feat: add new option for custom hash

README.md

@@ -213,6 +213,26 @@ app.use(session({
 }))
 ```
 
+##### hash 
+
+Function to call to generate a hash of session object for detect changes.
+
+```js
+app.use(session({
+  hash: function(sess) {
+    // serialize
+    const { cookie, ...sessWithoutCookie } = sess;
+    const str = JSON.stringify(sessWithoutCookie);
+    // hash
+    return crypto
+      .createHash('sha1')
+      .update(str, 'utf8')
+      .digest('hex')
+  },
+  secret: 'keyboard cat'
+}))
+```
+
 ##### name
 
 The name of the session ID cookie to set in the response (and read from in the

index.js

@@ -80,6 +80,7 @@ var defer = typeof setImmediate === 'function'
  * @param {String|Array} [options.secret] Secret for signing session ID
  * @param {Object} [options.store=MemoryStore] Session store
  * @param {String} [options.unset]
+ * @param {Function} [options.hash] Hash method to detect changes in the session object
  * @return {Function} middleware
  * @public
  */
@@ -114,10 +115,17 @@ function session(options) {
   // get the cookie signing secret
   var secret = opts.secret
 
+  // get the hash method
+  var hash = opts.hash || hashSession
+
   if (typeof generateId !== 'function') {
     throw new TypeError('genid option must be a function');
   }
 
+  if (typeof hash !== 'function') {
+    throw new TypeError('hash option must be a function');
+  }
+
   if (resaveSession === undefined) {
     deprecate('undefined resave option; provide resave option');
     resaveSession = true;
@@ -601,7 +609,7 @@ function getcookie(req, name, secrets) {
  * @private
  */
 
-function hash(sess) {
+function hashSession(sess) {
   // serialize
   var str = JSON.stringify(sess, function (key, val) {
     // ignore sess.cookie property

test/session.js

@@ -843,6 +843,42 @@ describe('session()', function(){
     });
   });
 
+  describe('hash option', function(){
+    it('should reject non-function values', function(){
+      assert.throws(session.bind(null, { hash: 'bogus!' }), /hash.*must/)
+    });
+
+    it('should provide default hash', function(done){
+      request(createServer())
+      .get('/')
+      .expect(shouldSetCookie('connect.sid'))
+      .expect(200, done)
+    });
+
+    it('should allow custom function', function(done){
+      var counter = 0;
+      function hash() {
+        counter++;
+        var str = JSON.stringify(sess, function (key, val) {
+          if (this === sess && key === 'cookie') {
+            return
+          }
+          return val
+        })
+        return str;
+      }
+
+      var server = createServer({ hash: hash }, function (req, res) {
+        res.end('counter ' + counter)
+      });
+
+      request(server)
+      .get('/')
+      .expect(shouldSetCookie('connect.sid'))
+      .expect(200, 'counter 1', done)
+    });
+  });
+
   describe('key option', function(){
     it('should default to "connect.sid"', function(done){
       request(createServer())