Websocket Client

A standalone WebSocket client: `new WebSocket(url)` performs its TCP connect + HTTP upgrade handshake *synchronously* (a documented V1 simplification — see WebSocketClientType in codegen/llvm/types.go), so `.onopen`/`.onmessage`/`.onclose`/`.onerror` only ever fire once the event loop gets a chance to run, which is why they're deferred to the very first loop iteration after construction — giving this script time to assign them before anything fires. One real consequence of "synchronous": a client can never connect to a server running in this *same* process/event loop (the blocking connect would need that very loop, which it's blocking, to ever accept and answer) — always a separate process, exactly like `websocket_server.ts` in this same directory.

Run websocket_server.ts in one terminal, then this file in another, to see a real round trip — comment out websocket_server.ts's own setTimeout first, since it self-closes after 300ms for `make examples`' own unattended run, too short a window to switch terminals: make run FILE=examples/websocket/websocket_server.ts & make run FILE=examples/websocket/websocket_client.ts

Run on its own (as `make examples` does, unattended, with no server actually listening), the connection is refused — a real, honestly reported outcome via onerror/onclose (never a thrown exception; real WebSocket never throws synchronously for a network-level failure either), not a hang or a crash.

const ws = new WebSocket('ws://127.0.0.1:8083/')
console.log('readyState right after construction: ' + ws.readyState)

ws.onopen = () => {
  console.log('connected, sending a message')
  ws.send('ping')
}
ws.onmessage = (ev) => {
  console.log('received: ' + ev.data)
  ws.close()
}
ws.onclose = () => {
  console.log('closed (readyState=' + ws.readyState + ')')
  process.exit(0)
}
ws.onerror = () => {
  console.log('connection failed — is websocket_server.ts running on :8083?')
}

setTimeout(() => {
  console.log('exiting')
  process.exit(0)
}, 2000)

View source on GitHubexamples/websocket/websocket_client.ts

← Weak Websocket Server →