ts-function

A proc-macro that generates TypeScript type aliases and wasm-bindgen ABI trait implementations for Rust function wrapper structs, enabling strongly-typed Typescript Functions types in ts-macro projects with little to no boilerplate.

Motivation

When using ts-macro to generate TypeScript interfaces for Rust structs, there is no built-in support for function types. A struct field like on_click: js_sys::Function produces an opaque Function type in TypeScript and forces you to manually handle raw JsValue conversions in Rust.

ts-function bridges this gap. It allows you to define function signatures in pure Rust, automatically generating the correct TypeScript types ((args: ...) => void) and implementing the wasm-bindgen ABI traits required to pass those functions across the Wasm boundary safely. In Javascript land, it's common for libaries to take an object of functions to customize behavior, and this libary makes it easy to write code like that.

Example: Unified #[ts] Attribute

The #[ts] attribute macro can generate TypeScript bindings for structs, enums, and functions. For enums it uses wasm_bindgen's enum handling, for structs it behaves simular to ts-macro (stil hoping to upstream oneday), and for functions which is the namesake and most complex case, they are generated from type aliases or impl blocks. So on the Rust side, Typescript functions are represented by a struct like: struct TypedFunction(Function) which implements a call pseudo-trait. Call provides strong typing and type conversion, as well as easy result chaining. The newtype struct allows us to hold the function handle.

use wasm_bindgen::prelude::*;
use ts_function::ts;

// 1. Define your enums - automatically applies #[wasm_bindgen]
#[ts]
pub enum UserStatus { Active, Inactive }

// 2. Define your function signatures using type aliases
#[ts]
pub type SingleArgFn = fn(msg: String);

#[ts]
pub type MultiArgFn = fn(a: f64, b: js_sys::Uint8Array);

// 3. Use `#[ts]` to create a struct containing your function wrappers
#[ts]
struct AppInit {
    on_ready: SingleArgFn,
    on_data: MultiArgFn,
    status: UserStatus,
}

// 4. Export a function that accepts your struct from JS
#[wasm_bindgen]
pub fn execute_callbacks(cbs: IAppInit) {
    // `.parse()` safely extracts the `js_sys::Function` objects into your strongly-typed wrappers
    let callbacks: AppCallbacks = cbs.parse();
    
    // Call the functions! Arguments are automatically converted to `JsValue`.
    // The `call` method returns a `Result<(), JsValue>` forwarding any JS exceptions.
    callbacks.on_ready.call("System is ready".to_string()).unwrap();
    
    let arr = js_sys::Uint8Array::new_with_length(3);
    arr.copy_from(&[1, 2, 3]);
    callbacks.on_data.call(42.5, arr).unwrap();
}

Generated TypeScript (index.d.ts)

wasm-bindgen outputs the types you would expect:

export enum UserStatus {
    Active = 0,
    Inactive = 1,
}

type SingleArgFn = (msg: string) => void;
type MultiArgFn = (a: number, b: Uint8Array) => void;

interface IAppCallbacks {
    onReady: SingleArgFn;
    onData: MultiArgFn;
    status: UserStatus;
}

export function execute_callbacks(cbs: IAppCallbacks): void;

Example: Returning a Value

#[ts]
pub type CalculateFn = fn(a: f64) -> f64;

// Usage:
let result: f64 = calculate_fn.call(10.0).unwrap();

Supported Types

ts-function supports a wide range of Rust types, automatically mapping them to their idiomatic TypeScript equivalents.

Rust Type	TypeScript Type	Notes
f32, f64, i8-i32, u8-u32	number
i64, u64	bigint	Mapped via js_sys::BigInt
bool	boolean
String, &str	string
Option<T>	`T	undefined`
Vec<u8>, &[u8]	Uint8Array	See Zero-Copy Performance Tip
Vec<f64>, &[f64]	Float64Array	Also supports i8, u16, i32, etc.
JsValue	any
js_sys::Object	object
Rust Struct	Typescript Object	Any JsCast type (including ts interfaces)
Enum	Enum	Limited to C-like enums, as ts-function uses wasm-binden for enums

Return Values & Performance

The #[ts] macro supports returning values from JavaScript back into Rust. It handles standard primitives, strings, options, and even numeric vectors (via TypedArray).

Rust Structs to Javascript Objects (Round-Trip)

Structs annotated with #[ts] automatically implement Into<JsValue>. This allows you to effortlessly construct Rust structs and pass them back to Javascript or return them from function calls natively:

#[ts]
pub struct BasicInfo {
    pub name: String,
    pub age: f64,
}

// In your wasm boundary:
let info = BasicInfo {
    name: "Alice".to_string(),
    age: 30.0,
};

// Converts 1-to-1 to a Javascript Object: `{ name: "Alice", age: 30.0 }`
let js_val: JsValue = info.into();

Zero-Copy Performance

When returning large arrays, using Vec<u8> or similar will force a memory copy from the JavaScript heap to the Rust heap. To achieve zero-copy performance, you can return a js_sys type directly:

#[ts]
pub type LargeDataFn = fn() -> js_sys::Uint8Array;

// Usage (Zero-Copy):
let arr: js_sys::Uint8Array = large_data_fn.call().unwrap();

Advanced Usage / The Escape Hatch

If you need completely custom serialization or want to embed specific side-effects and error handling directly into the function execution, you can use the escape hatch. By applying #[ts] to an impl block for a tuple-struct wrapping js_sys::Function, you can customize the call method's implementation.

ts-function will still parse your call method's signature and automatically generate the correct TypeScript interface for it!

use wasm_bindgen::prelude::*;
use ts_function::ts;

// 1. Define your own tuple struct
pub struct CustomLoggingFn(pub js_sys::Function);

// 2. Apply the macro to the impl block
#[ts]
impl CustomLoggingFn {
    // 3. Define the `call` signature exactly how you want it exposed to TypeScript
    pub fn call(&self, val: f64) {
        // You are responsible for all conversions and calling the JS function
        let result = self.0.call1(
            &wasm_bindgen::JsValue::NULL,
            &wasm_bindgen::JsValue::from_f64(val),
        );

        // Handle errors internally however you want
        if let Err(e) = result {
            if let Ok(err_obj) = e.dyn_into::<js_sys::Error>() {
                web_sys::console::error_1(&err_obj.message().into());
            }
        }
    }
}

License

Dual-licensed under either the MIT license or the Apache License, Version 2.0 at your option.