How to use the shifty.setBezierFunction function in shifty

it('sets up custom curves correctly', () => {
      setBezierFunction('custom', 0, 0.25, 0.5, 0.75);
      rekapi = setupTestRekapi();

      exportedTimeline = rekapi.exportTimeline();

      // Reset for a clean test
      unsetBezierFunction('custom');

      targetRekapi = new Rekapi();
      targetRekapi.importTimeline(exportedTimeline);

      assert.equal(typeof Tweenable.formulas.custom, 'function');
      assert.equal(Tweenable.formulas.custom.x1, 0);
      assert.equal(Tweenable.formulas.custom.y1, 0.25);
      assert.equal(Tweenable.formulas.custom.x2, 0.5);
      assert.equal(Tweenable.formulas.custom.y2, 0.75);

beforeEach(() => {
      setBezierFunction('testCurve', 0, 0, 0, 0);
      component.instance().updateEasingList();
    });

each(rekapiData.curves, (curve, curveName) =>
      setBezierFunction(
        curveName,
        curve.x1,
        curve.y1,
        curve.x2,
        curve.y2
      )
    );