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0.1.11 Sep 15, 2020
0.1.10 Apr 2, 2020

#277 in WebAssembly

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Exports serde-serializable structs and enums to Typescript definitions.


Good news everyone! Version 0.1.10 introduces a feature gated option to generate typescript type guards. Now you can:

    import {Record, isRecord} from "./server_defs";
    const a: any = JSON.parse(some_string_from_your_server)
    if (isRecord(a)) {
        // all the typescript type checking goodness plus a bit of safety
    } else {
        // something went wrong.

See Type Guards below.

Motivation 🦀

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 data types 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).


// #[cfg(target_arch="wasm32")]
use wasm_bindgen::prelude::*;

use serde::Serialize;
use typescript_definitions::TypescriptDefinition;

#[derive(Serialize, TypescriptDefinition)]
#[serde(tag = "tag", content = "fields")]
/// Important info about Enum
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:

// Important info about Enum
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 that these macros - by default - work only for the debug build since they pollute the code with strings and methods all of which are probably 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 might 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™ if you want to get started.

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"
serde = { version = "1.0", features = ["derive"] }

wasm-bindgen = "0.2"

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

$ WASM32=1 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

What just happened? This.

The WASM32=1 environment variable skirts around issue #1197.

Getting the toolchains

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).

$ curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh
$ WASM32=1 wasm-pack build --dev
$ 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::*;
use serde::Serialize;
use typescript_definitions::TypeScriptify;

#[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()`:

// main.rs

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

fn main() {
    if cfg!(any(debug_assertions, feature="export-typescript")) {

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

Use the cfg macro To protect any use of type_script_ify() if you need to.

If you have a generic struct such as:

use serde::Serialize;
use typescript_definitions::TypeScriptify;
#[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() 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 attributes.

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)

use serde::Serialize;
use typescript_definitions::{TypeScriptify, TypeScriptifyTrait};

#[derive(Serialize, TypeScriptify)]
struct S {
     image : Vec<u8>,
     buffer: &'static [u8],

println!("{}", S::type_script_ify());

prints export type S = { image: string, buffer: number[] };.

Serde attributes understood but rejected:

  • flatten (this will produce a panic). Probably will never be fixed.

All others are just ignored.

If you have specialized serialization then you will have to tell typescript-definitions what the result is ... see the next section.

typescript-definition attributes

There are 2 ways to intervene to correct the typescript output.

  • ts_as: a rust path to another rust type that this value serializes like:
  • ts_type: a typescript type that should be used.

e.g. some types, for example chrono::DateTime, will serializes themselves in an opaque manner. You need to tell typescript-definitions, viz:

use serde::Serialize;
use typescript_definitions::{TypeScriptify, TypeScriptifyTrait};
// with features=["serde"]
use chrono::{DateTime, Local, Utc};
// with features=["serde-1"]
use arrayvec::ArrayVec;

#[derive(Serialize, TypeScriptify)]
pub struct Chrono {
    pub local: DateTime<Local>,
    pub utc: DateTime<Utc>,
    pub ip4_addr1 : ArrayVec<[u8; 4]>,
    pub ip4_addr2 : ArrayVec<[u8; 4]>

Type Guards

See type guards.

typescript-definitions type guards provide a fail fast defensive check that a random json object agrees with the layout and types of a given typescript-definitions type.

To enable them change your dependency to:

typescript-definitions = { version="^0.1.10", features=["type-guards"] }

With the feature on you can turn guard generation off for any struct/enum with the #[ts(guard=false)] attribute.

If your struct has a long list of data as Vec<data> then you can prevent a type check of the entire array with a field attribute #[ts(array_check="first")] which will check only the first row.


use serde::Serialize;
use typescript_definitions::{TypeScriptify, TypeScriptifyTrait};
pub struct Maybe {
    maybe : Option<String>

println!("{}", Maybe::type_script_guard().unwrap());

will print (after passing through prettier):

export const isMaybe = (obj: any): obj is Maybe => {
  if (obj == undefined) return false;
  if (obj.maybe === undefined) return false;
    const val = obj.maybe;
    if (!(val === null)) {
      if (!(typeof val === "string")) return false;
  return true;


Limitations of JSON

e.g. Maps with non string keys: This

use wasm_bindgen::prelude::*;
use serde::Serialize;
use std::collections::HashMap;
use typescript_definitions::TypescriptDefinition;
#[derive(Serialize, TypescriptDefinition)]
pub struct IntMap {
    pub intmap: HashMap<i32, i32>,

will generate:

export type IntMap = { intmap: { [key: number]: number } };

But the typescript compiler will type check this:

let v : IntMap = { intmap: {  "6": 6, 4: 4 } };

So the generated guard also checks for integer keys with (+key !== NaN).

You can short circuit any field with some attribute markup

  • ts_type specify the serialization.
  • ts_guard: verify the type as if it was this typescript type.

Limitations of Generics

typescript-definitions has limited support for verifing generics.

Rust and typescript diverge a lot on what genericity means. Generic Rust structs don't map well to generic typescript types. However we don't give up totally.

This will work:

use wasm_bindgen::prelude::*;
use serde::Serialize;
use typescript_definitions::TypescriptDefinition;

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

#[derive(Serialize, TypescriptDefinition)]
pub struct DependsOnValue {
    pub value: Vec<Value<i32>>,

Since the monomorphization of Value in DependsOnValue is one of number, string or boolean.

Beyond this you will have to write your own guards e.g.:

use wasm_bindgen::prelude::*;
use serde::Serialize;
use typescript_definitions::TypescriptDefinition;

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

#[derive(Serialize, TypescriptDefinition)]
pub struct DependsOnValue {
    #[ts(ts_guard="{value: number[]}")]
    pub value: Value<Vec<i32>>,

OR you will have to rewrite the generated guard for generic type value: T yourself. viz:

const isT = <T>(o: any, typename: string): o is T => {
    // typename is the stringified type that we are
    // expecting e.g. `number` or `{a: number, b: string}[]` etc.
    if (typename !== "number[]") return false;
    if (!Array.isArray(o)) return false;
    for (let v of o) {
        if (typeof v !== "number") return false;
    return true

Watch out for function name collisions especially if you use simple names such as T, for a generic type name.

The generated output file should really be passed through something like prettier.


Top level doc (/// or //! ) comments are converted to javascript (line) comments:

use serde::Serialize;
use typescript_definitions::{TypeScriptify, TypeScriptifyTrait};
#[derive(Serialize, TypeScriptify)]
/// This is some API Event.
struct Event {
    what : String,
    pos : Vec<(i32,i32)>

assert_eq!(Event::type_script_ify(), "\
// This is some API Event.
export type Event = { what: string; pos: [ number , number ][] };"


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.

// #[cfg(target_arch="wasm32")]
use wasm_bindgen::prelude::*;
use serde::Serialize;
use typescript_definitions::{TypescriptDefinition};
struct B<T> {q: T}

#[derive(Serialize, TypescriptDefinition)]
struct A {
    x : f64,
    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.

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 band-aid regex patches to pretty things up. But use prettier.

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;
use typescript_definitions::{TypeScriptify,TypeScriptifyTrait};

struct S<'a> {
    pig: Pig<'a, str>,
println!("{}", S::type_script_ify());

gives export type S = { pig : Pig<string> } instead of export type S = { pig : string } Use #[ts(ts_as="Cow")] to fix this.

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 re-implement 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.


For initial 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.


~147K SLoC