JavaScript to Rust and Back Again: The wasm-bindgen Tale

Recently we’ ve seen exactly how WebAssembly is incredibly fast to compile , speeding up JS libraries , and generating even smaller binaries . We’ ve even got a high-level plan for much better interoperability between the Rust and JavaScript communities, as well as other web programming dialects. As alluded to in that previous post , I’ d like to dive straight into more detail about a specific element, wasm-bindgen .

These days the WebAssembly specification only defines four types: 2 integer types and two floating-point types. Most of the time, however , JS plus Rust developers are working with a lot richer types! For example , JS designers often interact with document to add or modify HTML nodes, while Rust developers work with varieties like Result designed for error handling, and almost all developers work with strings.

Becoming limited only to the types that will WebAssembly provides today would be way too restrictive, and this is where wasm-bindgen comes into the particular picture. The goal of wasm-bindgen is to provide a link between the types of JS and Corrosion. It allows JS to contact a Rust API with a chain, or a Rust function to capture a JS exception. wasm-bindgen erases the particular impedance mismatch between WebAssembly plus JavaScript, ensuring that JavaScript can invoke WebAssembly functions efficiently and without boilerplate, and that WebAssembly can do the same along with JavaScript functions.

The particular wasm-bindgen project has some more description in the README . To get started here, let’ s jump into an example of using wasm-bindgen and then discover what else it has to offer.

Hello, World!

Always a classic, one of the best ways to a new tool is to explore the equivalent of printing “ Hello there, World! ” In this case we’ lmost all explore an example that will just that — alert “ Hi there, World! ” to the page.

The goal here is basic, we’ d like to define the Rust function that, given the name, will create a dialog in the page saying Hi, $ name! . In JavaScript we might define this function as:

  export functionality greet(name) 
    alert(`Hello, $ name !`);


The caveat for this example, even though, is that we’ d like to create it in Rust. Already there’ s a number of pieces happening right here that we’ ll have to use:

  • JavaScript will probably call into a WebAssembly module, specifically the greet export.
  • The particular Rust function will take a thread as an input, the name that we’ re greeting.
  • In house Rust will create a new chain with the name interpolated inside.
  • And finally Rust will invoke the JavaScript alert function with the string that it has established.

To get started, we all create a fresh new Rust project:

  $ cargo brand new wasm-greet --lib

This initializes a new wasm-greet folder, which usually we work inside of. Next upward we modify our Cargo. toml (the equivalent of package deal. json for Rust) with the following information:

crate-type = ["cdylib"]

wasm-bindgen = "0. 2"

We ignore the [lib] business for now, but the next component declares a dependency on the wasm-bindgen crate . The cage here contains all the support we have to use wasm-bindgen in Rust.

Next up, it’ s time for you to write some code! We substitute the auto-created src/lib. rs with these items:

 #![feature(proc_macro, wasm_custom_section, wasm_import_module)]

extern crate wasm_bindgen;

make use of wasm_bindgen:: prelude:: *;

    fn alert(s: &str);

bar fn greet(name: & str) 
    alert(&format!("Hello, !", name));


If you’ re unfamiliar with Rust this may appear a bit wordy, but fear not really! The wasm-bindgen project is continually enhancing over time, and it’ s sure that all this won’ t always be required. The most important piece to notice is the #[wasm_bindgen] attribute , an annotation in Rust program code which here means “ please process this having a wrapper as necessary” . Each our import of the alert function plus export of the welcome function are annotated with this attribute. In a moment, we’ ll see what’ s taking place under the hood.

However, we cut to the chase and open this particular up in a browser! Let’ t compile our wasm code:

  $ rustup focus on add wasm32-unknown-unknown --toolchain nightly # only needed once
$ consignments +nightly build --target wasm32-unknown-unknown

This gives us the wasm file at target/wasm32-unknown-unknown/debug/wasm_greet. wasm . If we appearance inside this wasm file making use of tools like wasm2wat it might be a bit scary. It turns out this particular wasm file isn’ t in fact ready to be consumed just yet simply by JS! Instead we need one more stage to make it usable:

  $ cargo install wasm-bindgen-cli # only needed once
$ wasm-bindgen target/wasm32-unknown-unknown/debug/wasm_greet. wasm --out-dir.

