findKey(map, test)

This returns the key of the first entry in the map-like object for which the test function returns true. This is analogous to the standard JavaScript Array.prototype.findKey function but extended to cover objects and other types of maps.

The test function will be called with the value, key, and map of each key/value in the map. The test function should return a truthy result if the entry passes the test.

Example: if bunnies.tsv contains

breed	fur	color	origin
American Fuzzy Lop	Long	Various	United States
Belgian Hare	Short	Black, Chestnut	Belgium
Meissner Lop	Short	Various	Germany
Thrianta	Short	Chestnut	Netherlands

then:

$ ori "Tree.findKey(samples/help/bunnies.tsv, (entry) => entry.fur === 'Short')"
1

because 1 is the index of the first matching entry.

To get the value of the first matching entry, see the related Tree.find.