#cargo #subcommand #web #asmjs #webasm

bin+lib cargo-web

A Cargo subcommand for the client-side Web

46 releases

0.6.25 May 8, 2019
0.6.23 Dec 15, 2018
0.6.21 Nov 22, 2018
0.6.15 Jul 28, 2018
0.1.1 Mar 8, 2017

#11 in Cargo plugins

Download history 412/week @ 2019-01-23 309/week @ 2019-01-30 282/week @ 2019-02-06 293/week @ 2019-02-13 294/week @ 2019-02-20 413/week @ 2019-02-27 305/week @ 2019-03-06 347/week @ 2019-03-13 625/week @ 2019-03-20 434/week @ 2019-03-27 394/week @ 2019-04-03 440/week @ 2019-04-10 466/week @ 2019-04-17 455/week @ 2019-04-24 397/week @ 2019-05-01

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Build Status

A cargo subcommand for the client-side Web

This cargo subcommand aims to make it easy and convenient to build, develop and deploy client-side Web applications written in Rust.


Become a patron


This software was brought to you thanks to these wonderful people:

  • Embark Studios
  • Joe Narvaez
  • Eduard Knyshov
  • Anselm Eickhoff
  • Johan Andersson
  • Stephen Sugden
  • is8ac

Thank you!


Currently it supports the following features:

  • cargo web build - will build your project using one of Rust's three Web backends:
    • WebAssembly using Rust's native WebAssembly backend (when you pass --target=wasm32-unknown-unknown; default)
    • WebAssembly using Emscripten (when you pass --target=wasm32-unknown-emscripten)
    • asm.js using Emscripten (when you pass --target=asmjs-unknown-emscripten)
  • cargo web check - will typecheck your project
  • cargo web test - will run your tests either under:
    • Under a headless instance of Google Chrome (default)
    • Under Node.js (when you pass --nodejs)
  • cargo web start - will build your project, start an embedded webserver and will continuously rebuild it if necessary; supports automatic reloading with --auto-reload.
  • cargo web deploy - will build your project and emit all of the necessary files so that you can easily serve them statically.
  • Will automatically download and install Emscripten for you (if necessary) on the following platforms:
    • Linux x86-64
    • Linux x86
  • Will automatically install the relevant Rust target through rustup

It's also highly recommended that you check out the stdweb crate if you want to interact with the JavaScript world in your project. (In fact, cargo-web is what makes it possible to use stdweb's js! macro on Rust's native WebAssembly backend.)


$ cargo install cargo-web

To upgrade:

$ cargo install --force cargo-web

Or clone and build with $ cargo build --release then place in your $PATH.

On Linux the installation can fail with a message that it can't find OpenSSL, in which case you most likely need to install the -dev package for OpenSSL from your distribution's repositories. (On Ubuntu it's called libssl-dev.)


cargo-web has its own configuration file which you can put next to cargo's Cargo.toml.

Here's an example configuration showing every supported key:

# The default value of `--target` used when building this crate
# in cases where it's not specified on the command line.
default-target = "wasm32-unknown-unknown"

# This will prepend a given JavaScript file to the resulting `.js` artifact.
# You can put any initialization code here which you'd like to have executed
# when your `.js` file first loads.
# This accepts either a string (as shown here), or an array of strings,
# in which case it will prepend all of the specified files in their
# order of appearance.
prepend-js = "src/runtime.js"

# Asserts the minimum required version of `cargo-web` necessary
# to compile this crate; supported since 0.6.0.
minimum-version = "0.6.0"

# These will only take effect on *-emscripten targets.
# You can have a target-specific `prepend-js` key.
prepend-js = "src/emscripten_runtime.js"
# This will enable Emscripten's SDL2 port. Consult Emscripten's documentation
# for more details.
link-args = ["-s", "USE_SDL=2"]

# You can also specify the target by its full name.
prepend-js = "src/native_runtime.js"

If you use any external crates which have a Web.toml then cargo-web will load it and use it.

A few restrictions concerning the Web.toml:

  • You can't have overlapping prepend-js keys. You can either define a single global prepend-js, or multiple per-target ones.
  • The link-args currently can't have any spaces in them.
  • The order in which cargo-web will process the Web.toml files from multiple crates is deterministic yet unspecified. This means that you shouldn't depend on this order in any way.

Static files

Any static files you'd like to have served when running cargo web start or deployed when running cargo web deploy can be put in a directory called static in the root of your crate. No static artifacts are required by default; an index.html file will be automatically generated for you if it's missing. You can, of course, put your own static/index.html file, in which case it will be used instead of the autogenerated one.

Detecting cargo-web during compilation

If during compilation you'd like to detect that your project is being built with cargo-web you can check the COMPILING_UNDER_CARGO_WEB environment variable, which will be set to 1.

