# Welcome to the Highcharts Maps JS (highmaps) Options Reference

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

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# plotOptions.contour.joinBy

What property to join the `mapData` to the value data. For example, if joinBy is "code", the mapData items with a specific code is merged into the data with the same code. For maps loaded from GeoJSON, the keys may be held in each point's `properties` object.

The joinBy option can also be an array of two values, where the first points to a key in the `mapData`, and the second points to another key in the `data`.

When joinBy is `null`, the map items are joined by their position in the array, which performs much better in maps with many data points. This is the recommended option if you are printing more than a thousand data points and have a backend that can preprocess the data into a parallel array of the mapData.