fix: typescript promise type (#160)

* fix: change Tweenable to implement Promise<unknown>

Co-authored-by: Jeremy Kahn <[email protected]>

src/tweenable.js

@@ -323,8 +323,13 @@ const remove = ((previousTween, nextTween) => tween => {
 })()
 
 const defaultPromiseCtor = typeof Promise === 'function' ? Promise : null
-
+/**
+ * @class
+ * @implements {Promise<unknown>}
+ */
 export class Tweenable {
+  //required for Promise implementation
+  [Symbol.toStringTag] = 'Promise'
   /**
    * @method Tweenable.now
    * @static
@@ -555,9 +560,9 @@ export class Tweenable {
   /**
    * Overrides any `finish` function passed via a {@link shifty.tweenConfig}.
    * @method Tweenable#then
-   * @param {function} onFulfilled Receives {@link shifty.promisedData} as the
+   * @param {function=} onFulfilled Receives {@link shifty.promisedData} as the
    * first parameter.
-   * @param {function} onRejected Receives {@link shifty.promisedData} as the
+   * @param {function=} onRejected Receives {@link shifty.promisedData} as the
    * first parameter.
    * @return {Promise<Object>}
    * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then
@@ -582,6 +587,15 @@ export class Tweenable {
   catch(onRejected) {
     return this.then().catch(onRejected)
   }
+  /**
+   * @method Tweenable#finally
+   * @param {function} onFinally
+   * @return {Promise<undefined>}
+   * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/finally
+   */
+  finally(onFinally) {
+    return this.then().finally(onFinally)
+  }
 
   /**
    * @method Tweenable#get

src/tweenable.test.js

@@ -225,6 +225,29 @@ describe('#tween', () => {
         expect(testState).toEqual({ x: 10 })
       })
     })
+    describe('finally', () => {
+      test('runs finally with no parameters', done => {
+        Tweenable.now = () => 0
+        tweenable = new Tweenable()
+
+        const tween = tweenable.tween({
+          from: { x: 0 },
+          to: { x: 10 },
+          duration: 500,
+        })
+        tween
+          .catch(() => {})
+          .finally(state => {
+            expect(state).toEqual(undefined)
+            done()
+          })
+
+        Tweenable.now = () => 250
+        processTweens()
+
+        tween.cancel()
+      })
+    })
 
     describe('rejection', () => {
       let testState