# caldemo: a simple wasm demo showing a calendar
Suppose I want to create a small calendar widget in Go and run it in the browser with WASM. What do I need to know to achieve this? Thought I document the most important bits for my own future reference and then provide a minimal demo of that calendar widget.
# Execution model
The WASM binary only runs when the JS engine yields to it. When WASM returns, the whole Go process (including all background goroutines like the GC) becomes inactive. It won't run until JS yields to it again. I think this is good, wastes fewer resources. But keep in mind that the WASM environment is single threaded so GC hiccups can be more noticeable. If that is a problem (e.g. realtime games) then consider using a GC-less language like Odin (see @/odinlang).
# One shot mode vs reactor mode
In one shot mode Go code runs, does its magic (modify the DOM or JS variables as needed), and then exits.
The other mode is the reactor mode. Basically, the binary sets up some callbacks in the DOM and then just keeps running by having a `select {}` at the end of the main function. At that point the WASM yields back the execution to the JS engine. The JS can then call back to it and the Go runtime can "react" to it, hence the name. It works quite well too. This is what I would use for a more complex, interactive application. https://golangbot.com/webassembly-using-go/ has an example for this.
# Go vs TinyGo
Unfortunately, by default Go creates pretty large wasm binaries. TinyGo can create a much smaller binary (about ~10%, see below). I compile it like this:
tinygo build -target=wasm -no-debug -gc=leaking -scheduler=none -o=cal.wasm cal.go
This disables GC and goroutine support. That leads to the smallest binaries. A non-trivial application usually needs both GC and goroutines but it's nice to start with fewer features and only enable them if really needed. It also needs wasm_exec.js (search for wasm_exec.js in https://tinygo.org/docs/guides/webassembly/wasm/) which I versioned as @/tinygo24wasm.js in this demo.
# Result
So with those in mind, I created a one shot mode binary that displays the current calendar with some minimal DOM interaction. The result with GC and goroutines is about 260 KB. Without GC and without scheduler (so with the flags above) the result is 150 KB. The official Go toolchain creates a 2.4 MB binary for this.
I might rely on this more if I'm ever writing a more complex web app because I find Go easier to work with than TypeScript.
published on 2025-09-01
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