diff --git a/api/wasm-interpreter/src/lib.rs b/api/wasm-interpreter/src/lib.rs
index 319042471..9946176c3 100644
--- a/api/wasm-interpreter/src/lib.rs
+++ b/api/wasm-interpreter/src/lib.rs
@@ -33,6 +33,17 @@ impl CompilationResult {
     }
 }
 
+#[wasm_bindgen(typescript_custom_section)]
+const IMPORT_CALLBACK_FUNCTION_SECTION: &'static str = r#"
+type ImportCallbackFunction = (url: string) => Promise<string>;
+"#;
+
+#[wasm_bindgen]
+extern "C" {
+    #[wasm_bindgen(typescript_type = "ImportCallbackFunction")]
+    pub type ImportCallbackFunction;
+}
+
 /// Compile the content of a string.
 ///
 /// Returns a promise to a compiled component which can be run with ".run()"
@@ -40,7 +51,7 @@ impl CompilationResult {
 pub async fn compile_from_string(
     source: String,
     base_url: String,
-    optional_import_callback: Option<js_sys::Function>,
+    optional_import_callback: Option<ImportCallbackFunction>,
 ) -> Result<CompilationResult, JsValue> {
     compile_from_string_with_style(source, base_url, String::new(), optional_import_callback).await
 }
@@ -51,7 +62,7 @@ pub async fn compile_from_string_with_style(
     source: String,
     base_url: String,
     style: String,
-    optional_import_callback: Option<js_sys::Function>,
+    optional_import_callback: Option<ImportCallbackFunction>,
 ) -> Result<CompilationResult, JsValue> {
     #[cfg(feature = "console_error_panic_hook")]
     console_error_panic_hook::set_once();
@@ -66,7 +77,7 @@ pub async fn compile_from_string_with_style(
             Box<dyn core::future::Future<Output = Option<std::io::Result<String>>>>,
         > {
             Box::pin({
-                let load_callback = load_callback.clone();
+                let load_callback = js_sys::Function::from(load_callback.clone());
                 let file_name: String = file_name.to_string_lossy().into();
                 async move {
                     let result = load_callback.call1(&JsValue::UNDEFINED, &file_name.into());
diff --git a/tools/lsp/wasm_main.rs b/tools/lsp/wasm_main.rs
index d23389139..2c3623dda 100644
--- a/tools/lsp/wasm_main.rs
+++ b/tools/lsp/wasm_main.rs
@@ -127,6 +127,17 @@ impl Drop for ReentryGuardLock {
     }
 }
 
+#[wasm_bindgen(typescript_custom_section)]
+const IMPORT_CALLBACK_FUNCTION_SECTION: &'static str = r#"
+type ImportCallbackFunction = (url: string) => Promise<Uint8Array>;
+"#;
+
+#[wasm_bindgen]
+extern "C" {
+    #[wasm_bindgen(typescript_type = "ImportCallbackFunction")]
+    pub type ImportCallbackFunction;
+}
+
 #[wasm_bindgen]
 pub struct SlintServer {
     document_cache: Rc<RefCell<DocumentCache>>,
@@ -139,7 +150,7 @@ pub struct SlintServer {
 pub fn create(
     init_param: JsValue,
     send_notification: Function,
-    load_file: Function,
+    load_file: ImportCallbackFunction,
 ) -> Result<SlintServer, JsError> {
     console_error_panic_hook::set_once();
 
@@ -148,7 +159,7 @@ pub fn create(
     let mut compiler_config =
         CompilerConfiguration::new(i_slint_compiler::generator::OutputFormat::Interpreter);
     compiler_config.open_import_fallback = Some(Rc::new(move |path| {
-        let load_file = load_file.clone();
+        let load_file = Function::from(load_file.clone());
         Box::pin(async move { Some(self::load_file(path, &load_file).await) })
     }));