Code review feedback

Add a non-null assertion, as requested in code review.

Add JSDoc to clarify that `getCanvasPosition` now expects a native `Event`, not a `ChartEvent`.  Add `@ts-ignore`; `getCanvasPosition` relied on some loose conversions between `Event`, `TouchEvent`, and `Touch` that would require several changes to make TypeScript happy.

src/helpers/helpers.dom.js

@@ -59,7 +59,13 @@ function getPositionedStyle(styles, style, suffix) {
 
 const useOffsetPos = (x, y, target) => (x > 0 || y > 0) && (!target || !target.shadowRoot);
 
+/**
+ * @param {Event} e
+ * @param {HTMLCanvasElement} canvas
+ * @returns {{x: number, y: number, box: boolean}}
+ */
 function getCanvasPosition(e, canvas) {
+  // @ts-ignore
   const touches = e.touches;
   const source = touches && touches.length ? touches[0] : e;
   const {offsetX, offsetY} = source;

test/specs/helpers.dom.tests.js

@@ -449,6 +449,7 @@ describe('DOM helpers tests', function() {
       const nativeEvent = await jasmine.triggerMouseEvent(chart, 'mousemove', point);
       const nativePosition = Chart.helpers.getRelativePosition(nativeEvent, chart);
 
+      expect(chartPosition).not.toBeNull();
       expect(nativePosition).toEqual(chartPosition);
     });
   });