Mastering TypeScript 3 - Third Edition

Const enums

A slight variant of the enum type is the const enum, which simply adds the keyword const before the enum definition, as follows:

const enum DoorStateConst { 
    Open, 
    Closed, 
    Ajar 
} 
var constDoorOpen = DoorStateConst.Open; 
console.log(`constDoorOpen is : ${constDoorOpen}`); 

const enums have been introduced largely for performance reasons. Let's take a quick look at the JavaScript that is generated from this DoorStateConst enum:

var constDoorOpen = 0 /* Open */; 

Note how the compiler has simply resolved the DoorStateConst.Open enum to its internal value of 0, and removed the const enum definition entirely.