code: function encodeStyleFill_(fillStyle, encodedStyles, opt_propertyName) {
    const propertyName = opt_propertyName !== undefined ?
      opt_propertyName : 'fillColor';
    const fillColor = fillStyle.getColor();
    if (fillColor !== null) {
      if (Array.isArray(fillColor)) {
        const fillColorHex = rgbArrayToHex(fillColor);
        if (encodedStyles.length > 0) {
          encodedStyles.push('\'');
        }
        encodedStyles.push(
          encodeURIComponent(`${propertyName}*${fillColorHex}`));
      } else {
        console.assert(false, 'only supporting fill colors');
      }
    }
  }
docs: Transform an {@link import("ol/style/Fill.js").default} and optionally its properties into
  a logical sequence of characters and put the result into the given encoded
  styles's array.
  @param {import("ol/style/Fill.js").default} fillStyle Fill style.
  @param {Array<string>} encodedStyles Encoded styles array.
  @param {string=} opt_propertyName Property name.
  @private
  @hidden