FEATURE: add Ruby-to-JS Uint8Array support (#406)

* FEATURE: add Ruby-to-JS Uint8Array support

Introduce MiniRacer::Binary for attached Ruby callbacks that need to
return raw bytes to JavaScript. The C extension now serializes this
wrapper as a Uint8Array, and TruffleRuby is updated to mirror the same
conversion path.

Also add documentation, tests, and bump the version to 0.20.1.

* version bump

Author: SamSaffron

CHANGELOG

@@ -1,3 +1,6 @@
+- 0.21.0 - 16-04-2026
+  - Add MiniRacer::Binary for returning Uint8Array to JavaScript from attached Ruby callbacks
+
 - 0.20.0 - 24-02-2026
   - Add Snapshot.load to restore snapshots from binary data, enabling disk persistence
 

README.md

@@ -60,6 +60,25 @@ puts context.eval("array_and_hash()")
 # => {"a" => 1, "b" => [1, {"a" => 1}]}
 ```
 
+### Return binary data from Ruby to JavaScript
+
+Attached Ruby functions can return binary data as `Uint8Array` using `MiniRacer::Binary`:
+
+```ruby
+require "digest"
+
+context = MiniRacer::Context.new
+context.attach("sha256_raw", ->(data) {
+  MiniRacer::Binary.new(Digest::SHA256.digest(data))
+})
+
+# Inside JavaScript the return value is a Uint8Array
+context.eval("sha256_raw('hello') instanceof Uint8Array") # => true
+context.eval("sha256_raw('hello').length")                 # => 32
+```
+
+This is useful when you need to pass raw bytes (e.g., cryptographic digests, compressed data, binary file contents) from Ruby to JavaScript. The `MiniRacer::Binary` wrapper tells the bridge to serialize the data as a `Uint8Array` on the JavaScript side rather than a string.
+
 ### GIL free JavaScript execution
 
 The Ruby Global interpreter lock is released when scripts are executing:

ext/mini_racer_extension/mini_racer_extension.c

@@ -193,6 +193,7 @@ static VALUE terminated_error;
 static VALUE context_class;
 static VALUE snapshot_class;
 static VALUE date_time_class;
+static VALUE binary_class;
 static VALUE js_function_class;
 
 static pthread_mutex_t flags_mtx = PTHREAD_MUTEX_INITIALIZER;
@@ -688,10 +689,17 @@ static int serialize1(Ser *s, VALUE refs, VALUE v)
         // entirely new objects
         if (rb_respond_to(v, rb_intern("to_time"))) {
             v = rb_funcall(v, rb_intern("to_time"), 0);
-        }
-        if (rb_obj_is_kind_of(v, rb_cTime)) {
-            struct timeval tv = rb_time_timeval(v);
-            ser_date(s, tv.tv_sec*1e3 + tv.tv_usec/1e3);
+            if (rb_obj_is_kind_of(v, rb_cTime)) {
+                struct timeval tv = rb_time_timeval(v);
+                ser_date(s, tv.tv_sec*1e3 + tv.tv_usec/1e3);
+            } else {
+                snprintf(s->err, sizeof(s->err), "unsupported type %s", rb_class2name(CLASS_OF(v)));
+                return -1;
+            }
+        } else if (!NIL_P(binary_class) && rb_obj_is_kind_of(v, binary_class)) {
+            t = rb_ivar_get(v, rb_intern("@data"));
+            Check_Type(t, T_STRING);
+            ser_uint8array(s, RSTRING_PTR(t), RSTRING_LEN(t));
         } else {
             snprintf(s->err, sizeof(s->err), "unsupported type %s", rb_class2name(CLASS_OF(v)));
             return -1;
@@ -1111,6 +1119,11 @@ static VALUE context_alloc(VALUE klass)
         if (Qtrue == rb_funcall(rb_cObject, f, 1, a))
             date_time_class = rb_const_get(rb_cObject, rb_intern("DateTime"));
     }
+    if (NIL_P(binary_class)) {
+        VALUE m = rb_const_get(rb_cObject, rb_intern("MiniRacer"));
+        if (Qtrue == rb_funcall(m, rb_intern("const_defined?"), 1, rb_str_new_cstr("Binary")))
+            binary_class = rb_const_get(m, rb_intern("Binary"));
+    }
