fetch

Fetch API example

A self-contained, offline demo of host/fetch. main.go starts a tiny HTTP server on a random loopback port, builds a VM with a print provider and the Fetch API installed, hands the script the server's base URL, and runs main.js.

Run it from the repository root:

go run ./examples/fetch

Expected output:

GET /hello -> 200 OK
  body: Hello from the local server!
GET /time -> service=demo version=1 tags=gojs,fetch
  content-type: application/json
POST /echo -> 200
  method seen by server: POST
  api key seen by server: secret-123
  echoed payload: {"hello":"world","items":[1,2,3]}
Headers demo -> accept: text/plain, application/json
Error handling -> caught TypeError: Failed to fetch: dial tcp 127.0.0.1:1: connect: connection refused
Abort demo -> caught AbortError
Done.

main.js demonstrates:

1. GET + text — await fetch(...) then await res.text().
2. GET + JSON — await res.json() and reading a response header.
3. POST JSON with custom headers — a JSON body and an X-Api-Key header echoed back by the server.
4. Headers — case-insensitive, comma-combined values.
5. Error handling — a refused connection rejects with a TypeError.
6. Abort — an AbortController rejects the pending fetch with an AbortError.

Because the demo server runs on 127.0.0.1, no network access is required.

Walling off the network — NetProvider

Every dial made by fetch (and sse/websocket) — and therefore every DNS lookup — passes through the VM's NetProvider, the single egress wall. This example installs the pass-through default; wrap DialContext to enforce a policy:

// Allowlist: the script may only reach api.mygame.com.
type allowlist struct{ d net.Dialer }
func (a allowlist) DialContext(ctx context.Context, network, addr string) (net.Conn, error) {
    host, _, _ := net.SplitHostPort(addr)
    if host != "api.mygame.com" {
        return nil, fmt.Errorf("egress to %s denied", host)
    }
    return a.d.DialContext(ctx, network, addr)
}

vm := gojs.New(gojs.WithNetProvider(allowlist{}))

The same seam makes network tests hermetic: a NetProvider that dials every address to a local httptest.Server lets a script request https://api.example.com and actually reach your loopback server — no real DNS, no outbound sockets (see netprovider_test.go).