Why do enums transpile into weird, large JS?

Vanilla JS has no concept of enums, just primitives, functions, and objects, so transpilers do their best at turning it into a thing that behaves exactly like one.

enum RealEnum {
  Primary,
  Secondary,
}

transpiles into:

var RealEnum;
(function (RealEnum) {
  RealEnum[RealEnum["Primary"] = 0] = "Primary";
  RealEnum[RealEnum["Secondary] = 1] = "Secondary";
}) (RealEnum || RealEnum = {}));

Big thanks to my team’s tech lead for explaining this.