Get a triad chord for a given degree in the scale

Fix saebekassebil#106
renatomartins · Dec 1, 2019 · cbd74dd · cbd74dd
1 parent 0f4bbe8
commit cbd74dd
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diff --git a/lib/scale.js b/lib/scale.js
@@ -97,6 +97,22 @@ Scale.prototype = {
     return this.tonic.interval(interval).interval(new Interval([octaves, 0]));
   },
 
+  getTriadChord: function(index) {
+    if (this.scale.length !== 7) {
+      throw new Error('Chord degrees are only supported for diatonic scales');
+    }
+    var root = this.get(index);
+    var third = root.interval(this.get(index + 2)).toString();
+    var fifth = root.interval(this.get(index + 4)).toString();
+    var quality;
+    if (third === 'M3') {
+      quality = fifth === 'P5' ? 'M' : 'aug';
+    } else {
+      quality = fifth === 'P5' ? 'm' : 'dim';
+    }
+    return root.chord(quality);
+  },
+
   solfege: function(index, showOctaves) {
     if (index)
       return this.get(index).solfege(this, showOctaves);

diff --git a/test/scales.js b/test/scales.js
@@ -120,5 +120,33 @@ vows.describe('Scales').addBatch({
     '-13 is two octaves down': function(topic) {
       assert.deepEqual(topic.get(-13), teoria.note('A2'));
     }
+  },
+
+  'Chord Degrees': {
+    'D major': function() {
+      var scale = teoria.note('D').scale('major');
+      var expected = ['DM', 'Em', 'F#m', 'GM', 'AM', 'Bm', 'C#dim'];
+      var actual = scale.notes().map((n, i) => scale.getTriadChord(i+1).name);
+      assert.deepEqual(actual, expected);
+    },
+
+    'Bb minor': function() {
+      var scale = teoria.note('Bb').scale('minor');
+      var expected = ['Bbm', 'Cdim', 'DbM', 'Ebm', 'Fm', 'GbM', 'AbM'];
+      var actual = scale.notes().map((n, i) => scale.getTriadChord(i + 1).name);
+      assert.deepEqual(actual, expected);
+    },
+
+    'G mixolydian': function () {
+      var scale = teoria.note('G').scale('mixolydian');
+      var expected = ['GM', 'Am', 'Bdim', 'CM', 'Dm', 'Em', 'FM'];
+      var actual = scale.notes().map((n, i) => scale.getTriadChord(i + 1).name);
+      assert.deepEqual(actual, expected);
+    },
+
+    'only supports diatonic scales': function() {
+      var scale = teoria.note('C').scale('minorpentatonic');
+      assert.throws(() => scale.getTriadChord(1), Error);
+    }
   }
 }).export(module);