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Carl Riis edited this page Oct 15, 2021 · 69 revisions
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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.

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.

Getting Started

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

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)/misc/wasm/wasm_exec.js" .

Create an index.html file:

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

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 get -u
$ goexec 'http.ListenAndServe(`:8080`, http.FileServer(http.Dir(`.`)))'

Or use your own basic HTTP server command.

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.

With Node installed and in your PATH, set the -exec flag to the location of go_js_wasm_exec when you execute go run or go test.

By default, go_js_wasm_exec is in the misc/wasm directory of your Go installation.

$ GOOS=js GOARCH=wasm go run -exec="$(go env GOROOT)/misc/wasm/go_js_wasm_exec" .
Hello, WebAssembly!
$ GOOS=js GOARCH=wasm go test -exec="$(go env GOROOT)/misc/wasm/go_js_wasm_exec"
ok	0.800s

Adding go_js_wasm_exec to your PATH will allow go run and go test to work for js/wasm without having to manually provide the -exec flag each time:

$ export PATH="$PATH:$(go env GOROOT)/misc/wasm"
$ GOOS=js GOARCH=wasm go run .
Hello, WebAssembly!
$ GOOS=js GOARCH=wasm go test
ok	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 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
ok	0.800s

Alternatively, use the exec test flag.

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

Go WebAssembly talks

Interacting with the DOM


  • app: A PWA-compatible, React-based framework with custom tooling.

  • dom: A library for streamlining DOM manipulation is in development.

  • dom: Go bindings for the JavaScript DOM APIs.

  • domui: A pure Go framework for creating complete GUI application.

  • gas: Components based framework for WebAssembly applications.

  • GoWebian: A library to build pages with pure Go and add WebAssembly bindings.

  • VECTY: Build responsive and dynamic web frontends in Go using WebAssembly, competing with modern web frameworks like React & VueJS.

  • vert: WebAssembly interop between Go and JS values.

  • vue: The progressive framework for WebAssembly applications.

  • Vugu: A wasm web UI library featuring HTML layout with Go for app logic, single-file components, rapid dev and prototyping workflow.

  • webapi: A binding generator and generated bindings for DOM, HTML, WebGL, and more.

  • webgen: Define components in HTML and generate Go types and constructor functions for them using webapi.


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 -

  • js.fetch:mode: An option to the Fetch API mode setting. Valid values are: "cors", "no-cors", "same-origin", navigate". The default is "same-origin".

  • js.fetch:credentials: An option to the Fetch API credentials setting. Valid values are: "omit", "same-origin", "include". The default is "same-origin".

  • js.fetch:redirect: An option to the Fetch API redirect setting. Valid values are: "follow", "error", "manual". The default is "follow".

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 {
resp, err := http.DefaultClient.Do(req)
if err != nil {
defer resp.Body.Close()
// handle the response

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

Editor configuration

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.


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.

  • Clicking on a hex value to the left will highlight the section it is part of, and the corresponding text representation on the right

  • Clicking a line on the right will highlight the hex byte representations for it on the left

Known bug(s)

Go releases prior to 1.11.2 have a bug which can generate incorrect wasm code in some (rare) circumstances.

If your Go code compiles to wasm without problem, but produces an error like this when run in the browser:

CompileError: wasm validation error: at offset 1269295: type mismatch: expression has type i64 but expected f64

Then you’re probably hitting this error.

The solution is to upgrade to Go 1.11.2 or later.

Further examples


Canvas (2D)


WebGL canvas (3D)

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.

    1. 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.

    2. 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


(uncompressed size)



brotli -o test.wasm



go-zopfli test.wasm

3m 2.6s


gzip --best test.wasm



gzip test.wasm


Example 2

Size Command Compression time


(uncompressed size)



brotli -o main.wasm



go-zopfli main.wasm



gzip --best main.wasm



gzip main.wasm


Use something like 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 for more information on using WebAssembly with TinyGo.

Other WebAssembly resources

  • Awesome-Wasm - An extensive list of further Wasm resources. Not Go specific.