[feature] serve sourcemap for socket.io-client (#2482)

lib/index.js

@@ -26,6 +26,7 @@ module.exports = Server;
  */
 
 var clientSource = undefined;
+var clientSourceMap = undefined;
 
 /**
  * Server constructor.
@@ -98,6 +99,11 @@ Server.prototype.serveClient = function(v){
 
   if (v && !clientSource) {
     clientSource = read(require.resolve('socket.io-client/socket.io.js'), 'utf-8');
+    try {
+      clientSourceMap = read(require.resolve('socket.io-client/socket.io.js.map'), 'utf-8');
+    } catch(err) {
+      debug('could not load sourcemap file');
+    }
   }
 
   return this;
@@ -255,11 +261,14 @@ Server.prototype.attach = function(srv, opts){
 Server.prototype.attachServe = function(srv){
   debug('attaching client serving req handler');
   var url = this._path + '/socket.io.js';
+  var urlMap = this._path + '/socket.io.js.map';
   var evs = srv.listeners('request').slice(0);
   var self = this;
   srv.removeAllListeners('request');
   srv.on('request', function(req, res) {
-    if (0 === req.url.indexOf(url)) {
+    if (0 === req.url.indexOf(urlMap)) {
+      self.serveMap(req, res);
+    } else if (0 === req.url.indexOf(url)) {
       self.serve(req, res);
     } else {
       for (var i = 0; i < evs.length; i++) {
@@ -299,6 +308,36 @@ Server.prototype.serve = function(req, res){
   res.end(clientSource);
 };
 
+/**
+ * Handles a request serving `/socket.io.js.map`
+ *
+ * @param {http.Request} req
+ * @param {http.Response} res
+ * @api private
+ */
+
+Server.prototype.serveMap = function(req, res){
+  // Per the standard, ETags must be quoted:
+  // https://tools.ietf.org/html/rfc7232#section-2.3
+  var expectedEtag = '"' + clientVersion + '"';
+
+  var etag = req.headers['if-none-match'];
+  if (etag) {
+    if (expectedEtag == etag) {
+      debug('serve client 304');
+      res.writeHead(304);
+      res.end();
+      return;
+    }
+  }
+
+  debug('serve client sourcemap');
+  res.setHeader('Content-Type', 'application/json');
+  res.setHeader('ETag', expectedEtag);
+  res.writeHead(200);
+  res.end(clientSourceMap);
+};
+
 /**
  * Binds socket.io to an engine.io instance.
  *