xet

xet ([ʃɛt]) retrieves a map's value for a given key, setting it with a provided factory if it isn't present.

xet( map, key, factory )
// where
const factory = (key, value) => value

This is useful when writing in a point-free declarative dialect - virtual DOM view templates for example - and want to write functions that can conditionally run setup in the form of the factory.