wasm-pack build
The wasm-pack build command creates the files necessary for JavaScript
interoperability and for publishing a package to npm. This involves compiling
your code to wasm and generating a pkg folder. This pkg folder will contain the
wasm binary, a JS wrapper file, your README, and a package.json file.
The pkg directory is automatically .gitignored by default, since it contains
build artifacts which are not intended to be checked into version
control.0
Path
The wasm-pack build command can be given an optional path argument, e.g.:
wasm-pack build examples/js-hello-world
This path should point to a directory that contains a Cargo.toml file. If no
path is given, the build command will run in the current directory.
Output Directory
By default, wasm-pack will generate a directory for its build output called pkg.
If you'd like to customize this you can use the --out-dir flag.
wasm-pack build --out-dir out
The above command will put your build artifacts in a directory called out, instead
of the default pkg.
Generated file names
Flag --out-name sets the prefix for output file names. If not provided, package name is used instead.
Usage examples, assuming our crate is named dom:
wasm-pack build
# will produce files
# dom.d.ts dom.js dom_bg.d.ts dom_bg.wasm package.json README.md
wasm-pack build --out-name index
# will produce files
# index.d.ts index.js index_bg.d.ts index_bg.wasm package.json README.md
Profile
The build command accepts an optional profile argument: one of --dev,
--profiling, or --release. If none is supplied, then --release is used.
This controls whether debug assertions are enabled, debug info is generated, and which (if any) optimizations are enabled.
| Profile | Debug Assertions | Debug Info | Optimizations | Notes |
|---|---|---|---|---|
--dev | Yes | Yes | No | Useful for development and debugging. |
--profiling | No | Yes | Yes | Useful when profiling and investigating performance issues. |
--release | No | No | Yes | Useful for shipping to production. |
The --dev profile will build the output package using cargo's default
non-release profile. Building this way is
faster but applies few optimizations to the output, and enables debug assertions
and other runtime correctness checks. The --profiling and --release profiles
use cargo's release profile, but the former enables debug info as well, which
helps when investigating performance issues in a profiler.
The exact meaning of the profile flags may evolve as the platform matures.
Target
The build command accepts a --target argument. This will customize the JS
that is emitted and how the WebAssembly files are instantiated and loaded. For
more documentation on the various strategies here, see the documentation on
using the compiled output.
wasm-pack build --target nodejs
| Option | Usage | Description |
|---|---|---|
not specified or bundler | Bundler | Outputs JS that is suitable for interoperation with a Bundler like Webpack. You'll import the JS and the module key is specified in package.json. sideEffects: false is by default. |
nodejs | Node.js | Outputs JS that uses CommonJS modules, for use with a require statement. main key in package.json. |
web | Native in browser | Outputs JS that can be natively imported as an ES module in a browser, but the WebAssembly must be manually instantiated and loaded. |
no-modules | Native in browser | Same as web, except the JS is included on a page and modifies global state, and doesn't support as many wasm-bindgen features as web |
deno | Deno | Outputs JS that can be natively imported as an ES module in deno. |
Scope
The build command also accepts an optional --scope argument. This will scope
your package name, which is useful if your package name might conflict with
something in the public registry. For example:
wasm-pack build examples/js-hello-world --scope test
This command would create a package.json file for a package called
@test/js-hello-world. For more information about scoping, you can refer to
the npm documentation here.
Mode
The build command accepts an optional --mode argument.
wasm-pack build examples/js-hello-world --mode no-install
| Option | Description |
|---|---|
no-install | wasm-pack build implicitly and create wasm binding without installing wasm-bindgen. |
normal | do all the stuffs of no-install with installed wasm-bindgen. |
Extra options
The build command can pass extra options straight to cargo build even if
they are not supported in wasm-pack. To use them simply add the extra arguments
at the very end of your command, just as you would for cargo build. For
example, to build the previous example using cargo's offline feature:
wasm-pack build examples/js-hello-world --mode no-install -- --offline
0 If you need to include additional assets in the pkg directory and your NPM package, we intend to have a solution for your use case soon. ↩