diff --git a/CHANGES.md b/CHANGES.md
index 8ffd45871f..f93122abdc 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -14,6 +14,7 @@
 * Runtime: change Sys.os_type on windows (Cygwin -> Win32)
 * Runtime: backtraces are really expensive, they need to be be explicitly
   requested at compile time (--enable with-js-error) or at startup (OCAMLRUNPARAM=b=1)
+* Runtime: allow dynlink of precompiled js with separate compilation (#1676)
 * Lib: Modify Typed_array API for compatibility with WebAssembly
 
 
diff --git a/compiler/tests-dynlink-js/dune b/compiler/tests-dynlink-js/dune
index 064ea7b5a8..37a0f7561f 100644
--- a/compiler/tests-dynlink-js/dune
+++ b/compiler/tests-dynlink-js/dune
@@ -2,12 +2,48 @@
  (name main)
  (modules main)
  (libraries js_of_ocaml)
- (modes byte))
+ (link_flags
+  (:standard -linkall))
+ ;; Until dune is fixed https://github.com/ocaml/dune/pull/10935
+ (js_of_ocaml
+  (flags
+   (:standard)
+   --linkall
+   (:include effects_flags.sexp))
+  (build_runtime_flags
+   (:standard)
+   (:include effects_flags.sexp))
+  (link_flags (:standard)))
+ (modes js byte))
+
+(executable
+ (name effects_flags)
+ (modules effects_flags))
+
+(rule
+ (target effects_flags.sexp)
+ (action
+  (with-stdout-to
+   %{target}
+   (run ./effects_flags.exe sexp %{profile}))))
+
+(rule
+ (target effects_flags.txt)
+ (action
+  (with-stdout-to
+   %{target}
+   (run ./effects_flags.exe txt %{profile}))))
 
 (rule
  (target main.js)
  (action
-  (run %{bin:js_of_ocaml} --linkall -o %{target} %{dep:main.bc})))
+  (run
+   %{bin:js_of_ocaml}
+   --linkall
+   %{read-strings:effects_flags.txt}
+   -o
+   %{target}
+   %{dep:main.bc})))
 
 (rule
  (target plugin.cmo)
@@ -17,17 +53,52 @@
 (rule
  (target plugin.js)
  (action
-  (run %{bin:js_of_ocaml} %{dep:./plugin.cmo})))
+  (run
+   %{bin:js_of_ocaml}
+   %{read-strings:effects_flags.txt}
+   %{dep:./plugin.cmo})))
+
+(rule
+ (target plugin2.cma)
+ (action
+  (run
+   %{bin:ocamlc}
+   -a
+   %{dep:./plugin2a.ml}
+   %{dep:./plugin2b.ml}
+   -o
+   plugin2.cma)))
+
+(rule
+ (target plugin2.js)
+ (action
+  (run
+   %{bin:js_of_ocaml}
+   %{read-strings:effects_flags.txt}
+   %{dep:./plugin2.cma})))
 
 (rule
  (target main.out)
- (deps plugin.js)
+ (deps plugin.js plugin2.js)
  (action
   (with-outputs-to
    %{target}
    (run %{bin:node} %{dep:./main.js}))))
 
+(rule
+ (target main.out2)
+ (deps plugin.js plugin2.js)
+ (action
+  (with-outputs-to
+   %{target}
+   (run %{bin:node} %{dep:./main.bc.js}))))
+
 (rule
  (alias runtest)
  (action
   (diff main.out.expected main.out)))
+
+(rule
+ (alias runtest)
+ (action
+  (diff main.out.expected main.out2)))
diff --git a/compiler/tests-dynlink-js/effects_flags.ml b/compiler/tests-dynlink-js/effects_flags.ml
new file mode 100644
index 0000000000..c0fce23c94
--- /dev/null
+++ b/compiler/tests-dynlink-js/effects_flags.ml
@@ -0,0 +1,19 @@
+let enable b n =
+  let f = if b then "--enable" else "--disable" in
+  [ f; n ]
+
+let () =
+  let major = String.split_on_char '.' Sys.ocaml_version |> List.hd |> int_of_string in
+  let has_effect l =
+    match l with
+    | [ "using-effects" ] -> major >= 5
+    | _ -> false
+  in
+  let aux l = enable (has_effect l) "effects" in
+  match Sys.argv |> Array.to_list |> List.tl with
+  | "txt" :: rest -> List.iter print_endline (aux rest)
+  | "sexp" :: rest ->
+      print_endline "(";
+      List.iter print_endline (aux rest);
+      print_endline ")"
+  | _ -> assert false
diff --git a/compiler/tests-dynlink-js/main.ml b/compiler/tests-dynlink-js/main.ml
index 98cf2f9a47..31825e4a26 100644
--- a/compiler/tests-dynlink-js/main.ml
+++ b/compiler/tests-dynlink-js/main.ml
@@ -5,3 +5,5 @@ let require s =
   Js.Unsafe.fun_call (Js.Unsafe.js_expr "require") [| Js.Unsafe.inject (Js.string s) |]
 
 let () = require "./plugin.js"
+
+let () = require "./plugin2.js"
diff --git a/compiler/tests-dynlink-js/main.out.expected b/compiler/tests-dynlink-js/main.out.expected
index ba390cb600..587f51d01e 100644
--- a/compiler/tests-dynlink-js/main.out.expected
+++ b/compiler/tests-dynlink-js/main.out.expected
@@ -1,2 +1,4 @@
 hello
 plugin loaded
+plugin 2a loaded
+plugin 2b loaded
diff --git a/compiler/tests-dynlink-js/plugin2a.ml b/compiler/tests-dynlink-js/plugin2a.ml
new file mode 100644
index 0000000000..28d230b361
--- /dev/null
+++ b/compiler/tests-dynlink-js/plugin2a.ml
@@ -0,0 +1 @@
+let () = print_endline "plugin 2a loaded"
diff --git a/compiler/tests-dynlink-js/plugin2b.ml b/compiler/tests-dynlink-js/plugin2b.ml
new file mode 100644
index 0000000000..3c05267241
--- /dev/null
+++ b/compiler/tests-dynlink-js/plugin2b.ml
@@ -0,0 +1 @@
+let () = print_endline "plugin 2b loaded"
diff --git a/runtime/stdlib.js b/runtime/stdlib.js
index 3e878681cf..e537b382a8 100644
--- a/runtime/stdlib.js
+++ b/runtime/stdlib.js
@@ -150,11 +150,15 @@ var caml_global_data = [0];
 //Requires: caml_jsstring_of_string
 function caml_build_symbols(symb) {
   var r = {};
+  var max = -1;
   if (symb) {
     for (var i = 1; i < symb.length; i++) {
-      r[caml_jsstring_of_string(symb[i][1])] = symb[i][2];
+      var idx = symb[i][2];
+      max = Math.max(max, idx);
+      r[caml_jsstring_of_string(symb[i][1])] = idx;
     }
   }
+  r.next_idx = max + 1;
   return r;
 }
 
@@ -173,7 +177,10 @@ function caml_register_global(n, v, name_opt) {
       var nid = caml_global_data.symidx[name];
       if (nid >= 0) n = nid;
       else {
-        caml_failwith("caml_register_global: cannot locate " + name);
+        // The unit is unknown, this can happen when dynlinking a precompiled js,
+        // let's allocate a fresh idx.
+        var n = caml_global_data.symidx.next_idx++;
+        caml_global_data.symidx[name] = n;
       }
     }
   }