This step is where a lot of the miracle happens: the wasm-bindgen CLI tool postprocesses the input wasm file, making it appropriate to use. We’ ll see a little bit later what “ suitable” indicates, for now it suffices to say that when we import the freshly made wasm_greet. js file (created by the wasm-bindgen tool) we’ ve got the greet functionality that we defined in Rust.

Finally, all we’ ve got to do is package up with a bundler, and create an CODE page to run our code. During the time of this writing, only Webpack’ s 4. zero release has sufficient WebAssembly support to work out-of-the container (although it also has a Chrome caveat for the time being). In time, a lot more bundlers will surely follow. I’ lmost all skip the details here, but you can the actual example configuration in the Github repo. If we look inside though, our own JS for the page looks like:

 const corrosion = import(". /wasm_greet");
rust. then(m => m. greet("World! "));

… and that’ s it! Opening up our web page should now show a nice “ Hello, World! ” dialog, powered by Rust.

Exactly how wasm-bindgen works

Phew, which was a bit of a hefty “ Hello, Planet! ” Let’ s dive in to a bit more detail as to what’ s i9000 happening under the hood and how the particular tooling works.

Probably the most important aspects of wasm-bindgen is that its incorporation is fundamentally built on the idea that a wasm module is just a different type of ES module. For example , above all of us wanted an ES module with an unique (in Typescript) that looks like:

 export perform greet(s: string);

WebAssembly has no way to natively try this (remember that it only supports figures today), so we’ re depending on wasm-bindgen to fill in the gaps. Within our final step above, when we happened to run the wasm-bindgen tool you’ ll observe that a wasm_greet. js file was released alongside a wasm_greet_bg. wasm file. The previous is the actual JS interface we’ d like, performing any essential glue to call Rust. The particular *_bg. wasm file contains the actual execution and all our compiled code.

When we import the . /wasm_greet component we get what the Rust program code wanted to expose but couldn’ t very do so natively today. Now that we’ ve seen how the integration functions, let’ s follow the execution of our own script to see what happens. First upward, our example ran:

 const rust sama dengan import(". /wasm_greet");
rust. then(m => m. greet("World! "));

Here we’ re asynchronously importing the interface we needed, waiting for it to be resolved (by downloading and compiling the wasm). Then we call the greet perform on the module.

Note : The asynchronous loading here is required today along with Webpack , but this will likely not at all times be the case and may not be the requirement for other bundlers.

If we take a look inside the wasm_greet. js file that the wasm-bindgen tool generated we’ ll see something that looks like:

 import 2. as wasm from '. /wasm_greet_bg';


export function greet(arg0) 
    const  [ptr0, len0]  = passStringToWasm(arg0);
        const ret = wasm.greet(ptr0, len0);
        return ret;
        wasm.__wbindgen_free(ptr0, len0);

export function __wbg_f_alert_alert_n(ptr0, len0) 
    // ...


Note : Keep in mind this really is unoptimized and generated code, it might not be pretty or small! Whenever compiled with LTO (Link Period Optimization) and a release build within Rust, and after going through JS bundler pipelines (minification) this should be smaller.

Here you observe how the greet function is being generated for all of us by wasm-bindgen . Under the hood it’ s nevertheless calling the wasm’ s greet perform, but it is called with a pointer as well as a length rather than a string. For more fine detail about passStringToWasm you can see Lin Clark’ s previous publish . This is all boilerplate you’ d otherwise need to write except the particular wasm-bindgen tool is taking care of it for all of us! We’ ll get to the __wbg_f_alert_alert_n functionality in a moment.

