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WebAssembly

Justin Clift edited this page Oct 25, 2018 · 86 revisions

WebAssembly

Introduction

Go 1.11 added an experimental port to WebAssembly.

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 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.

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:

<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:

$ goexec 'http.ListenAndServe(":8080", http.FileServer(http.Dir(".")))'

Or use your own basic HTTP server command.

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"
PASS
ok  	example.org/my/pkg	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
PASS
ok  	example.org/my/pkg	0.800s

Interacting with the DOM

Alternatively, a library for streamlining DOM manipulation is in development.

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. 😄

Go WebAssembly talks

Further examples

General

  • Shimmer - Image transformation in wasm using Go

Canvas (2D)

WebGL canvas (3D)

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.

Known bug(s)

The Go 1.11 and 1.11.1 releases 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 fix has been added to both the 1.11.x release and master branches on GitHub, so re-compiling Go from either of those will solve the problem.

Other WebAssembly resources

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

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