perf: memoise the Content-Type resolution in res.set, res.type and setCharset

lib/response.js

@@ -49,6 +49,39 @@ var res = Object.create(http.ServerResponse.prototype)
 
 module.exports = res
 
+/**
+ * The results of `mime.contentType`, by the value it was given: a response
+ * sets one of a handful of types, and each lookup through the mime database
+ * costs a few hundred nanoseconds.
+ * @private
+ */
+
+var contentTypeCache = new Map();
+
+/**
+ * `mime.contentType`, memoised.
+ *
+ * @param {String} value
+ * @return {String|Boolean}
+ * @private
+ */
+
+function cachedContentType(value) {
+  var resolved = contentTypeCache.get(value);
+
+  if (resolved === undefined) {
+    resolved = mime.contentType(value);
+
+    if (contentTypeCache.size >= 100) {
+      contentTypeCache.clear();
+    }
+
+    contentTypeCache.set(value, resolved);
+  }
+
+  return resolved;
+}
+
 /**
  * Set the HTTP status code for the response.
  *
@@ -507,7 +540,7 @@ res.download = function download (path, filename, options, callback) {
 res.contentType =
 res.type = function contentType(type) {
   var ct = type.indexOf('/') === -1
-    ? (mime.contentType(type) || 'application/octet-stream')
+    ? (cachedContentType(type) || 'application/octet-stream')
     : type;
 
   return this.set('Content-Type', ct);
@@ -678,7 +711,7 @@ res.header = function header(field, val) {
       if (Array.isArray(value)) {
         throw new TypeError('Content-Type cannot be set to an Array');
       }
-      value = mime.contentType(value)
+      value = cachedContentType(value)
     }
 
     this.setHeader(field, value);

lib/utils.js

@@ -213,6 +213,16 @@ exports.compileTrust = function(val) {
   return proxyaddr.compile(val || []);
 }
 
+/**
+ * The results of setCharset, a map of type to result per charset: the same
+ * few types come through on every response, and parsing and formatting each
+ * costs a few hundred nanoseconds. One map per charset, since a key made by
+ * joining the two strings would be hashed again on every call.
+ * @private
+ */
+
+var setCharsetCache = new Map();
+
 /**
  * Set the charset in a given Content-Type string.
  *
@@ -227,14 +237,33 @@ exports.setCharset = function setCharset(type, charset) {
     return type;
   }
 
-  // parse type
-  var parsed = contentType.parse(type);
+  var byType = setCharsetCache.get(charset);
+
+  if (byType === undefined) {
+    byType = new Map();
+    setCharsetCache.set(charset, byType);
+  }
+
+  var formatted = byType.get(type);
+
+  if (formatted === undefined) {
+    // parse type
+    var parsed = contentType.parse(type);
+
+    // set charset
+    parsed.parameters.charset = charset;
+
+    // format type
+    formatted = contentType.format(parsed);
 
-  // set charset
-  parsed.parameters.charset = charset;
+    if (byType.size >= 100) {
+      byType.clear();
+    }
+
+    byType.set(type, formatted);
+  }
 
-  // format type
-  return contentType.format(parsed);
+  return formatted;
 };
 
 /**