If Expression

An if expression is constructed with the if keyword followed by two expressions. The first expression is enclosed in parenthesis () and the second is enclosed with curly braces {}. They both evaluate to a value of a certain type.

The first expression after the if keyword has to evaluate to a value of type Bool. Based on the boolean value of the first expression, the if expression will either evaluate the second expression or it doesn't.

let condition = true;

var number = 0;

if (condition) { number += 1 };

// number is now 1

The first expression evaluates to true so the second expression is evaluated and the code inside it is executed. Note we used the += assignment operator to increment the mutable variable number.

If the first expression evaluates to false, then the second expression is not evaluated and the whole if expression will evaluate to the unit type () and the program continues.