What's the difference between let and var?

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

Swift lets you create both variables and constants as ways to reference your data, but there's a strong push (even Xcode warnings!) if you create things as variables then never change them. To make a constant, use let like this:

let x = 10

To make a variable, use var like this:

var y = 20

The reason Swift strongly encourages you to use constants wherever possible is because it's safer: if you say "this value will never change," then Swift will refuse to let you change it even by accident. It also opens the possibility of compiler optimizations if the system knows certain data will not change.