Chart.js/test/specs/helpers.interpolation.tests.js

const {_pointInLine, _steppedInterpolation, _bezierInterpolation} = Chart.helpers;

describe('helpers.interpolation', function() {
  it('Should interpolate a point in line', function() {
    expect(_pointInLine({x: 10, y: 10}, {x: 20, y: 20}, 0)).toEqual({x: 10, y: 10});
    expect(_pointInLine({x: 10, y: 10}, {x: 20, y: 20}, 0.5)).toEqual({x: 15, y: 15});
    expect(_pointInLine({x: 10, y: 10}, {x: 20, y: 20}, 1)).toEqual({x: 20, y: 20});
  });

  it('Should interpolate a point in stepped line', function() {
    expect(_steppedInterpolation({x: 10, y: 10}, {x: 20, y: 20}, 0, 'before')).toEqual({x: 10, y: 10});
    expect(_steppedInterpolation({x: 10, y: 10}, {x: 20, y: 20}, 0.4, 'before')).toEqual({x: 14, y: 20});
    expect(_steppedInterpolation({x: 10, y: 10}, {x: 20, y: 20}, 0.5, 'before')).toEqual({x: 15, y: 20});
    expect(_steppedInterpolation({x: 10, y: 10}, {x: 20, y: 20}, 1, 'before')).toEqual({x: 20, y: 20});

    expect(_steppedInterpolation({x: 10, y: 10}, {x: 20, y: 20}, 0, 'middle')).toEqual({x: 10, y: 10});
    expect(_steppedInterpolation({x: 10, y: 10}, {x: 20, y: 20}, 0.4, 'middle')).toEqual({x: 14, y: 10});
    expect(_steppedInterpolation({x: 10, y: 10}, {x: 20, y: 20}, 0.5, 'middle')).toEqual({x: 15, y: 20});
    expect(_steppedInterpolation({x: 10, y: 10}, {x: 20, y: 20}, 1, 'middle')).toEqual({x: 20, y: 20});

    expect(_steppedInterpolation({x: 10, y: 10}, {x: 20, y: 20}, 0, 'after')).toEqual({x: 10, y: 10});
    expect(_steppedInterpolation({x: 10, y: 10}, {x: 20, y: 20}, 0.4, 'after')).toEqual({x: 14, y: 10});
    expect(_steppedInterpolation({x: 10, y: 10}, {x: 20, y: 20}, 0.5, 'after')).toEqual({x: 15, y: 10});
    expect(_steppedInterpolation({x: 10, y: 10}, {x: 20, y: 20}, 1, 'after')).toEqual({x: 20, y: 20});
  });

  it('Should interpolate a point in curve', function() {
    const pt1 = {x: 10, y: 10, cp2x: 12, cp2y: 12};
    const pt2 = {x: 20, y: 30, cp1x: 18, cp1y: 28};

    expect(_bezierInterpolation(pt1, pt2, 0)).toEqual({x: 10, y: 10});
    expect(_bezierInterpolation(pt1, pt2, 0.2)).toBeCloseToPoint({x: 11.616, y: 12.656});
    expect(_bezierInterpolation(pt1, pt2, 1)).toEqual({x: 20, y: 30});
  });
});