prev(loop = false) {
    let index = this.index - 1;

    if (index < 0) {
      index = loop ? this.length - 1 : 0;
    }

    this.view(index);
    return this;
  }
View the previous image
@param {boolean} [loop=false] - Indicate if view the last one
when it is the first one at present.
@returns {Viewer} this