Variables that can change their value after declaring them are called mutable variables. They are declared using a var declaration:

var x = 1;

x is now the name of a mutable variable with the value 1. If we want to change the value, in other words mutate it, we assign a new value to it:

x := 2;

x now has the value 2.

The := is called the assignment operator. It is used to assign a new value to an already declared variable.

We can change the value of a mutable variable as many times as we like. For example, we declare a mutable variable named city with text value "Amsterdam"

var city = "Amsterdam";

And we also declare two immutable variables.

let newYork = "New York";
let berlin = "Berlin";

Now we mutate city three times:

city := newYork;
city := berlin;
city := "Paris";

The last mutation was achieved by assigning a string literal to the variable name. The first two mutations were achieved by assigning the value of other immutable variables. It is also possible to assign the value of another mutable variable.