Using cargo-web on Travis

Precompiled binaries

You can use the following script to download and install the latest cargo-web:

CARGO_WEB_RELEASE=$(curl -L -s -H 'Accept: application/json' https://github.com/koute/cargo-web/releases/latest)
CARGO_WEB_VERSION=$(echo $CARGO_WEB_RELEASE | sed -e 's/.*"tag_name":"\([^"]*\)".*/\1/')
if [ "$(uname -s)" == "Darwin" ]; then

echo "Downloading cargo-web from: $CARGO_WEB_URL"
curl -L $CARGO_WEB_URL | gzip -d > cargo-web
chmod +x cargo-web

mkdir -p ~/.cargo/bin
mv cargo-web ~/.cargo/bin

Running tests under headless Chrome

By default cargo web test will run your tests under headless Chrome. To be able to use this on Travis you need to add something like this to your .travis.yml:

  chrome: stable

Custom runtime (wasm32-unknown-unknown-only)

When building a project by default cargo-web generates a standalone runtime runtime for you. What this means is that the .js file which is generated can be immediately put inside of a <script> tag or launched with Node.js without having to load it manually or do anything extra, however this does limit you when it comes to customizability.

If you'd like to have a little more control on how your module is loaded then you can tell cargo-web to generate a non-standalone, library-like module for you with the --runtime library-es6 option. This will result in a .js file which exports a factory function with the following interface:

export default function() {
    return {
        imports: { ... },
        initialize: function( instance ) { ... }

Here you have to instantiate the WebAssembly module yourself; in this case you have to pass imports as its imports, and then immediately after instantiating it call initialize.

For example, assuming you'll name your module generated by the cargo-web as my-module.mjs and my-module.wasm you can instantiate it like this from Node.js:

import fs from "fs";
import factory from "my-module.mjs";

// We read in the `.wasm` module.
const bytecode = fs.readFileSync( "my-module.wasm" );
const wasm = new WebAssembly.Module( bytecode );

// We instantiate it.
const instance = factory();
const compiled = new WebAssembly.Instance( wasm, instance.imports );

// This will initialize the module and call your `main`, if you have one.
const exports = instance.initialize( compiled );

// In the object it returns you can find any functions which
// you've exported with `stdweb`'s `#[js_export]` macro.
console.log( exports.add( 1, 2 ) );

Then you can run it with node --experimental-modules run.mjs.

This is useful if you want to load your .wasm file from a custom URL or you want to integrate the output with a JavaScript bundler, or anything else which requires you to load the module yourself.


  • 0.6.25
    • cargo web start will now try to not trigger superfluous rebuilds when the project's files are modified in quick succession
    • The vendored OpenSSL copy was updated to the newest version
  • 0.6.24
    • Conditional dependencies of form [target.'cfg(...)'.dependencies] are now properly supported
    • You can now use cfg(cargo_web) to detect whenever your crate is being compiled under cargo-web
    • Artifacts matching target/wasm32-unknown-unknown/*/deps/*.wasm are now ignored; this should prevent cargo-web from processing superfluous .wasm artifacts generated due to dependencies also being cdylibs
    • cargo-web is now available as a library through a structopt-based interface
  • 0.6.23
    • New subcommand: cargo web check
    • The wasm32-unknown-unknown target is now the default
  • 0.6.22
    • Running tests through Chrome should now work out-of-box on macOS
    • The deploy subcommand can now be told where to deploy using the -o/--output parameter
    • Static files with spaces in their names are now properly served
    • Access-Control-Allow-Origin: * is now always sent by the embedded webserver
    • Debug builds on wasm32-unknown-unknown are now supported provided a recent enough stdweb is used
  • 0.6.21
    • Emscripten was updated to 1.38.19; the Emscripten-based targets should now work again on nightly
    • Broken output redirection in the test runner is now fixed
    • The generated JS snippets and imports under wasm32-unknown-unknown are now sorted
    • Compatibility with really old nightlies was removed for wasm32-unknown-unknown
  • 0.6.20
    • Installation through cargo install should now work again
    • Most of the dependencies were updated to newer versions
    • deploy should not panic when it doesn't find a valid target
  • 0.6.19
    • cargo install should now compile instead of failing in some environments
    • Minimum required Rust version is now 1.26.2
  • 0.6.18
    • Default index.html doesn't have a newline before its doctype anymore
  • 0.6.17
    • OpenSSL is now vendored; this should fix the compilation in some environments
    • Minimum required Rust version is now 1.25.0
  • 0.6.16
    • The runtime for wasm32-unknown-unknown now uses WebAssembly.instantiateStreaming when available
    • Running tests under headless Chromium is now supported for the wasm32-unknown-unknown target
    • Color codes are no longer emitted when the output of cargo-web is redirected
    • Improved coloring; a lot more messages should now be colored
    • Initial experimental support for asynchronous tests


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


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