diff --git a/lib/js_of_ocaml/js_of_ocaml_stubs.c b/lib/js_of_ocaml/js_of_ocaml_stubs.c
index 0e1c1b22db..9577aa127c 100644
--- a/lib/js_of_ocaml/js_of_ocaml_stubs.c
+++ b/lib/js_of_ocaml/js_of_ocaml_stubs.c
@@ -232,6 +232,10 @@ void caml_read_file_content () {
   fprintf(stderr, "Unimplemented Javascript primitive caml_read_file_content!\n");
   exit(1);
 }
+void caml_register_lazy () {
+  fprintf(stderr, "Unimplemented Javascript primitive caml_register_lazy!\n");
+  exit(1);
+}
 void caml_string_of_array () {
   fprintf(stderr, "Unimplemented Javascript primitive caml_string_of_array!\n");
   exit(1);
diff --git a/lib/js_of_ocaml/sys_js.ml b/lib/js_of_ocaml/sys_js.ml
index 79455100fe..9288569bab 100644
--- a/lib/js_of_ocaml/sys_js.ml
+++ b/lib/js_of_ocaml/sys_js.ml
@@ -61,3 +61,5 @@ let js_of_ocaml_version =
   if String.equal Lib_version.git_version ""
   then Lib_version.s
   else Lib_version.s ^ "+" ^ Lib_version.git_version
+
+external register_lazy : string -> unit = "caml_register_lazy"
diff --git a/lib/js_of_ocaml/sys_js.mli b/lib/js_of_ocaml/sys_js.mli
index 0c06137acc..84b66e27f8 100644
--- a/lib/js_of_ocaml/sys_js.mli
+++ b/lib/js_of_ocaml/sys_js.mli
@@ -56,6 +56,12 @@ val create_file : name:string -> content:string -> unit
       [create_file ~name ~content] register the file [name] with content [content]
       so it can be be opened with [open_in name] *)
 
+val register_lazy : string -> unit
+(** Register a lazy file to a Pseudo Filesystem.
+    [register_lazy name] causes an entry to be made in the filesystem such that it
+    will be present in a directory list, but the callback will be called when the
+    file is opened. *)
+
 val update_file : name:string -> content:string -> unit
 (** Update a file in the Pseudo Filesystem.
       [update_file ~name ~content] update the file [name] with content [content] *)
diff --git a/runtime/fs.js b/runtime/fs.js
index 2ee2a47d4d..e8ecafa3b2 100644
--- a/runtime/fs.js
+++ b/runtime/fs.js
@@ -314,7 +314,6 @@ function caml_create_file(name,content) {
   return 0;
 }
 
-
 //Provides: jsoo_create_file
 //Requires: caml_create_file, caml_string_of_jsbytes
 function jsoo_create_file(name,content) {
@@ -324,6 +323,16 @@ function jsoo_create_file(name,content) {
 }
 
 
+//Provides: caml_register_lazy
+//Requires: caml_failwith, resolve_fs_device, caml_string_of_jsbytes
+function caml_register_lazy(name) {
+  var name = (typeof name == "string")?caml_string_of_jsbytes(name):name;
+  var root = resolve_fs_device(name);
+  if(! root.device.register_lazy) caml_failwith("cannot register lazy file");
+  root.device.register_lazy(root.rest);
+  return 0;
+}
+
 //Provides: caml_read_file_content
 //Requires: resolve_fs_device, caml_raise_no_such_file, caml_create_bytes, caml_string_of_bytes
 //Requires: caml_string_of_jsbytes
diff --git a/runtime/fs_fake.js b/runtime/fs_fake.js
index cf71290193..3f78932cf4 100644
--- a/runtime/fs_fake.js
+++ b/runtime/fs_fake.js
@@ -27,6 +27,7 @@
 //Requires: make_unix_err_args
 function MlFakeDevice (root, f) {
   this.content={};
+  this.lazyfiles = [];
   this.root = root;
   this.lookupFun = f;
 }
@@ -60,13 +61,15 @@ MlFakeDevice.prototype.exists = function(name) {
   // Check if a directory exists
   var name_slash = this.slash(name);
   if(this.content[name_slash]) return 1;
+  // If it's a lazy file, let's say it exists without doing a lookup.
+  if(this.lazyfiles.indexOf(name) >= 0) return 1;
   // Check if a file exists
   this.lookup(name);
   return this.content[name]?1:0;
 }
 MlFakeDevice.prototype.mkdir = function(name,mode, raise_unix) {
   var unix_error = raise_unix && caml_named_value('Unix.Unix_error');
-  if(this.exists(name)) {
+  if(this.exists(name) || (this.lazyfiles.indexOf(name) >= 0)) {
     if (unix_error) {
       caml_raise_with_args(unix_error, make_unix_err_args("EEXIST", "mkdir", this.nm(name)));
     }
@@ -140,6 +143,10 @@ MlFakeDevice.prototype.readdir = function(name) {
     var m = n.match(r);
     if(m && !seen[m[1]]) {seen[m[1]] = true; a.push(m[1])}
   }
+  this.lazyfiles.forEach(function(n) {
+    var m = n.match(r);
+    if(m && !seen[m[1]]) {seen[m[1]] = true; a.push(m[1])}
+  })
   return a;
 }
 MlFakeDevice.prototype.is_dir = function(name) {
@@ -148,7 +155,7 @@ MlFakeDevice.prototype.is_dir = function(name) {
   return this.content[name_slash]?1:0;
 }
 MlFakeDevice.prototype.unlink = function(name) {
-  var ok = this.content[name]?true:false;
+  var ok = (this.content[name]?true:false) || (this.lazyfiles.indexOf(name) >= 0);
   delete this.content[name];
   return ok;
 }
@@ -195,6 +202,10 @@ MlFakeDevice.prototype.register= function (name,content){
   else caml_raise_sys_error(this.nm(name) + " : registering file with invalid content type");
 }
 
+MlFakeDevice.prototype.register_lazy = function (name) {
+  this.lazyfiles.push(name)
+}
+
 MlFakeDevice.prototype.constructor = MlFakeDevice
 
 //Provides: MlFakeFile