feat: remove dynamic require() with wsEngine

This change is necessary to get rid of:

> Critical dependency: the request of a dependency is an expression

when bundling the server with webpack.

BREAKING CHANGE: the syntax of the "wsEngine" option is updated

Before:

```js
const eioServer = require("engine.io")(httpServer, {
  wsEngine: "eiows"
});
```

After:

```js
const eioServer = require("engine.io")(httpServer, {
  wsEngine: require("eiows").Server
});
```

Related: #609

README.md

@@ -188,7 +188,7 @@ server.on('connection', /* ... */);
 const engine = require('engine.io');
 const httpServer = require('http').createServer().listen(3000);
 const server = engine.attach(httpServer, {
-  wsEngine: 'uws' // requires having uws as dependency
+  wsEngine: require('eiows').Server // requires having eiows as dependency
 });
 
 server.on('connection', /* ... */);
@@ -249,7 +249,7 @@ to a single process.
         contains the client sid to send as part of handshake response
         headers. This cookie might be used for sticky-session. Defaults to not sending any cookie (`false`).
         See [here](https://github.com/jshttp/cookie#options-1) for all supported options.
-      - `wsEngine` (`String`): what WebSocket server implementation to use. Specified module must conform to the `ws` interface (see [ws module api docs](https://github.com/websockets/ws/blob/master/doc/ws.md)). Default value is `ws`. An alternative c++ addon is also available by installing `uws` module.
+      - `wsEngine` (`Function`): what WebSocket server implementation to use. Specified module must conform to the `ws` interface (see [ws module api docs](https://github.com/websockets/ws/blob/master/doc/ws.md)). Default value is `ws`. An alternative c++ addon is also available by installing `eiows` module.
       - `cors` (`Object`): the options that will be forwarded to the cors module. See [there](https://github.com/expressjs/cors#configuration-options) for all available options. Defaults to no CORS allowed.
       - `initialPacket` (`Object`): an optional packet which will be concatenated to the handshake packet emitted by Engine.IO.
       - `allowEIO3` (`Boolean`): whether to support v3 Engine.IO clients (defaults to `false`)

lib/server.js

@@ -7,6 +7,8 @@ const Socket = require("./socket");
 const debug = require("debug")("engine");
 const cookieMod = require("cookie");
 
+const DEFAULT_WS_ENGINE = require("ws").Server;
+
 class Server extends EventEmitter {
   /**
    * Server constructor.
@@ -22,7 +24,7 @@ class Server extends EventEmitter {
 
     this.opts = Object.assign(
       {
-        wsEngine: process.env.EIO_WS_ENGINE || "ws",
+        wsEngine: DEFAULT_WS_ENGINE,
         pingTimeout: 20000,
         pingInterval: 25000,
         upgradeTimeout: 10000,
@@ -76,10 +78,7 @@ class Server extends EventEmitter {
 
     if (this.ws) this.ws.close();
 
-    // add explicit require for bundlers like webpack
-    const wsModule =
-      this.opts.wsEngine === "ws" ? require("ws") : require(this.opts.wsEngine);
-    this.ws = new wsModule.Server({
+    this.ws = new this.opts.wsEngine({
       noServer: true,
       clientTracking: false,
       perMessageDeflate: this.opts.perMessageDeflate,

test/common.js

@@ -16,6 +16,10 @@ exports.listen = (opts, fn) => {
 
   opts.allowEIO3 = true;
 
+  if (process.env.EIO_WS_ENGINE) {
+    opts.wsEngine = require(process.env.EIO_WS_ENGINE).Server;
+  }
+
   const e = eio.listen(0, opts, () => {
     fn(e.httpServer.address().port);
   });

test/server.js

@@ -3121,7 +3121,7 @@ describe("server", () => {
   describe("wsEngine option", () => {
     it("should allow loading of other websocket server implementation like eiows", done => {
       const engine = listen(
-        { allowUpgrades: false, wsEngine: "eiows" },
+        { allowUpgrades: false, wsEngine: require("eiows").Server },
         port => {
           expect(engine.ws instanceof require("eiows").Server).to.be.ok();
           const socket = new eioc.Socket("ws://localhost:%d".s(port));