What is the difference between `let` and `var` in Swift?

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The let keyword defines a constant:

let theAnswer = 42

The theAnswer cannot be changed afterwards. This is why anything weak can’t be written using let. They need to change during runtime and you must be using var instead.

The var defines an ordinary variable.

What is interesting:

The value of a constant doesn’t need to be known at compile time, but you must assign the value exactly once.

Another strange feature:

You can use almost any character you like for constant and variable
names, including Unicode characters:

let 🐶🐮 = "dogcow"

Excerpts From: Apple Inc. “The Swift Programming Language.” iBooks. https://itunes.apple.com/WebObjects/MZStore.woa/wa/viewBook?id=881256329

Community Wiki

Because comments are asking for adding other facts to the answer, converting this to community wiki answer. Feel free edit the answer to make it better.

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