perf: skip unnecessary parsing of entire header

HISTORY.md

@@ -7,6 +7,7 @@ unreleased
     - perf: unroll the "fast-path" `RegExp`
   * deps: vary@~1.1.2
     - perf: improve header token parsing speed
+  * perf: skip unnecessary parsing of entire header
 
 2.3.9 / 2017-05-19
 ==================

index.js

@@ -110,14 +110,26 @@ function createQueryGetter (key) {
  */
 
 function createHeaderGetter (str) {
-  var header = str.toLowerCase()
+  var name = str.toLowerCase()
 
   return function (req, res) {
     // set appropriate Vary header
     vary(res, str)
 
+    // get header
+    var header = req.headers[name]
+
+    if (!header) {
+      return undefined
+    }
+
     // multiple headers get joined with comma by node.js core
-    return (req.headers[header] || '').split(/ *, */)
+    var index = header.indexOf(',')
+
+    // return first value
+    return index !== -1
+      ? header.substr(0, index).trim()
+      : header.trim()
   }
 }