     c = ruby_xmalloc(sizeof(*c));
     memset(c, 0, sizeof(*c));
     c->exception = Qnil;
@@ -1763,5 +1776,6 @@ void Init_mini_racer_extension(void)
     rb_define_singleton_method(c, "set_flags!", platform_set_flags, -1);
 
     date_time_class = Qnil; // lazy init
+    binary_class = Qnil; // lazy init
     js_function_class = rb_define_class_under(m, "JavaScriptFunction", rb_cObject);
 }

ext/mini_racer_extension/serde.c

@@ -322,6 +322,19 @@ static void ser_string16(Ser *s, const void *p, size_t n)
     w(s, p, n);
 }
 
+// Uint8Array: ArrayBuffer header + data + typed array view descriptor
+static void ser_uint8array(Ser *s, const void *p, size_t n)
+{
+    w_byte(s, 'B');     // ArrayBuffer tag
+    w_varint(s, n);     // byte length
+    w(s, p, n);         // raw bytes
+    w_byte(s, 'V');     // typed array view tag
+    w_byte(s, 'B');     // Uint8Array type
+    w_varint(s, 0);     // byteOffset
+    w_varint(s, n);     // byteLength
+    w_varint(s, 0);     // flags
+}
+
 static void ser_object_begin(Ser *s)
 {
     w_byte(s, 'o');

lib/mini_racer.rb

@@ -1,6 +1,17 @@
 require "mini_racer/version"
 require "pathname"
 
+module MiniRacer
+  class Binary
+    attr_reader :data
+
+    def initialize(data)
+      raise TypeError, "wrong argument type #{data.class} (expected String)" unless data.is_a?(String)
+      @data = data
+    end
+  end
+end
+
 if RUBY_ENGINE == "truffleruby"
   require "mini_racer/truffleruby"
 else

lib/mini_racer/truffleruby.rb

@@ -102,7 +102,7 @@ def init_unsafe(isolate, snapshot)
       else
         @snapshot = nil
       end
-      @is_object_or_array_func, @is_map_func, @is_map_iterator_func, @is_time_func, @js_date_to_time_func, @is_symbol_func, @js_symbol_to_symbol_func, @js_new_date_func, @js_new_array_func = eval_in_context <<-CODE
+      @is_object_or_array_func, @is_map_func, @is_map_iterator_func, @is_time_func, @js_date_to_time_func, @is_symbol_func, @js_symbol_to_symbol_func, @js_new_date_func, @js_new_array_func, @js_new_uint8array_func = eval_in_context <<-CODE
         [
           (x) => { return (x instanceof Object || x instanceof Array) && !(x instanceof Date) && !(x instanceof Function) },
           (x) => { return x instanceof Map },
@@ -113,6 +113,7 @@ def init_unsafe(isolate, snapshot)
           (x) => { var r = x.description; return r === undefined ? 'undefined' : r },
           (x) => { return new Date(x) },
           (x) => { return new Array(x) },
+          (x) => { return new Uint8Array(x) },
         ]
       CODE
     end
@@ -329,6 +330,8 @@ def convert_ruby_to_js(value)
         js_new_date(value.to_f * 1000)
       when DateTime
         js_new_date(value.to_time.to_f * 1000)
+      when MiniRacer::Binary
+        @js_new_uint8array_func.call(value.data.bytes)
       else
         "Undefined Conversion"
       end

lib/mini_racer/version.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
 module MiniRacer
-  VERSION = "0.20.0"
+  VERSION = "0.21.0"
   LIBV8_NODE_VERSION = "~> 24.12.0.1"
 end

test/mini_racer_test.rb

@@ -1224,6 +1224,19 @@ def test_large_integer
     end
   end
 
+  def test_binary_returns_uint8array
+    context = MiniRacer::Context.new
+    context.attach("add_one", ->(data) {
+      MiniRacer::Binary.new(data.bytes.map { _1 + 1 }.pack("C*"))
+    })
+
+    result = context.eval <<~JS
+      var output = add_one(new Uint8Array([0, 1, 2, 3]));
+      (output instanceof Uint8Array) && Array.from(output).join(",") === "1,2,3,4";
+    JS
+    assert_equal true, result
+  end
+
   def test_exception_message_encoding
     e = nil
     begin