toString() {
    if (this._serialized !== null) {
      return this._serialized;
    }

    let svg = this.xmlDeclaration + this.doctypeDeclaration;
    svg += '<svg';
    for (const [attr, value] of Object.entries(this.rootAttributes)) {
      svg += ` ${attr}="${escape(value)}"`;
    }

    svg += '>';
    svg += this.content.join('');
    svg += '</svg>';

    // Apply post-processing transformations
    for (const transform of this.transform) {
      if (isFunction(transform)) {
        svg = transform(svg) || '';
      }
    }

    this._serialized = svg;

    return this._serialized;
  }
Serialize the SVG sprite

@returns {string} SVG sprite