Go Wiki: WebAssembly

Introduction

Go 1.11 added an experimental port to WebAssembly. Go 1.12 has improved some parts of it, with further improvements expected in Go 1.13. Go 1.21 added a new port targeting the WASI syscall API.

WebAssembly is described on its home page as:

WebAssembly (abbreviated Wasm) is a binary instruction format for a stack-based virtual machine. Wasm is designed as a portable target for compilation of high-level languages like C/C++/Rust, enabling deployment on the web for client and server applications.

If you’re new to WebAssembly read the Getting Started section, watch some of the Go WebAssembly talks, then take a look at the Further examples below.

JavaScript (GOOS=js) port

Getting Started

This page assumes a functional Go 1.11 or newer installation. For troubleshooting, see the Install Troubleshooting page.

If you are on Windows, we suggest to follow this tutorial using a BASH emulation system such as Git Bash.

For Go 1.23 and earlier, the wasm support files needed in this article are located in misc/wasm, and the path should be replaced when performing operations with files such as lib/wasm/wasm_exec.js.

To compile a basic Go package for the web:

package main

import "fmt"

func main() {
    fmt.Println("Hello, WebAssembly!")
}

Set GOOS=js and GOARCH=wasm environment variables to compile for WebAssembly:

$ GOOS=js GOARCH=wasm go build -o main.wasm

That will build the package and produce an executable WebAssembly module file named main.wasm. The .wasm file extension will make it easier to serve it over HTTP with the correct Content-Type header later on.

Note that you can only compile main packages. Otherwise, you will get an object file that cannot be run in WebAssembly. If you have a package that you want to be able to use with WebAssembly, convert it to a main package and build a binary.

To execute main.wasm in a browser, we’ll also need a JavaScript support file, and a HTML page to connect everything together.

Copy the JavaScript support file:

cp "$(go env GOROOT)/lib/wasm/wasm_exec.js" .

Create an index.html file:

<html>
    <head>
        <meta charset="utf-8"/>
        <script src="wasm_exec.js"></script>
        <script>
            const go = new Go();
            WebAssembly.instantiateStreaming(fetch("main.wasm"), go.importObject).then((result) => {
                go.run(result.instance);
            });
        </script>
    </head>
    <body></body>
</html>

If your browser doesn’t yet support WebAssembly.instantiateStreaming, you can use a polyfill.

Then serve the three files (index.html, wasm_exec.js, and main.wasm) from a web server. For example, with goexec:

# install goexec: go install github.com/shurcooL/goexec@latest
goexec 'http.ListenAndServe(`:8080`, http.FileServer(http.Dir(`.`)))'

Or use your own basic HTTP server command.

Note: The same major Go version of the compiler and wasm_exec.js support file must be used together. That is, if main.wasm file is compiled using Go version 1.N, the corresponding wasm_exec.js file must also be copied from Go version 1.N. Other combinations are not supported.

Note: for the goexec command to work on Unix-like systems, you must add the path environment variable for Go to your shell’s profile. Go’s getting started guide explains this:

Add /usr/local/go/bin to the PATH environment variable. You can do this by adding this line to your /etc/profile (for a system-wide installation) or $HOME/.profile:

export PATH=$PATH:/usr/local/go/bin

Note: changes made to a profile file may not apply until the next time you log into your computer

Finally, navigate to http://localhost:8080/index.html, open the JavaScript debug console, and you should see the output. You can modify the program, rebuild main.wasm, and refresh to see new output.

Executing WebAssembly with Node.js

It’s possible to execute compiled WebAssembly modules using Node.js rather than a browser, which can be useful for testing and automation.

First, make sure Node is installed and in your PATH.

Then, add $(go env GOROOT)/lib/wasm to your PATH. This will allow go run and go test find go_js_wasm_exec in a PATH search and use it to just work for js/wasm:

$ export PATH="$PATH:$(go env GOROOT)/lib/wasm"
$ GOOS=js GOARCH=wasm go run .
Hello, WebAssembly!
$ GOOS=js GOARCH=wasm go test
PASS
ok      example.org/my/pkg  0.800s

If you’re running working on Go itself, this will also allow you to run run.bash seamlessly.

go_js_wasm_exec is a wrapper that allows running Go Wasm binaries in Node. By default, it may be found in the lib/wasm directory of your Go installation.

If you’d rather not add anything to your PATH, you may also set the -exec flag to the location of go_js_wasm_exec when you execute go run or go test manually.

$ GOOS=js GOARCH=wasm go run -exec="$(go env GOROOT)/lib/wasm/go_js_wasm_exec" .
Hello, WebAssembly!
$ GOOS=js GOARCH=wasm go test -exec="$(go env GOROOT)/lib/wasm/go_js_wasm_exec"
PASS
ok      example.org/my/pkg  0.800s

Finally, the wrapper may also be used to directly execute a Go Wasm binary:

$ GOOS=js GOARCH=wasm go build -o mybin .
$ $(go env GOROOT)/lib/wasm/go_js_wasm_exec ./mybin
Hello, WebAssembly!
$ GOOS=js GOARCH=wasm go test -c
$ $(go env GOROOT)/lib/wasm/go_js_wasm_exec ./pkg.test
PASS
ok      example.org/my/pkg  0.800s

Running tests in the browser

You can also use wasmbrowsertest to run tests inside your browser. It automates the job of spinning up a webserver and uses headless Chrome to run the tests inside it and relays the logs to your console.

