diff --git a/packages/webr/NAMESPACE b/packages/webr/NAMESPACE
index f9ca3f0f..25c1ee09 100644
--- a/packages/webr/NAMESPACE
+++ b/packages/webr/NAMESPACE
@@ -14,6 +14,7 @@ export(mount)
 export(pager_install)
 export(require_shim)
 export(shim_install)
+export(syncfs)
 export(test_package)
 export(unmount)
 useDynLib(webr, .registration = TRUE)
diff --git a/packages/webr/R/library.R b/packages/webr/R/library.R
index ba1861ef..3ebb7740 100644
--- a/packages/webr/R/library.R
+++ b/packages/webr/R/library.R
@@ -21,8 +21,10 @@
 #' `show_menu` argument. By default, if no global option is set and no argument
 #' is provided, the menu will not be shown.
 #'
+#' @param pkg Character vector of package names
 #' @param show_menu Show a menu asking the user if they would like to install
 #'   the package if it is missing. Defaults to `getOption("webr.show_menu")`.
+#' @param ... Other arguments to be passed to `library` and `require`.
 #'
 #' @export
 library_shim <- function(pkg, ..., show_menu = getOption("webr.show_menu")) {
diff --git a/packages/webr/R/mount.R b/packages/webr/R/mount.R
index 5a790ab6..fc882cf1 100644
--- a/packages/webr/R/mount.R
+++ b/packages/webr/R/mount.R
@@ -17,10 +17,11 @@
 #' under Node.
 #'
 #' When mounting an Emscripten "idbfs" type filesystem, files will be persisted
-#' or populated from/to a browser-based IndexedDB database when the JavaScript
-#' function `Module.FS.syncfs` is invoked. See the Emscripten `IDBFS`
+#' to or populated from a browser-based IndexedDB database whenever the
+#' JavaScript function `Module.FS.syncfs` is invoked. See the Emscripten `IDBFS`
 #' documentation for more information. This filesystem type can only be used
-#' when webR is running in a web browser.
+#' when webR is running in a web browser and using the `PostMessage`
+#' communication channel.
 #'
 #' @param mountpoint a character string giving the path to a directory to mount
 #'   onto in the Emscripten virtual filesystem.
@@ -52,3 +53,19 @@ mount <- function(mountpoint, source, type = "workerfs") {
 unmount <- function(mountpoint) {
   invisible(.Call(ffi_unmount, mountpoint))
 }
+
+#' Synchronise the Emscripten virtual filesystem
+#'
+#' @description
+#' Uses the Emscripten filesystem API to synchronise all mounted virtual
+#' filesystems with their backing storage, where it exists. The `populate`
+#' argument controls the direction of the synchronisation between Emscripten's
+#' internal data and the file system's persistent store.
+#'
+#' @param populate A boolean. When `true`, initialises the filesystem with data
+#'   from persistent storage. When `false`, writes current filesystem data to
+#'   the persistent storage.
+#' @export
+syncfs <- function(populate) {
+  invisible(.Call(ffi_syncfs, populate))
+}
diff --git a/packages/webr/man/library_shim.Rd b/packages/webr/man/library_shim.Rd
index a1fe01a6..f5a33163 100644
--- a/packages/webr/man/library_shim.Rd
+++ b/packages/webr/man/library_shim.Rd
@@ -10,6 +10,10 @@ library_shim(pkg, ..., show_menu = getOption("webr.show_menu"))
 require_shim(pkg, ..., show_menu = getOption("webr.show_menu"))
 }
 \arguments{
+\item{pkg}{Character vector of package names}
+
+\item{...}{Other arguments to be passed to \code{library} and \code{require}.}
+
 \item{show_menu}{Show a menu asking the user if they would like to install
 the package if it is missing. Defaults to \code{getOption("webr.show_menu")}.}
 }
diff --git a/packages/webr/man/mount.Rd b/packages/webr/man/mount.Rd
index 7a6d77f6..956d1de6 100644
--- a/packages/webr/man/mount.Rd
+++ b/packages/webr/man/mount.Rd
@@ -36,8 +36,9 @@ will be mapped into the virtual filesystem and mounted onto the directory
 under Node.
 
