This is the Go compiler ("gc") compiled for WASM, running in your browser! It can be used to run a simple playground, à la play.golang.org entirely in your browser, but generally means you can compile any pure Go in the browser to run in the browser.

You can try it out here: https://progrium.github.io/wasm-go-playground

This fork is updated with a newer version of Go (1.21.1) and is for experimentation and demonstration. It has some but not all of the standard library available. Also, since the creation of the original upstream, Safari now seems to work reasonably fast, though still slower than Chrome and Firefox. Build time is now output in the console.

Most of the top level source files are modified bits and pieces from play.golang.org for the web interface. The wasm_exec.js file is the standard wasm_exec.js that comes with Go, but with an extension in wasm_extra.js for a simple in-memory virtual filesystem implementation. The index.html file orchestrates the build with an exec function that works pretty similar to a typical exec, but taking a WASM binary executable blob fetched from the cmd directory.

The files under cmd are made from compile, link, and gofmt in $GOROOT/src/cmd by running GOOS=js GOARCH=wasm go build . in each directory. These are tools under the go tool command, which give you a lower level interface to the go build process. These are orchestrated almost like you would on the command line in index.html using exec, running compile and link with a generated importcfg file. All these WASM modules are run using the virtual filesystem. The playground user source is put into a main.go and then after compile and link are run, an a.out executable is written to the virtual filesystem and then run itself with exec.

Also written into the virtual filesystem are the pre-compiled packages under pkg. These archives are of the standard library packages, but are a bit trickier to produce with the new Go build caching system added since the upstream version. The included build_pkgs.sh script takes a standard library package name and runs go install, leaving the temporary work directory around to get all the package archive files made in the process, using their importcfg files to identify them. These files are currently hardcoded in index.html. Most notably, the first importcfg file created specifies what packages are needed for the playground user code. At the moment, runtime, fmt, os, and syscall/js are hardcoded, so those are the only packages that can be imported. You can add more to the importcfg but if the package archive isn't already under pkg you'll have to run build_pkgs.sh to produce them.