$ show sockjs/sockjs-client
SockJS-client is a mature browser/Node JavaScript library that exposes a WebSocket-compatible API (SockJS constructor with onopen/onmessage/onclose, send/close) over a protocol that degrades gracefully across dozens of transports. In production it first tries native WebSockets, then falls back through xhr-streaming, xdr-streaming, eventsource, htmlfile, xhr-polling, xdr-polling, iframe-mediated variants, and jsonp-polling. The connection flow is: validate the URL against the HTML5 WebSocket constructor spec, fetch the server's /info endpoint to measure RTT and capability flags (cookie_needed, websocket support, base_url), compute a dynamic RFC 2988-style retransmission timeout, then walk an ordered transport list with per-transport round-trip timeouts until one opens. Incoming frames are type-prefixed ('o' open, 'h' heartbeat, 'a' batched messages, 'm' message, 'c' close); outgoing messages are JSON-array framed with aggressive Unicode escaping. Architecturally it is a modular CommonJS library bundled with browserify into a UMD global (dist/sockjs.js) plus a newer ESM build (dist/sockjs-esm.js) and handwritten TypeScript declarations. lib/main.js is the core state machine; each transport is split into senders (xhr-cors, xhr-local, xdr, jsonp) and receivers (incremental XHR streaming parser, eventsource, htmlfile, jsonp) composed by shared glue in lib/transport/lib (ajax-based, sender-receiver, buffered-sender, polling). A DOM2-style EventTarget/EventEmitter layer provides addEventListener and on* semantics. Browser versus Node differences are resolved via package.json 'browser' field remapping: faye-websocket <-> native WebSocket, eventsource package <-> global EventSource, Node crypto <-> window.crypto. Because the library must support IE6-9 era browsers, the source is deliberately ES5 with vendored es5-shim pieces. The engineering craftsmanship is genuine: the XHR receiver tracks a persistent buffer offset so streaming readyState=3 chunks are parsed incrementally without re-scanning; lib/utils/escape.js lazily unrolls a 65,536-character lookup table only when a message contains characters that browsers mangle on the wire; the JSONP receiver handles IE9's onerror/onreadystatechange race with a loadedOkay flag and the htmlFor/onclick script-execution trick; iframe transports enforce postMessage origin equality and window-ID correlation; unload/pagehide lifecycle hooks clean up sockets and iframes. The Node WebSocket transport propagates real backpressure (send() returning false plus a drain event) and the test suite proves it against an actual faye-websocket socket. Maintenance is low-velocity but real: the last commit is recent, CI runs Node 18/20/22, and the 1.6.x history is dominated by CVE-driven dependency bumps (url-parse, eventsource, minimist, faye-websocket) plus the 1.6.2 ESM/types addition. Drift is medium: the gulp+browserify build chain is dated, .nvmrc pins Node 12 while CI tests 18/20/22, lib/version.js lags package.json (1.6.1 vs 1.6.2), and several TODO comments remain. None of this affects the library's contract; it is still the de-facto SockJS client and remains safe to depend on.
The LLM identified 5 files worth highlighting for reuse:
Quotes strings with JSON.stringify, then replaces a curated set of Unicode characters (lone surrogates, combining marks, bidi controls, and ranges Chrome and other browsers silently mangle on the wire) with \uXXXX escapes. The lookup table is lazily unrolled from a 65,536-character scratch string only on first match, so the common fast path stays cheap. Zero internal dependencies - directly extractable as a safe-transmission quoting utility for any protocol carrying raw Unicode through a browser.
Parses newline-delimited frames from an incrementally growing XHR responseText. A persistent bufferPosition tracks how much of the accumulated text has already been scanned across repeated readyState=3 'chunk' callbacks, so partial frames survive chunk boundaries and each slice is examined only once. Depends only on inherits and EventEmitter. Reusable for any chunked stream protocol (ndjson over HTTP, SSE framing, log tailing) where the reader emits overlapping or incremental text slices.
A minimal DOM2 EventTarget implementation with addEventListener/removeEventListener/dispatchEvent plus on* handler invocation. Listener arrays are copied on every mutation (concat/slice), so add/remove calls made during dispatch never corrupt the in-progress iteration, and duplicate listener registrations are ignored. Zero dependencies and fully self-contained; it is the foundation of the library's EventEmitter and is directly extractable for any ES5-era API needing WebSocket-style events.
URL helpers for security-sensitive identity checks. getOrigin normalizes away default ports (http->80, https->443) and returns null for file: URLs; isOriginEqual backs all postMessage origin validation in the iframe transports. Also provides addPath/addQuery that preserve query strings, and isLoopbackAddr matching 127.0.0.0/8 and [::1] to exempt loopback from the HTTPS mixed-content error. Depends only on url-parse - reusable in any client gating cross-window messaging or mixed-content rules by origin.
Coalesces outbound messages into a single JSON-array payload and posts it through an injected sender function. A sendStop re-entrancy guard prevents overlapping requests while a POST is in flight, and sendScheduleWait inserts a 25ms delay after each successful send so a polling transport's receive connection can establish before the next batch - avoiding network saturation on slow links. Depends only on inherits and EventEmitter. The batching-plus-delay pattern transfers to any latency-sensitive polling channel where request fan-out must be minimized.