# JS API Reference

# Welcome to the Highcharts JS Options Reference

These pages outline the chart configuration options, and the methods and properties of Highcharts objects.

Feel free to search this API through the search bar or the navigation tree in the sidebar.

# series.areasplinerange.fillOpacity

Fill opacity for the area. When you set an explicit `fillColor`, the `fillOpacity` is not applied. Instead, you should define the opacity in the `fillColor` with an rgba color definition. The `fillOpacity` setting, also the default setting, overrides the alpha component of the `color` setting.

In styled mode, the fill opacity can be set with the `.highcharts-area` class name.