const enum vs enum

An enum can be declared with or without the const keyword. Here are the examples of a regular enum:

enum Direction {
    Up,
    Down,
    Left,
    Right
}

and a const enum:

const enum Light {
    Red,
    Green,
    Blue
}

Differences

TypeScript compiles regular enum to JavaScript objects. Given the Direction enum above, it will be transpiled to the following JavaScript code:
```
var Direction;
(function (Direction) {
    Direction[(Direction['Up'] = 0)] = 'Up';
    Direction[(Direction['Down'] = 1)] = 'Down';
    Direction[(Direction['Left'] = 2)] = 'Left';
    Direction[(Direction['Right'] = 3)] = 'Right';
})(Direction || (Direction = {}));
```
On the other hand, the Light enum is not transpiled at all. You will see nothing if the enum is not used.

In the other cases, all the enum references are replaced by the inline codes. For example, console.log(Light.Red) is compiled as console.log(0 /* