Add isThird

README.md

@@ -598,6 +598,23 @@ console.log(isTone(getIntervals(['C3', 'D3'])[0]));
 // > true;
 ```
 
+#### + `isThird(interval):boolean`
+
+Returns `true` if the interval is a third (diminished, minor, major or augmented) `(2, 3, 4, 5)`.
+
+```js
+import { getIntervals, isThird } from 'music-fns';
+
+console.log(isThird(getIntervals(['C4', 'E4'])[0]));
+// > true;
+
+console.log(isThird(getIntervals(['A4', 'C4'])[0]));
+// > true;
+
+console.log(isThird(getIntervals(['C4', 'E#4'])[0]));
+// > true;
+```
+
 #### + `isFifth(interval):boolean`
 
 Returns `true` if the interval is a fifth (diminished, perfect or augmented) `(6, 7, 8)`.

src/isThird/index.js

@@ -0,0 +1,15 @@
+import { MINOR_THIRD, MAJOR_THIRD } from '../constants/Interval/MinorMajor';
+import {
+  AUGMENTED_THIRD,
+  DIMINISHED_THIRD
+} from '../constants/Interval/AugmentedDiminished';
+
+// @flow
+
+const isThird = (interval: Interval) =>
+  interval === DIMINISHED_THIRD ||
+  interval === MINOR_THIRD ||
+  interval === MAJOR_THIRD ||
+  interval === AUGMENTED_THIRD;
+
+export default isThird;

src/isThird/test.js

@@ -0,0 +1,31 @@
+import isThird from './';
+
+describe('isThird', () => {
+  it('should return true when interval is a diminished third', () => {
+    expect(isThird(2)).toBe(true);
+  });
+
+  it('should return true when interval is a minor third', () => {
+    expect(isThird(3)).toBe(true);
+  });
+
+  it('should return true when interval is a major third', () => {
+    expect(isThird(4)).toBe(true);
+  });
+
+  it('should return true when interval is a augmented third', () => {
+    expect(isThird(5)).toBe(true);
+  });
+
+  it('should return false when interval is not a third', () => {
+    expect(isThird(7)).toBe(false);
+  });
+
+  it('should return false when interval is not a third', () => {
+    expect(isThird(1)).toBe(false);
+  });
+
+  it('should return false on no input', () => {
+    expect(isThird()).toBe(false);
+  });
+});