function units(a, b, u, v) { // number1, number2, unit, progress
    var A = +a;
    var B = b - a;
    // a = +a; b -= a;
    return (A + B * v) + u;
  }
Units Interpolation Function.

@param {number} a start value
@param {number} b end value
@param {string} u unit
@param {number} v progress
@returns {string} the interpolated value + unit string