Same as before, just go get github.com/agnivade/wasmbrowsertest to get a binary. Rename that to go_js_wasm_exec and place it to your PATH

$ mv $GOPATH/bin/wasmbrowsertest $GOPATH/bin/go_js_wasm_exec
$ export PATH="$PATH:$GOPATH/bin"
$ GOOS=js GOARCH=wasm go test
PASS
ok      example.org/my/pkg  0.800s

Alternatively, use the exec test flag.

GOOS=js GOARCH=wasm go test -exec="$GOPATH/bin/wasmbrowsertest"

Interacting with the DOM

See https://pkg.go.dev/syscall/js.

Also:

Canvas

Configuring fetch options while using net/http

You can use the net/http library to make HTTP requests from Go, and they will be converted to fetch calls. However, there isn’t a direct mapping between the fetch options and the http client options. To achieve this, we have some special header values that are recognized as fetch options. They are -

So as an example, if we want to set the mode as “cors” while making a request, it will be something like:

req, err := http.NewRequest("GET", "http://localhost:8080", nil)
req.Header.Add("js.fetch:mode", "cors")
if err != nil {
  fmt.Println(err)
  return
}
resp, err := http.DefaultClient.Do(req)
if err != nil {
  fmt.Println(err)
  return
}
defer resp.Body.Close()
// handle the response

Please feel free to subscribe to #26769 for more context and possibly newer information.

WebAssembly in Chrome

If you run a newer version of Chrome there is a flag (chrome://flags/#enable-webassembly-baseline) to enable Liftoff, their new compiler, which should significantly improve load times. Further info here.

Further examples

General

Canvas (2D)

Database

WebGL canvas (3D)

WASI (GOOS=wasip1) port

Getting Started (WASI)

Go 1.21 introduced WASI as a supported platform. To build for WASI, use the wasip1 port:

$ GOOS=wasip1 GOARCH=wasm go build -o main.wasm

The official blog has a helpful introduction to using the WASI port: https://go.dev/blog/wasi.

Go WebAssembly talks

Editor configuration

Debugging

WebAssembly doesn’t yet have any support for debuggers, so you’ll need to use the good ‘ol println() approach for now to display output on the JavaScript console.

An official WebAssembly Debugging Subgroup has been created to address this, with some initial investigation and proposals under way:

Please get involved and help drive this if you’re interested in the Debugger side of things. :smile:

Analysing the structure of a WebAssembly file

WebAssembly Code Explorer is useful for visualising the structure of a WebAssembly file.

Reducing the size of Wasm files

At present, Go generates large Wasm files, with the smallest possible size being around ~2MB. If your Go code imports libraries, this file size can increase dramatically. 10MB+ is common.

There are two main ways (for now) to reduce this file size:

  1. Manually compress the .wasm file.

    • Using gz compression reduces the ~2MB (minimum file size) example WASM file down to around 500kB. It may be better to use Zopfli to do the gzip compression, as it gives better results than gzip --best, however it does take much longer to run.
    • Using Brotli for compression, the file sizes are markedly better than both Zopfli and gzip --best, and compression time is somewhere in between the two, too. This (new) Brotli compressor looks reasonable.

    Examples from @johanbrandhorst

    Example 1

    Size Command Compression time
    16M (uncompressed size) N/A
    2.4M brotli -o test.wasm.br test.wasm 53.6s
    3.3M go-zopfli test.wasm 3m 2.6s
    3.4M gzip --best test.wasm 2.5s
    3.4M gzip test.wasm 0.8s

    Example 2

    Size Command Compression time
    2.3M (uncompressed size) N/A
    496K brotli -o main.wasm.br main.wasm 5.7s
    640K go-zopfli main.wasm 16.2s
    660K gzip --best main.wasm 0.2s
    668K gzip main.wasm 0.2s

    Use something like https://github.com/lpar/gzipped to automatically serve compressed files with correct headers, when available.

  2. Use TinyGo to generate the Wasm file instead.

    TinyGo supports a subset of the Go language targeted for embedded devices, and has a WebAssembly output target.

    While it does have limitations (not yet a full Go implementation), it is still fairly capable and the generated Wasm files are… tiny. ~10kB isn’t unusual. The “Hello world” example is 575 bytes. If you gz -6 that, it drops down to 408 bytes. :wink:

    This project is also very actively developed, so its capabilities are expanding out quickly. See https://tinygo.org/docs/guides/webassembly/ for more information on using WebAssembly with TinyGo.

Other WebAssembly resources

This content is part of the Go Wiki.

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