mutable

There are two kinds of mutability:

• mutable variables can point to different objects during their lifetime.
• mutable objects can change during their lifetime.

x="hello"
x+=" world" # ok, value was mutable
x="bye" # ok, variable was mutable

• immutable variables point to the same object during their lifetime.
• immutable objects stay completely unchanged during their lifetime.

The first variant is called 'let' variable:

let x="hello"
x+=" world"  # ok, value was mutable
x="bye" # error: variable was immutable

Both properties can be combined with the constant keyword or with the := sigil:

constant x = "hello"
x := "hello" # ok redundant redeclaration
x+="ok" # error: immutable value
x="hi" # error: constant variable

This is near identical to JavaScript's:
Uncaught SyntaxError: Identifier 'x' has already been declared Uncaught TypeError: Assignment to constant variable.

With the only difference being: If the value is identical then it is OK to re-declare a variable. In JavaScript it is allowed to re-declare variables without let:

let x=1
x=2 # ok in JavaScript
let x=2 # error