Go

Default: current Go release, modules, standard library first (flag, net/http, encoding/json, log/slog), go test, go vet, gofmt.

Setup

1. go mod init example.com/tool; keep go.mod's go line at the version you test with.
2. Layout: cmd/<tool>/main.go for binaries, internal/ for code no one else should import. Do not create pkg/ by habit.
3. A user-level install (unpack the official tarball into a home directory) is enough; add it to PATH only for the shell that needs it.

Command-line tools

• flag handles most tools; switch to a library only for nested subcommands.
• Let configuration come from a flag with an environment-variable default, e.g. -file defaulting to $APP_DATA; document both in -h with an example.
• Print an actionable message on empty input ("no records yet — add one with ...") rather than an empty table.
• Exit codes: os.Exit(1) for failures, 2 for usage errors.

HTTP services

• Start with net/http and the method-aware ServeMux patterns (GET /items/{id}); add a framework only when middleware needs grow.
• Always set server timeouts (ReadHeaderTimeout, ReadTimeout, WriteTimeout); the zero value means none.
• Pass context.Context from the request into every downstream call.

WebAssembly

• Standard Go WASM includes the runtime and is several MB. TinyGo produces binaries in the hundreds of KB for compute-only code; check that the packages you use are supported by TinyGo before choosing it.
• Expose functions with syscall/js (or //export in TinyGo) and keep the JS/WASM boundary coarse: pass a whole problem, not one number per call.

Testing

• Table-driven tests with t.Run(name, ...); include argument-count and empty-input cases.
• go test -race ./... for anything with goroutines.
• Use t.TempDir() and t.Setenv(); they clean up automatically.

Verification loop

1. gofmt -l . prints nothing; go vet ./... is clean.
2. go test ./... passes (with -race where relevant).
3. go build the binary and run it with -h and with a real input.

Pitfalls

• Loop variable capture changed in Go 1.22 (each iteration has its own variable); old code and old toolchains differ — the go line in go.mod decides.
• Nil interfaces: an interface holding a typed nil pointer is not nil.
• Maps are not safe for concurrent writes; guard with a mutex or use channels.
• defer in loops runs at function end, not iteration end; wrap the body in a function.
• JSON: unexported fields are ignored silently; use struct tags and DisallowUnknownFields for strict input.
• Time: time.Now() includes a monotonic reading; compare with Sub, format with layouts based on 2006-01-02.