# Welcome to the Highcharts Maps JS (highmaps) 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.flowmap.tooltip.nullFormatter

Callback function to format the text of the tooltip for visible null points. Works analogously to formatter, including the `ctx` callback argument added in v12.5.0.