TSConfig

preserveConstEnums

Do not erase const enum declarations in generated code. const enums provide a way to reduce the overall memory footprint of your application at runtime by emitting the enum value instead of a reference.

For example with this TypeScript:

ts
const enum Album {
  JimmyEatWorldFutures = 1,
  TubRingZooHypothesis = 2,
  DogFashionDiscoAdultery = 3,
}
 
const selectedAlbum = Album.JimmyEatWorldFutures;
if (selectedAlbum === Album.JimmyEatWorldFutures) {
  console.log("That is a great choice.");
}
Try

The default const enum behavior is to convert any Album.Something to the corresponding number literal, and to remove a reference to the enum from the JavaScript completely.

ts
"use strict";
const selectedAlbum = 1 /* Album.JimmyEatWorldFutures */;
if (selectedAlbum === 1 /* Album.JimmyEatWorldFutures */) {
    console.log("That is a great choice.");
}
 
Try

With preserveConstEnums set to true, the enum exists at runtime and the numbers are still emitted.

ts
"use strict";
var Album;
(function (Album) {
    Album[Album["JimmyEatWorldFutures"] = 1] = "JimmyEatWorldFutures";
    Album[Album["TubRingZooHypothesis"] = 2] = "TubRingZooHypothesis";
    Album[Album["DogFashionDiscoAdultery"] = 3] = "DogFashionDiscoAdultery";
})(Album || (Album = {}));
const selectedAlbum = 1 /* Album.JimmyEatWorldFutures */;
if (selectedAlbum === 1 /* Album.JimmyEatWorldFutures */) {
    console.log("That is a great choice.");
}
 
Try

This essentially makes such const enums a source-code feature only, with no runtime traces.