 When mounting an Emscripten "idbfs" type filesystem, files will be persisted
-or populated from/to a browser-based IndexedDB database when the JavaScript
-function \code{Module.FS.syncfs} is invoked. See the Emscripten \code{IDBFS}
+to or populated from a browser-based IndexedDB database whenever the
+JavaScript function \code{Module.FS.syncfs} is invoked. See the Emscripten \code{IDBFS}
 documentation for more information. This filesystem type can only be used
-when webR is running in a web browser.
+when webR is running in a web browser and using the \code{PostMessage}
+communication channel.
 }
diff --git a/packages/webr/man/syncfs.Rd b/packages/webr/man/syncfs.Rd
new file mode 100644
index 00000000..4d95fc13
--- /dev/null
+++ b/packages/webr/man/syncfs.Rd
@@ -0,0 +1,19 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/mount.R
+\name{syncfs}
+\alias{syncfs}
+\title{Synchronise the Emscripten virtual filesystem}
+\usage{
+syncfs(populate)
+}
+\arguments{
+\item{populate}{A boolean. When \code{true}, initialises the filesystem with data
+from persistent storage. When \code{false}, writes current filesystem data to
+the persistent storage.}
+}
+\description{
+Uses the Emscripten filesystem API to synchronise all mounted virtual
+filesystems with their backing storage, where it exists. The \code{populate}
+argument controls the direction of the synchronisation between Emscripten's
+internal data and the file system's persistent store.
+}
diff --git a/packages/webr/src/init.c b/packages/webr/src/init.c
index 1839a826..e1dcf96a 100644
--- a/packages/webr/src/init.c
+++ b/packages/webr/src/init.c
@@ -11,6 +11,7 @@ extern SEXP ffi_dev_canvas_destroy(SEXP);
 extern SEXP ffi_mount_workerfs(SEXP, SEXP);
 extern SEXP ffi_mount_nodefs(SEXP, SEXP);
 extern SEXP ffi_mount_idbfs(SEXP);
+extern SEXP ffi_syncfs(SEXP);
 extern SEXP ffi_unmount(SEXP);
 
 static
@@ -25,6 +26,7 @@ const R_CallMethodDef CallEntries[] = {
   { "ffi_mount_workerfs",         (DL_FUNC) &ffi_mount_workerfs,         2},
   { "ffi_mount_nodefs",           (DL_FUNC) &ffi_mount_nodefs,           2},
   { "ffi_mount_idbfs",            (DL_FUNC) &ffi_mount_idbfs,            1},
+  { "ffi_syncfs",                 (DL_FUNC) &ffi_syncfs,                 1},
   { "ffi_unmount",                (DL_FUNC) &ffi_unmount,                1},
   { NULL,                         NULL,                                  0}
 };
diff --git a/packages/webr/src/mount.c b/packages/webr/src/mount.c
index c2f2bfa1..9289a646 100644
--- a/packages/webr/src/mount.c
+++ b/packages/webr/src/mount.c
@@ -15,6 +15,14 @@
     Rf_error("`" #arg "` can't be `NA`.");               \
   }
 
+#define CHECK_LOGICAL(arg) \
+  if (!Rf_isLogical(arg) || LENGTH(arg) != 1) {          \
+    Rf_error("`" #arg "` must be a logical.");           \
+  }                                                      \
+  if (LOGICAL(arg)[0] == NA_LOGICAL){                    \
+    Rf_error("`" #arg "` can't be `NA`.");               \
+  }
+
 SEXP ffi_mount_workerfs(SEXP source, SEXP mountpoint) {
 #ifdef __EMSCRIPTEN__
   CHECK_STRING(source);
@@ -105,6 +113,16 @@ SEXP ffi_mount_idbfs(SEXP mountpoint) {
 #endif
 }
 
+SEXP ffi_syncfs(SEXP populate) {
+#ifdef __EMSCRIPTEN__
+  CHECK_LOGICAL(populate);
+  EM_ASM({ Module.FS.syncfs($0, () => {}) }, LOGICAL(populate)[0]);
+  return R_NilValue;
+#else
+  Rf_error("Function must be running under Emscripten.");
+#endif
+}
+
 SEXP ffi_unmount(SEXP mountpoint) {
 #ifdef __EMSCRIPTEN__
   CHECK_STRING(mountpoint);