diff --git a/javascript/JZZ.js b/javascript/JZZ.js
index cea36c6..f52dfed 100644
--- a/javascript/JZZ.js
+++ b/javascript/JZZ.js
@@ -2847,7 +2847,7 @@
   // JZZ.UMP
 
   function UMP(arg) {
-    var self = this instanceof UMP ? this : self = new UMP();
+    var self = this instanceof UMP ? this : self = new UMP([0, 0, 0, 0]);
     var i;
     if (arg instanceof UMP) {
       self._from = arg._from.slice();
@@ -2857,11 +2857,13 @@
     else self._from = [];
     if (typeof arg == 'undefined') return self;
     var arr = arg instanceof Array ? arg : arguments;
+    self.length = 0;
     for (i = 0; i < arr.length; i++) {
       n = arr[i];
       if (n != parseInt(n) || n < 0 || n > 255) _throw(arr[i]);
       self.push(n);
     }
+    if (self.length != [4, 4, 4, 8, 8, 16, 4, 4, 8, 8, 8, 12, 12, 16, 16, 16][self[0] >> 4]) throw RangeError('Wrong UMP size');
     return self;
   }
   UMP.prototype = [];
@@ -2885,6 +2887,11 @@
     ret._gr = g;
     return ret;
   };
+  UMP.prototype.getGroup = function() {
+    if (!this[0]) return;
+    var m = this[0] >> 4;
+    if (m == 1 || m == 2 || m == 3 || m == 4 || m == 5 || m == 13) return this[0] & 15;
+  };
 
   UMP.prototype.toString = function() {
     return _hexx(this);
diff --git a/test/common.js b/test/common.js
index 7dc0bc1..1783c84 100644
--- a/test/common.js
+++ b/test/common.js
@@ -763,6 +763,38 @@ describe('MIDI messages', function() {
   });
 });
 
+describe('UMP messages', function() {
+  it('throw', function() {
+    assert.throws(function() { JZZ.UMP([1]); });
+    assert.throws(function() { JZZ.UMP([1, 2, 3, 'error']); });
+    assert.throws(function() { JZZ.UMP([1, 2, 3, 4, 5, 6, 7, 8]); });
+  });
+  it('noop', function() {
+    var msg =JZZ.UMP.noop();
+    assert.equal(msg.toString(), '00000000');
+    assert.equal(typeof msg.getGroup(), 'undefined');
+  });
+  it('noteOn', function() {
+    var s = '21923d7f';
+    var msg = JZZ.UMP.noteOn(1, 2, 'C#5');
+    assert.equal(msg.getGroup(), 1);
+    assert.equal(JZZ.UMP.noteOn(1, 2, 'C#5').toString(), s);
+    assert.equal(JZZ.UMP.ch().noteOn(1, 2, 'C#5').toString(), s);
+    assert.equal(JZZ.UMP.ch(5).ch().noteOn(1, 2, 'C#5').toString(), s);
+    assert.equal(JZZ.UMP.gr().noteOn(1, 2, 'C#5').toString(), s);
+    assert.equal(JZZ.UMP.gr(5).gr().noteOn(1, 2, 'C#5').toString(), s);
+    assert.equal(JZZ.UMP.gr(1).noteOn(2, 'C#5').toString(), s);
+    assert.equal(JZZ.UMP.ch(2).noteOn(1, 'C#5').toString(), s);
+    assert.equal(JZZ.UMP.gr(1).ch(2).noteOn('C#5').toString(), s);
+    assert.equal(JZZ.UMP.ch(2).gr(1).noteOn('C#5', 127).toString(), s);
+    assert.equal(JZZ.UMP.ch(2).gr(1).ch(2).gr(1).noteOn('C#5', 127).toString(), s);
+  });
+  it('noteOff', function() {
+    assert.equal(JZZ.UMP.noteOff(1, 2, 61).toString(), '21823d40');
+    assert.equal(JZZ.UMP.noteOff(1, 2, 'C#5', 64).toString(), '21823d40');
+  });
+});
+
 describe('SMF events', function() {
   it('empty', function() {
     assert.equal(JZZ.MIDI.smf(1).toString(), 'ff01 -- Text');