TypeScript Language Specification v1.5

9.4 Constant Enum Declarations
An enum declaration that specifies a const modifier is a constant enum declaration. In a constant enum
declaration, all members must have constant values and it is an error for a member declaration to specify
an expression that isn't classified as a constant enum expression.
Unlike regular enum declarations, constant enum declarations are completely erased in the emitted
JavaScript code. For this reason, it is an error to reference a constant enum object in any other context
than a property access that selects one of the enum's members. For example:
const enum Comparison {
LessThan = -1,
EqualTo = 0,
GreaterThan = 1
}
var x = Comparison.EqualTo; // Ok, replaced with 0 in emitted code
var y = Comparison[Comparison.EqualTo]; // Error
var z = Comparison; // Error
The entire const enum declaration is erased in the emitted JavaScript code. Thus, the only permitted
references to the enum object are those that are replaced with an enum member value.
9.5 Code Generation
An enum declaration generates JavaScript equivalent to the following:
var ;
(function () {
})(||(={}));
EnumName is the name of the enum.
EnumMemberAssignments is a sequence of assignments, one for each enum member, in order they are
declared, of the form
[[""] = ] = "";
where MemberName is the name of the enum member and Value is the assigned constant value or the
code generated for the computed value expression.
For example, the 'Color' enum example from section 9.1 generates the following JavaScript:
132