fix(websocket): fix timer blocking writes (#670)

An immediate setTimeout was used to unblock the WebSocket write.
Unfortunately, this setTimeout can be throttled by browsers when the
tab is backgrounded.

This can lead to missed pong responses to server pings, which
eventually leads to disconnection.

Related: #649

Author: hyperlink

lib/transports/websocket-constructor.browser.js

@@ -1,7 +1,17 @@
 const globalThis = require("../globalThis");
+const nextTick = (() => {
+  const isPromiseAvailable =
+    typeof Promise === "function" && typeof Promise.resolve === "function";
+  if (isPromiseAvailable) {
+    return cb => Promise.resolve().then(cb);
+  } else {
+    return cb => setTimeout(cb, 0);
+  }
+})();
 
 module.exports = {
   WebSocket: globalThis.WebSocket || globalThis.MozWebSocket,
   usingBrowserWebSocket: true,
-  defaultBinaryType: "arraybuffer"
+  defaultBinaryType: "arraybuffer",
+  nextTick
 };

lib/transports/websocket-constructor.js

@@ -1,5 +1,6 @@
 module.exports = {
   WebSocket: require("ws"),
   usingBrowserWebSocket: false,
-  defaultBinaryType: "nodebuffer"
+  defaultBinaryType: "nodebuffer",
+  nextTick: process.nextTick
 };

lib/transports/websocket.js

@@ -6,7 +6,8 @@ const { pick } = require("../util");
 const {
   WebSocket,
   usingBrowserWebSocket,
-  defaultBinaryType
+  defaultBinaryType,
+  nextTick
 } = require("./websocket-constructor");
 
 const debug = require("debug")("engine.io-client:websocket");
@@ -161,10 +162,10 @@ class WS extends Transport {
         if (lastPacket) {
           // fake drain
           // defer to next tick to allow Socket to clear writeBuffer
-          setTimeout(() => {
+          nextTick(() => {
             this.writable = true;
             this.emit("drain");
-          }, 0);
+          });
         }
       });
     }