code: function collinear(a,b,c,thresholdAngle) {
      if(!thresholdAngle){
          return triangleArea(a, b, c) === 0;
      } else {
          var ab = tmpPoint1,
              bc = tmpPoint2;
  
          ab[0] = b[0]-a[0];
          ab[1] = b[1]-a[1];
          bc[0] = c[0]-b[0];
          bc[1] = c[1]-b[1];
  
          var dot = ab[0]*bc[0] + ab[1]*bc[1],
              magA = Math.sqrt(ab[0]*ab[0] + ab[1]*ab[1]),
              magB = Math.sqrt(bc[0]*bc[0] + bc[1]*bc[1]),
              angle = Math.acos(dot/(magA*magB));
          return angle < thresholdAngle;
      }
  }
docs: Check if three points are collinear
  @method collinear
  @param  {Array} a
  @param  {Array} b
  @param  {Array} c
  @param  {Number} [thresholdAngle=0] Threshold angle to use when comparing the vectors. The function will return true if the angle between the resulting vectors is less than this value. Use zero for max precision.
  @return {Boolean}