diff --git a/crates/cli-support/src/js/mod.rs b/crates/cli-support/src/js/mod.rs
index 4b375e4b1ed..632fab2819f 100644
--- a/crates/cli-support/src/js/mod.rs
+++ b/crates/cli-support/src/js/mod.rs
@@ -461,7 +461,13 @@ impl<'a> Context<'a> {
         Ok(imports)
     }
 
-    fn ts_for_init_fn(has_memory: bool, has_module_or_path_optional: bool) -> String {
+    fn ts_for_init_fn(
+        &self,
+        has_memory: bool,
+        has_module_or_path_optional: bool,
+    ) -> Result<String, Error> {
+        let output = crate::wasm2es6js::interface(&self.module)?;
+
         let (memory_doc, memory_param) = if has_memory {
             (
                 "* @param {WebAssembly.Memory} maybe_memory\n",
@@ -471,22 +477,28 @@ impl<'a> Context<'a> {
             ("", "")
         };
         let arg_optional = if has_module_or_path_optional { "?" } else { "" };
-        format!(
+        Ok(format!(
             "\n\
+            export type InitInput = RequestInfo | URL | Response | BufferSource | WebAssembly.Module;\n\
+            \n\
+            export interface InitOutput {{\n\
+            {output}}}\n\
+            \n\
             /**\n\
-            * If `module_or_path` is {{RequestInfo}}, makes a request and\n\
+            * If `module_or_path` is {{RequestInfo}} or {{URL}}, makes a request and\n\
             * for everything else, calls `WebAssembly.instantiate` directly.\n\
             *\n\
-            * @param {{RequestInfo | BufferSource | WebAssembly.Module}} module_or_path\n\
+            * @param {{InitInput | Promise<InitInput>}} module_or_path\n\
             {}\
             *\n\
-            * @returns {{Promise<any>}}\n\
+            * @returns {{Promise<InitOutput>}}\n\
             */\n\
             export default function init \
-                (module_or_path{}: RequestInfo | BufferSource | WebAssembly.Module{}): Promise<any>;
+                (module_or_path{}: InitInput | Promise<InitInput>{}): Promise<InitOutput>;
         ",
-            memory_doc, arg_optional, memory_param
-        )
+            memory_doc, arg_optional, memory_param,
+            output = output,
+        ))
     }
 
     fn gen_init(
@@ -541,7 +553,7 @@ impl<'a> Context<'a> {
             _ => "",
         };
 
-        let ts = Self::ts_for_init_fn(has_memory, !default_module_path.is_empty());
+        let ts = self.ts_for_init_fn(has_memory, !default_module_path.is_empty())?;
 
         // Initialize the `imports` object for all import definitions that we're
         // directed to wire up.
diff --git a/crates/cli-support/src/wasm2es6js.rs b/crates/cli-support/src/wasm2es6js.rs
index f50a1b38ac5..e67a6394b3e 100644
--- a/crates/cli-support/src/wasm2es6js.rs
+++ b/crates/cli-support/src/wasm2es6js.rs
@@ -44,6 +44,49 @@ impl Config {
     }
 }
 
+pub fn interface(module: &Module) -> Result<String, Error> {
+    let mut exports = String::new();
+
+    for entry in module.exports.iter() {
+        let id = match entry.item {
+            walrus::ExportItem::Function(i) => i,
+            walrus::ExportItem::Memory(_) => {
+                exports.push_str(&format!("  readonly {}: WebAssembly.Memory;\n", entry.name,));
+                continue;
+            }
+            walrus::ExportItem::Table(_) => {
+                exports.push_str(&format!("  readonly {}: WebAssembly.Table;\n", entry.name,));
+                continue;
+            }
+            walrus::ExportItem::Global(_) => continue,
+        };
+
+        let func = module.funcs.get(id);
+        let ty = module.types.get(func.ty());
+        let mut args = String::new();
+        for (i, _) in ty.params().iter().enumerate() {
+            if i > 0 {
+                args.push_str(", ");
+            }
+            args.push((b'a' + (i as u8)) as char);
+            args.push_str(": number");
+        }
+
+        exports.push_str(&format!(
+            "  readonly {name}: ({args}) => {ret};\n",
+            name = entry.name,
+            args = args,
+            ret = match ty.results().len() {
+                0 => "void",
+                1 => "number",
+                _ => "Array",
+            },
+        ));
+    }
+
+    Ok(exports)
+}
+
 pub fn typescript(module: &Module) -> Result<String, Error> {
     let mut exports = format!("/* tslint:disable */\n/* eslint-disable */\n");