Relocating a level deeper, the next item appealing is the greet function in WebAssembly. To check out that, let’ s see exactly what code the Rust compiler views. Note that like the JS wrapper produced above, you’ re not composing the greet exported symbol here, but instead the #[wasm_bindgen] attribute can be generating a shim, which really does the translation for you, namely:

 pub fn greet(name: & str) 
    alert(&format!("Hello, !", name));

#[export_name = "greet"]
pub extern fn __wasm_bindgen_generated_greet(arg0_ptr: *mut u8, arg0_len: usize) 
    let arg0 = unsafe  ::std::slice::from_raw_parts(arg0_ptr as *const u8, arg0_len) 
    let arg0 = unsafe  ::std::str::from_utf8_unchecked(arg0) ;


Here you observe our original code, greet , but the #[wasm_bingen] attribute inserted this funny searching function __wasm_bindgen_generated_greet . This is an exported function (specified along with #[export_name] and the extern keyword) plus takes the pointer/length pair the particular JS passed in. Internally after that it converts the pointer/length to a & str (a Thread in Rust) and forwards towards the greet function we defined.

Put a different way, the #[wasm_bindgen] attribute is generating two packages: One in JavaScript which requires JS types to convert in order to wasm, and one in Rust which usually receives wasm types and changes to Rust types.

Ok let’ s look at a single last set of wrappers, the alert perform. The greet function in Rust utilizes the standard format! macro to create a new string and then goes by it to notify . Recall though that when all of us declared the notify function we announced it with #[wasm_bindgen] , so let’ s see what rustc views for this function:

 fn alert(s: & str) 
    # [wasm_import_module = "__wbindgen_placeholder__"] 
        fn __wbg_f_alert_alert_n(s_ptr: *const u8, s_len: usize);
        let s_ptr = s.as_ptr();
        let s_len = s.len();
        __wbg_f_alert_alert_n(s_ptr, s_len);


Now that’ s not quite what we wrote, yet we can sort of see how this is arriving together. The notify function is actually a slim wrapper which takes the Corrosion & str after which converts it to wasm sorts (numbers). This calls our __wbg_f_alert_alert_n funny-looking function we saw above, yet a curious aspect of this is the #[wasm_import_module] attribute.

All of imports of function in WebAssembly have a module that they come from, plus since wasm-bindgen is built on ES quests, this too will be interpreted being an ES module import! Now the particular __wbindgen_placeholder__ module doesn’ t actually can be found, but it indicates that this import is going to be rewritten by the wasm-bindgen tool to transfer from the JS file we created as well.

And finally, the past piece of the puzzle, we’ ve got our generated JS document which contains:

 export function __wbg_f_alert_alert_n(ptr0, len0) 
    let arg0 = getStringFromWasm(ptr0, len0);


Whoa! It turns out that quite a bit is happening underneath the hood here and we’ ve got a relatively long chain through greet in JS to alert in the internet browser. Fear not though, the key facet of wasm-bindgen is that all this infrastructure is concealed! You only need to write Rust program code with a few #[wasm_bindgen] every now and then. Then your JS can use Rust just like if it were another JS package deal or module.

Exactly what else can wasm-bindgen do?

The wasm-bindgen project is ambitious within scope and we don’ t very have the time to go into all the details right here. A great way to explore the functionality in wasm-bindgen would be to explore the examples directory including Hello, Entire world! like we noticed above to manipulating DOM nodes entirely in Rust.

At a high-level the features of wasm-bindgen are usually:

  • Importing JS structs, functions, items, etc ., to call in wasm. You are able to call JS methods on a struct and access properties, giving a “ native” feel to the Corrosion you write once the #[wasm_bindgen] annotations are all hooked up.
  • Exporting Corrosion structures and functions to JS. Instead of having JS only work together with numbers you can export a Corrosion struct which turns into a class in JS. Then you can pass structs around rather than only having to pass integers close to. The smorgasboard example gives a great taste of the interop supported.

