Description sounds just like `.set`

[Tony Brix]

We propose the addition of a method that will add a value to a map if the map does not already have something at key, and will also update an existing value at key.

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According to MDN Map.prototype.set():

The set() method adds or updates an element with a specified key and a value to a Map object.

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So both will insert a value at key if it doesn't exist and update the value at key if it does exist?

[Jordan Harband]

.set does not provide a facility to differentiate how to produce the value for addition vs for updates; upsert does.

[Tony Brix]

I see, so the important part is running a given function that sets a key based on the current value (or lack there of) of the key.