Map.put_new

You're seeing just the function put_new, go back to Map module for more information.
Link to this function

put_new(map, key, value)

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Specs

put_new(map(), key(), value()) :: map()

Puts the given value under key unless the entry key already exists in map.

Examples

iex> Map.put_new(%{a: 1}, :b, 2)
%{a: 1, b: 2}
iex> Map.put_new(%{a: 1, b: 2}, :a, 3)
%{a: 1, b: 2}