typescript-definitions 0.1.8

serde support for exporting Typescript definitions


Exports serde-serializable structs and enums to Typescript definitions.



Now that rust 2018 has landed there is no question that people should be using rust to write server applications (what are you thinking!). But generating wasm from rust code to run in the browser is currently much too bleeding edge.

Since javascript will be dominant on the client for the forseeable future there remains the problem of communicating with your javascript from your rust server.

Fundamental to this is to keep the datatypes on either side of the connection (http/websocket) in sync.

Typescript is an incremental typing system for javascript that is as almost(!) as tricked as rust... so why not create a typescript definition library based on your rust code?

Please see Credits.

typescript-definitions (as of 0.1.7) uses edition=2018 (heh).



#[derive(Serialize, TypescriptDefinition)]
#[serde(tag = "tag", content = "fields")]
enum Enum {
    V1 {
        #[serde(rename = "Foo")]
        foo: bool,
    V2 {
        #[serde(rename = "Bar")]
        bar: i64,
        #[serde(rename = "Baz")]
        baz: u64,
    V3 {
        #[serde(rename = "Quux")]
        quux: String,
    Internal {
        err: String

Using wasm-bindgen this will output in your *.d.ts definition file:

export type Enum = 
      {tag: "V1", fields: { Foo: boolean } }
    | {tag: "V2", fields: { Bar: number, Baz: number } }
    | {tag: "V3", fields: { Quux: string } }

Using typescript-definitions

NB: Please note these macros - by default - work only for the debug build since they pollute the code with strings and methods all of which are proabably not useful in any release (Since you are only using them to extract information about your current types from your code). In release builds they become no-ops. This means that there is no cost to your release exes/libs or to your users by using these macros. Zero cost abstraction indeed. Beautiful.

Also, although you need nightly to run wasm-bingen your code can remain stable.

See features below if you really want them in your release build.

There is a very small example in the repository that works for me (TM) if you want to get started on your own.

This crate only exports two derive macros: TypescriptDefinition and TypeScriptify, a simple trait TypeScriptifyTrait and a (very simple) serializer for byte arrays.

In your crate create a lib target in Cargo.toml pointing to your "interfaces"

name = "mywasm" # whatever... you decide
path = "src/interface.rs"
crate-type = ["cdylib"]

typescript-definitions = "0.1"
wasm-bindgen = "0.2"
serde = "1"
serde_derive = "1"

Then you can run (see here if you don't want to go near WASM):

cargo +nightly build --target wasm32-unknown-unknown
mkdir pkg
wasm-bindgen target/wasm32-unknown-unknown/debug/mywasm.wasm --typescript --out-dir pkg/
cat pkg/mywasm.d.ts # here are your definitions

If you don't have these tools then see here (You might also need to get rustup first):

rustup target add wasm32-unknown-unknown --toolchain nightly
cargo +nightly install wasm-bindgen-cli

or use wasm-pack (the typescript library will be in pkg/mywasm.d.ts). (NB as of 1.0.7 this no longer seems to work)

curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh
wasm-pack build
cat pkg/mywasm.d.ts

Using type_script_ify

You can ignore WASM totally by deriving using TypeScriptify:

// interface.rs
// wasm_bindgen not needed
// use::wasm_bindgen::prelude::*;

#[derive(Serialize, TypeScriptify)]
pub struct MyStruct {
    v : i32,

Then in main.rs (say) you can generate your own typescript specification using MyStruct::type_script_ify():

mod interface;
// need to pull in trait
use typescript_definitions::TypeScriptifyTrait;

fn main() {
    println!("{}", interface::MyStruct::type_script_ify());
    // prints "export type MyStruct = { v: number };"

Use the cfg macro To protect any use of type_script_ify()

if cfg!(any(debug_assertions, feature="export-typescript") {
    let s = A::type_script_ify();

If you have a generic struct such as:

#[derive(Serialize, TypeScriptify)]
pub struct Value<T> {
    value: T

then you need to choose a concrete type to generate the typescript: Value<i32>::type_script_ify(). The concrete type doesn't matter as long as it obeys rust restrictions; the output will still be generic export type Value<T> { value: T }.

Currently type bounds are discarded in the typescript.

So basically with TypeScriptify you have to create some binary that, via println! or similar statements, will cough up a typescript library file. I guess you have more control here... at the expense of complicating your Cargo.toml file and your code.


As we said before typescript-descriptions macros pollute your code with static strings and other garbage. Hence, by default, they only work in debug mode.

If you actually want T::type_script_ify() (for TypeScriptify) available in your release code then change your Cargo.toml file to:

version = "0.1"
features = ["export-typescript"]

## OR

typescript-definitions = { version="0.1",  features=["export-typescript"]  }

AFAIK the strings generated by TypescriptDescription don't survive the invocation of wasm-bindgen even in debug mode. So your *.wasm files are clean. You still need to add --features=export-typescript to generate anything in release mode though.

