diff --git a/compiler/tests/channel.ml b/compiler/tests/channel.ml
new file mode 100644
index 0000000000..a35f27f5ee
--- /dev/null
+++ b/compiler/tests/channel.ml
@@ -0,0 +1,107 @@
+(* Js_of_ocaml compiler
+ * http://www.ocsigen.org/js_of_ocaml/
+ * Copyright (C) 2019 Hugo Heuzard
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, with linking exception;
+ * either version 2.1 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+*)
+
+(* https://github.com/ocsigen/js_of_ocaml/issues/777 *)
+
+let%expect_test _ =
+  Util.compile_and_run
+    {|
+    let oc = open_out "a.txt"
+    let () = print_int (out_channel_length oc)
+    let () = output_string oc "test"
+    let () = print_int (out_channel_length oc)
+    let () = flush oc
+    let () = print_int (out_channel_length oc)
+    let () = output_string oc "test"
+    let () = print_int (out_channel_length oc)
+    let () = seek_out oc 0
+    let () = print_int (out_channel_length oc)
+ |};
+  [%expect {| 00448 |}]
+
+let%expect_test _ =
+  let oc = open_out "b.txt" in
+  let () = print_int (out_channel_length oc) in
+  let () = output_string oc "test" in
+  let () = print_int (out_channel_length oc) in
+  let () = flush oc in
+  let () = print_int (out_channel_length oc) in
+  let () = output_string oc "test" in
+  let () = print_int (out_channel_length oc) in
+  let () = seek_out oc 0 in
+  let () = print_int (out_channel_length oc) in
+  [%expect {| 00448 |}]
+
+let%expect_test _ =
+  Util.compile_and_run
+    {|
+
+    let marshal_out ch v = Marshal.to_channel ch v []; flush ch
+
+    let marshal_out_segment f ch v =
+      let start = pos_out ch in
+      Format.printf "start=%d\n%!" start;
+      output_binary_int ch 0;  (* dummy value for stop *)
+      marshal_out ch v;
+      let stop = pos_out ch in
+      seek_out ch start;
+      output_binary_int ch stop;
+      seek_out ch stop;
+      Digest.output ch (Digest.file f)
+
+
+    let _ =
+        let filename = "out.txt" in
+        let chan = open_out filename in
+        output_binary_int chan 8900;
+        marshal_out_segment filename chan ["output";"data"];
+        marshal_out_segment filename chan ["more";"stuff"]
+|};
+  [%expect {|
+    start=4
+    start=59 |}]
+
+let%expect_test _ =
+  let module M = struct
+    let marshal_out ch v =
+      Marshal.to_channel ch v [];
+      flush ch
+
+    let marshal_out_segment f ch v =
+      let start = pos_out ch in
+      Format.printf "start=%d\n%!" start;
+      output_binary_int ch 0;
+      (* dummy value for stop *)
+      marshal_out ch v;
+      let stop = pos_out ch in
+      seek_out ch start;
+      output_binary_int ch stop;
+      seek_out ch stop;
+      Digest.output ch (Digest.file f)
+
+    let _ =
+      let filename = "out.txt" in
+      let chan = open_out filename in
+      output_binary_int chan 8900;
+      marshal_out_segment filename chan ["output"; "data"];
+      marshal_out_segment filename chan ["more"; "stuff"]
+  end in
+  [%expect {|
+    start=4
+    start=59 |}]
diff --git a/runtime/io.js b/runtime/io.js
index 8a972a5be6..e351c2e63d 100644
--- a/runtime/io.js
+++ b/runtime/io.js
@@ -394,26 +394,32 @@ function caml_output_value (chanid,v,_flags) {
 
 
 //Provides: caml_ml_seek_out
-//Requires: caml_ml_channels
+//Requires: caml_ml_channels, caml_ml_flush
 function caml_ml_seek_out(chanid,pos){
+  caml_ml_flush(chanid);
   caml_ml_channels[chanid].offset = pos;
   return 0;
 }
 
 //Provides: caml_ml_seek_out_64
-//Requires: caml_int64_to_float, caml_ml_channels
+//Requires: caml_int64_to_float, caml_ml_channels, caml_ml_flush
 function caml_ml_seek_out_64(chanid,pos){
+  caml_ml_flush(chanid);
   caml_ml_channels[chanid].offset = caml_int64_to_float(pos);
   return 0;
 }
 
 //Provides: caml_ml_pos_out
-//Requires: caml_ml_channels
-function caml_ml_pos_out(chanid) {return caml_ml_channels[chanid].offset}
+//Requires: caml_ml_channels, caml_ml_flush
+function caml_ml_pos_out(chanid) {
+  caml_ml_flush(chanid);
+  return caml_ml_channels[chanid].offset
+}
 
 //Provides: caml_ml_pos_out_64
-//Requires: caml_int64_of_float, caml_ml_channels
+//Requires: caml_int64_of_float, caml_ml_channels, caml_ml_flush
 function caml_ml_pos_out_64(chanid) {
+  caml_ml_flush(chanid);
   return caml_int64_of_float (caml_ml_channels[chanid].offset);
 }