Power Tools: Closures & Array Methods

Enemy 1 of 5

A closure is a small, self-contained block of code you can define right where you need it, without a separate func declaration. The general syntax is { (params) -> ReturnType in expression }: parameters and a return type in parentheses, the in keyword marking where the body starts, then the expression itself. You can assign a closure to a let and call it exactly like any other function — square(6) behaves identically whether square was defined with func or as a closure.

Closures are called that because they can capture — "close over" — variables and constants from the surrounding scope, which is part of what makes them so useful for the short, situational logic you hand to functions like .map { } and .filter { } below, instead of writing a whole separate named function for a one-off transformation.