Serde Internally or Adjacently tagged Enums

See Serde Docs.

typescript-definitions tries to adhere to the meaning of serde attributes like#[serde(tag="type")] and #[serde(tag="tag", content="fields")].

Before 0.1.8 we had an implicit default tag of "kind" for enums. Now we don't (although we still have a implicit transparent on NewTypes).

Serde attributes understood

  • rename, rename_all:
  • tag:
  • content:
  • skip: (typescript-definitions also skips - by default - PhantomData fields ... sorry ghost who walks)
  • serialize_with="typescript_definitions::as_byte_string"
  • transparent: Newtypes are automatically transparent. Structs with a single field can be marked transparent.

serialize_with, if placed on a [u8] or Vec<u8> field, will take that field to be a string. (And serde_json will output a \xdd encoded string of the array. or you can create your own... just ensure to name it as_byte_string)

Serde attributes understood but rejected

  • flatten (This will produce a panic). Currently on my TODO list.

All others are just ignored.


Oh yes there are problems...

Currently typescript-descriptions will not fail (AFAIK) even for structs and enums with function pointers fn(a:A, b: B) -> C (generates typescript lambda (a:A, b:B) => C) and closures Fn(A,B) -> C (generates (A,B) => C). These make no sense in the current context (data types, json serialization) so this might be considered a bug. Watchout!

This might change if use cases show that an error would be better.

If you reference another type in a struct e.g.

    struct B<T> {q: T}
    #[derive(Serialize, TypescriptDefinition)]
    struct A {
        x : f64, /* simple */
        b: B<f64>,

then this will "work" (producing export type A = { x: number ,b: B<number> })) but B will be opaque to typescript unless B is also #[derive(TypescriptDefinition)].

Currently there is no check for this omission.

Also Trait bounds are stripped out for typescript; you can't serialize Traits! However...

If using type_script_ify then anything other than simple trait bounds will fail to compile. (because the impl<...> TypeScriptify for T<...> {} that is automatically generated by typescript-descriptions will be garbled).

  • no where clauses.
  • no generic Traits.

Use WASM instead.

This might be relaxed in future.

The following types are rendered as:

  • Option<T> => T | null (can't use undefined because this will mess with object checking)
  • HashMap<K,V> => { [key:K]:V } (same for BTreeMap)
  • HashSet<V> => V[] (same for BTreeSet)
  • &[u8] and Vec<u8> are expected to be byte buffers but are still rendered as number[] since this is what serde_json does. However you can force the output to be a string using #[serde(serialize_with="typescript_defintions::as_byte_string")]

An enum that is all Unit types such as

enum Color {

is rendered as a typescript enum:

enum Color {
    Red = "Red",
    Green ="Green",
    Blue = "Blue"

because serde_json will render Color::Red as the string "Red" instead of Color.Red (because JSON).

Serde always seems to render Result (in json) as {"Ok": T } | {"Err": E} i.e as "External" so we do too.

Formatting is rubbish and won't pass tslint. This is due to the quote! crate taking control of the output token stream. I don't know what it does with whitespace for example... (is whitespace a token in rust?). Anyhow... this crate applies a few bandaid regex patches to pretty things up.

We are not as clever as serde or the compiler in determining the actual type. For example this won't "work":

use std::borrow::Cow as Pig;

struct S<'a> {
    pig: Pig<'a, str>,

gives export type S = { pig : Pig<string> } instead of export type S = { pig : string }

At a certain point typescript-definitions just assumes that the token identifier i32 (say) is really the rust signed 32 bit integer and not some crazy renamed struct in your code!

Complex paths are ignored std::borrow::Cow and mycrate::mod::Cow are the same to us. We're not going to reimplement the compiler to find out if they are actually different. A Cow is always "Clone on write".

We can't reasonably obey serde attributes like "flatten" since we would need to find the actual Struct object (from somewhere) and query its fields.


Generate a typescript verifier for each type (maybe). We really need some testing of the typescript types against serde_json.

export verify_A<T>(obj: any): boolean {/*... */ }
// *or*
export verify_A<T>(obj: any): {Ok: A<T>} | {Err: string} {/* ... */}
// *or* using guards https://www.typescriptlang.org/docs/handbook/advanced-types.html
export is_A<T>(obj: any): obj is A<T> { /* ... */ }

or something...

Then one could:

let o : any = JSON.parse(some_string_from_the_inet);
if verify_A<number>(o) {
    return o as A<number>
} else {
    // err....



For intial inspiration see http://timryan.org/2019/01/22/exporting-serde-types-to-typescript.html

Forked from wasm-typescript-definition by @tcr which was forked from rust-serde-schema by @srijs.

type_script_ify idea from typescriptify by @n3phtys

Probably some others...


MIT or Apache-2.0, at your option.