Constants and Variable Declarations

Constants and variables are declarations that bind a value and type to an identifier. Constants are initialized with a value and cannot be reassigned afterwards. Variables are initialized with a value and can be reassigned later. Declarations can be created in any scope, including the global scope.

Constant means that the identifier's association is constant, not the value itself — the value may still be changed if it is mutable. For example, you can change the values inside of a constant array, but you cannot replace the array assignment with a new array.

Constants are declared using the let keyword. Variables are declared using the var keyword. The keywords are followed by the identifier, an optional type annotation, an equals sign =, and the initial value:

// Declare a constant named `a`.
//
let a = 1

// Invalid: re-assigning to a constant.
//
a = 2

// Declare a variable named `b`.
//
var b = 3

// Assign a new value to the variable named `b`.
//
b = 4

Variables and constants must be initialized:

// Invalid: the constant has no initial value.
//
let a

The names of the variable or constant declarations in each scope must be unique. Declaring another variable or constant with a name that is already declared in the current scope is invalid, regardless of kind or type:

// Declare a constant named `a`.
//
let a = 1

// Invalid: cannot re-declare a constant with name `a`,
// as it is already used in this scope.
//
let a = 2

// Declare a variable named `b`.
//
var b = 3

// Invalid: cannot re-declare a variable with name `b`,
// as it is already used in this scope.
//
var b = 4

// Invalid: cannot declare a variable with the name `a`,
// as it is already used in this scope,
// and it is declared as a constant.
//
var a = 5

However, variables can be redeclared in sub-scopes:

// Declare a constant named `a`.
//
let a = 1

if true {
    // Declare a constant with the same name `a`.
    // This is valid because it is in a sub-scope.
    // This variable is not visible to the outer scope.

    let a = 2
}

// `a` is `1`

A variable cannot be used as its own initial value:

// Invalid: Use of variable in its own initial value.
let a = a