getText(self: *const Response, page: *Page) !js.Promise {
 pub fn getJson(self: *Response, page: *Page) !js.Promise {
     const body = self._body orelse "";
     const local = page.js.local.?;
-    const value = local.parseJSON(body) catch |err| {
-        return local.rejectPromise(.{@errorName(err)});
+    const value = local.parseJSON(body) catch {
+        return local.rejectPromise(.{ .syntax_error = "failed to parse" });
     };
     return local.resolvePromise(try value.persist());
 }
diff --git a/src/browser/webapi/streams/ReadableStream.zig b/src/browser/webapi/streams/ReadableStream.zig
index e4e5d0f9..56d5e9eb 100644
--- a/src/browser/webapi/streams/ReadableStream.zig
+++ b/src/browser/webapi/streams/ReadableStream.zig
@@ -255,7 +255,7 @@ pub fn pipeThrough(self: *ReadableStream, transform: PipeTransform, page: *Page)
 /// Returns a promise that resolves when piping is complete.
 pub fn pipeTo(self: *ReadableStream, destination: *WritableStream, page: *Page) !js.Promise {
     if (self.getLocked()) {
-        return page.js.local.?.rejectPromise("ReadableStream is locked");
+        return page.js.local.?.rejectPromise(.{ .type_error = "ReadableStream is locked" });
     }
 
     const local = page.js.local.?;
diff --git a/src/browser/webapi/streams/ReadableStreamDefaultReader.zig b/src/browser/webapi/streams/ReadableStreamDefaultReader.zig
index 2d3c5bbe..7bd848b7 100644
--- a/src/browser/webapi/streams/ReadableStreamDefaultReader.zig
+++ b/src/browser/webapi/streams/ReadableStreamDefaultReader.zig
@@ -58,12 +58,12 @@ pub const ReadResult = struct {
 
 pub fn read(self: *ReadableStreamDefaultReader, page: *Page) !js.Promise {
     const stream = self._stream orelse {
-        return page.js.local.?.rejectPromise("Reader has been released");
+        return page.js.local.?.rejectPromise(.{ .type_error = "Reader has been released" });
     };
 
     if (stream._state == .errored) {
-        const err = stream._stored_error orelse "Stream errored";
-        return page.js.local.?.rejectPromise(err);
+        //const err = stream._stored_error orelse "Stream errored";
+        return page.js.local.?.rejectPromise(.{ .type_error = "Stream errored" });
     }
 
     if (stream._controller.dequeue()) |chunk| {
@@ -95,7 +95,7 @@ pub fn releaseLock(self: *ReadableStreamDefaultReader) void {
 
 pub fn cancel(self: *ReadableStreamDefaultReader, reason_: ?[]const u8, page: *Page) !js.Promise {
     const stream = self._stream orelse {
-        return page.js.local.?.rejectPromise("Reader has been released");
+        return page.js.local.?.rejectPromise(.{ .type_error = "Reader has been released" });
     };
 
     self.releaseLock();
diff --git a/src/browser/webapi/streams/WritableStreamDefaultWriter.zig b/src/browser/webapi/streams/WritableStreamDefaultWriter.zig
index 8ed552ea..f5e5e610 100644
--- a/src/browser/webapi/streams/WritableStreamDefaultWriter.zig
+++ b/src/browser/webapi/streams/WritableStreamDefaultWriter.zig
@@ -32,11 +32,11 @@ pub fn init(stream: *WritableStream, page: *Page) !*WritableStreamDefaultWriter
 
 pub fn write(self: *WritableStreamDefaultWriter, chunk: js.Value, page: *Page) !js.Promise {
     const stream = self._stream orelse {
-        return page.js.local.?.rejectPromise("Writer has been released");
+        return page.js.local.?.rejectPromise(.{ .type_error = "Writer has been released" });
     };
 
     if (stream._state != .writable) {
-        return page.js.local.?.rejectPromise("Stream is not writable");
+        return page.js.local.?.rejectPromise(.{ .type_error = "Stream is not writable" });
     }
 
     try stream.writeChunk(chunk, page);
@@ -46,11 +46,11 @@ pub fn write(self: *WritableStreamDefaultWriter, chunk: js.Value, page: *Page) !
 
 pub fn close(self: *WritableStreamDefaultWriter, page: *Page) !js.Promise {
     const stream = self._stream orelse {
-        return page.js.local.?.rejectPromise("Writer has been released");
+        return page.js.local.?.rejectPromise(.{ .type_error = "Writer has been released" });
     };
 
     if (stream._state != .writable) {
-        return page.js.local.?.rejectPromise("Stream is not writable");
+        return page.js.local.?.rejectPromise(.{ .type_error = "Stream is not writable" });
     }
 
     try stream.closeStream(page);
@@ -67,7 +67,7 @@ pub fn releaseLock(self: *WritableStreamDefaultWriter) void {
 
 pub fn getClosed(self: *WritableStreamDefaultWriter, page: *Page) !js.Promise {
     const stream = self._stream orelse {
-        return page.js.local.?.rejectPromise("Writer has been released");
+        return page.js.local.?.rejectPromise(.{ .type_error = "Writer has been released" });
     };
 
     if (stream._state == .closed) {