Sanctuary is small functional programming library inspired by Haskell and PureScript. Sanctuary makes it possible to write safe code without null checks.

In JavaScript it's trivial to introduce a possible run-time type error:

words[0].toUpperCase()

If words is ['foo', 'bar', 'baz'] this expression will evaluate to 'FOO'. But what if words is []?

Sanctuary is stricter in its types than most JavaScript libraries. Its head function, for example, has type a -> Maybe a which means it never returns null or undefined. This forces one to consider the empty case.

S.head(words) evaluates to a value of type Maybe String. One may derive from it a value of type String by applying S.fromMaybe. The toUpperCase method can then be invoked safely:

// :: String
S.fromMaybe('', S.head(words)).toUpperCase()

Without Sanctuary, one might have written:

// :: String
(words.length > 0 ? words[0] : '').toUpperCase()

Maybe is a functor, so one can use its map method to produce another Maybe:

// :: Maybe String
S.head(words).map(function(word) { return word.toUpperCase(); })

This approach is even cleaner if one uses Ramda:

// :: Maybe String
R.map(R.toUpper, S.head(words))