code: function median (size) {
    if (!is.defined(size)) {
      // No arguments: default to 3x3
      this.options.medianSize = 3;
    } else if (is.integer(size) && is.inRange(size, 1, 1000)) {
      // Numeric argument: specific sigma
      this.options.medianSize = size;
    } else {
      throw new Error('Invalid median size ' + size);
    }
    return this;
  }
docs: Apply median filter.
  When used without parameters the default window is 3x3.
  @param {Number} [size=3] square mask size: size x size
  @returns {Sharp}
  @throws {Error} Invalid parameters