  • Enabling other miscellaneous features such as adding from the global scope (aka the particular alert function), catching JS exceptions utilizing a Result in Rust, and a generic method to simulate storing JS values in the Rust program.

If you’ re inquisitive to see more functionality, stay tuned specifically with the problem tracker !

What’ s next for wasm-bindgen ?

Prior to we close out I’ m like to take a moment and describe the near future vision for wasm-bindgen because I think it’ s one of the most exciting aspects of the particular project today.

Assisting more than just Rust

Through day 1, the wasm-bindgen CLI device was designed with multiple language supoprt in mind. While Rust is the just supported language today, the device is designed to plug in C or C++ as well. The #[wasm_bindgen] feature creates a custom section of the output *. wasm file which the wasm-bindgen tool parses plus later removes. This section describes exactly what JS bindings to generate and what their particular interface is. There’ s absolutely nothing Rust-specific about this description, so the C++ compiler plugin could just like easily create the section and obtain processed by the wasm-bindgen tool.

I find this aspect specifically exciting because I believe it allows tooling like wasm-bindgen to become a standard exercise for integration of WebAssembly plus JS. Hopefully, it is beneficial to all dialects compiling to WebAssembly and identified automatically by bundlers, to avoid the majority of the configuration and build tooling required above.

Automatically holding the JS ecosystem

One of the downsides today when adding functionality with the #[wasm_bindgen] macro is that you have to write everything away and make sure you haven’ t produced any mistakes. This can sometimes become a tedious (and error-prone) process which is ripe for automation.

All web APIs are specific with WebIDL and it should be very feasible to create #[wasm_bindgen] annotations from WebIDL . This would mean you wouldn’ t need to define the alert perform like we did above, rather you’ d just need to write something similar to:

pub fn greet(s: & str) 
    webapi::alert(&format!("Hello, !", s));


In this case, the webapi crate could immediately be generated entirely from the WebIDL descriptions of web APIs, ensuring no errors.

We are able to even take this a step further plus leverage the awesome work from the TypeScript community and generate #[wasm_bindgen] through TypeScript as well . This would permit automatic binding of any bundle with TypeScript available on npm at no cost!

Faster-than-JS DOM overall performance

And last, although not least, on the wasm-bindgen horizon: ultra-fast DEM manipulation — the holy grail of numerous JS frameworks. Today calls straight into DOM functions have to go through pricey shims as they transition from JavaScript to C++ engine implementations. Along with WebAssembly, however , these shims will never be necessary. WebAssembly is known to be properly typed… and, well, has forms!

The wasm-bindgen code era has been designed with the future host bindings proposal in mind from day 1 ) As soon as that’ s a feature obtainable in WebAssembly, we’ ll be able to straight invoke imported functions without any associated with wasm-bindgen ‘ s JS shims. Furthermore, this can allow JS engines to strongly optimize WebAssembly manipulating the DEM as invocations are well-typed with no longer need the argument affirmation checks that calls from JS need. At that point wasm-bindgen will not only make it simple to work with richer types like guitar strings, but it will also provide best-in-class DEM manipulation performance.


I’ ve individually found that WebAssembly is extremely exciting to work with not only because of the neighborhood but also the leaps and range of progress being made therefore quickly. The wasm-bindgen tool has a shiny future ahead. It’ s producing interoperability between JS and different languages like Rust a first-class encounter while also providing long-term advantages as WebAssembly continues to evolve.

Try giving wasm-bindgen a spin, open an issue for feature requests, and or else stay included with Rust plus WebAssembly!

Alex is a member of the particular Rust core team and has already been working on Rust since late this year. Nowadays he’s helping the assisting the WebAssembly Rust Working Team make Rust+Wasm the best possible experience. Alex also helps to maintain Cargo (Rust’s deal manager), the Rust standard collection, and Rust’s infrastructure for produces and CI.

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