9.1 Enum Declarations

An enum declaration declares an enum type and an enum object.

  EnumDeclaration:   const_opt enum BindingIdentifier { EnumBody_opt }

An EnumDeclaration introduces a named type (the enum type) and a named value (the enum object) in the containing declaration space. The enum type is a distinct subtype of the Number primitive type. The enum object is a value of an anonymous object type containing a set of properties, all of the enum type, corresponding to the values declared for the enum type in the body of the declaration. The enum object’s type furthermore includes a numeric index signature with the signature ‘[x: number]: string’.

The BindingIdentifier of an enum declaration may not be one of the predefined type names (section 3.8.1).

When an enum declaration includes a const modifier it is said to be a constant enum declaration. The members of a constant enum declaration must all have constant values that can be computed at compile time. Constant enum declarations are discussed in section 9.4.

The example

enum Color { Red, Green, Blue }

declares a subtype of the Number primitive type called ‘Color’ and introduces a variable ‘Color’ with a type that corresponds to the declaration

var Color: {  
    [x: number]: string;  
    Red: Color;  
    Green: Color;  
    Blue: Color;  
};

The numeric index signature reflects a “reverse mapping” that is automatically generated in every enum object, as described in section 9.5. The reverse mapping provides a convenient way to obtain the string representation of an enum value. For example

var c = Color.Red;  
console.log(Color[c]);  // Outputs "Red"