GopherJS - A compiler from Go to JavaScript

GopherJS compiles Go code (golang.org) to pure JavaScript code. Its main purpose is to give you the opportunity to write front-end code in Go which will still run in all browsers. Give GopherJS a try on the GopherJS Playground.

What is supported?

Nearly everything, including Goroutines (compatibility table). Performance is quite good in most cases, see HTML5 game engine benchmark.

Installation and Usage

Get or update GopherJS and dependencies with:

go get -u github.com/gopherjs/gopherjs

Now you can use gopherjs build [package], gopherjs build [files] or gopherjs install [package] which behave similar to the go tool. For main packages, these commands create a .js file and .js.map source map in the current directory or in $GOPATH/bin. The generated JavaScript file can be used as usual in a website. Use gopherjs help [command] to get a list of possible command line flags, e.g. for minification and automatically watching for changes.

If you want to use gopherjs run or gopherjs test to run the generated code locally, install Node.js 4.x and the module source-map-support:

npm install --global source-map-support

For system calls (file system access, etc.), see this page.

Note: GopherJS will try to write compiled object files of the core packages to your $GOROOT/pkg directory. If that fails, it will fall back to $GOPATH/pkg.

Performance Tips

• Use the -m command line flag to generate minified code.
• Apply gzip compression (https://en.wikipedia.org/wiki/HTTP_compression).
• Use int instead of (u)int8/16/32/64.
• Use float64 instead of float32.

Community

• #gopherjs Channel on Gophers Slack (invites to Gophers Slack are available here)
• Google Group
• Bindings to JavaScript APIs and libraries
• GopherJS on Twitter

Getting started

Interacting with the DOM

The package github.com/gopherjs/gopherjs/js (see documentation) provides functions for interacting with native JavaScript APIs. For example the line

document.write("Hello world!");

would look like this in Go:

js.Global.Get("document").Call("write", "Hello world!")

You may also want use the DOM bindings, the jQuery bindings (see TodoMVC Example) or the AngularJS bindings. Those are some of the bindings to JavaScript APIs and libraries by community members.

Providing library functions for use in other JavaScript code

Set a global variable to a map that contains the functions:

package main

import "github.com/gopherjs/gopherjs/js"

func main() {
	js.Global.Set("pet", map[string]interface{}{
		"New": New,
	})
}

type Pet struct {
	name string
}

func New(name string) *js.Object {
	return js.MakeWrapper(&Pet{name})
}

func (p *Pet) Name() string {
	return p.name
}

func (p *Pet) SetName(name string) {
	p.name = name
}

For more details see Jason Stone's blog post about GopherJS.

Architecture

General

GopherJS emulates a 32-bit environment. This means that int, uint and uintptr have a precision of 32 bits. However, the explicit 64-bit integer types int64 and uint64 are supported. The GOARCH value of GopherJS is "js". You may use it as a build constraint: // +build js.

Application Lifecycle

The main function is executed as usual after all init functions have run. JavaScript callbacks can also invoke Go functions, even after the main function has exited. Therefore the end of the main function should not be regarded as the end of the application and does not end the execution of other goroutines.

In the browser, calling os.Exit (e.g. indirectly by log.Fatal) also does not terminate the execution of the program. For convenience, it calls runtime.Goexit to immediately terminate the calling goroutine.

Goroutines

Goroutines are fully supported by GopherJS. The only restriction is that you need to start a new goroutine if you want to use blocking code called from external JavaScript:

js.Global.Get("myButton").Call("addEventListener", "click", func() {
  go func() {
    [...]
    someBlockingFunction()
    [...]
  }()
})

How it works:

JavaScript has no concept of concurrency (except web workers, but those are too strictly separated to be used for goroutines). Because of that, instructions in JavaScript are never blocking. A blocking call would effectively freeze the responsiveness of your web page, so calls with callback arguments are used instead.

GopherJS does some heavy lifting to work around this restriction: Whenever an instruction is blocking (e.g. communicating with a channel that isn't ready), the whole stack will unwind (= all functions return) and the goroutine will be put to sleep. Then another goroutine which is ready to resume gets picked and its stack with all local variables will be restored. This is done by preserving each stack frame inside a closure.