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:

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// Declare a constant named `a`.

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//

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let a = 1

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// Invalid: re-assigning to a constant.

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//

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a = 2

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// Declare a variable named `b`.

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//

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var b = 3

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

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//

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b = 4

Variables and constants must be initialized:

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// Invalid: the constant has no initial value.

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//

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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:

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// Declare a constant named `a`.

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//

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let a = 1

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// Invalid: cannot re-declare a constant with name `a`,

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// as it is already used in this scope.

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//

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let a = 2

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// Declare a variable named `b`.

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//

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var b = 3

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// Invalid: cannot re-declare a variable with name `b`,

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// as it is already used in this scope.

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//

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var b = 4

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// Invalid: cannot declare a variable with the name `a`,

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// as it is already used in this scope,

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// and it is declared as a constant.

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//

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var a = 5

However, variables can be redeclared in sub-scopes:

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// Declare a constant named `a`.

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//

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let a = 1

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if true {

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// Declare a constant with the same name `a`.

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// This is valid because it is in a sub-scope.

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// This variable is not visible to the outer scope.

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let a = 2

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}

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// `a` is `1`

A variable cannot be used as its own initial value:

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

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let a = a