How does this extension property to capitalize the first letter of every word in a string work?

I'm recently trying to get into kotlin (coming from java) and did some beginner coding challenges.

The task is to write a function that capitalizes the first letter of every word in a string

I could solve the task in my own way, but I still don't understand this sample solution:

fun capitalizeSentence(str: String) {
     println(str.split(" ").joinToString(" ") { it.capitalize() })
}

Can someone explain to me, why this lambda expressions manages to capitalize the first letter of every word, even when using the joinToString method right after the split method?

Solution

joinToString() accepts an optional transform function which it applies to each item before joining. It might be confusing to you, because this code performs operations in a different order than the code flow itself. This code is effectively an equivalent of this one:

str.split(" ")
    .map { it.capitalize() }
    .joinToString(" ")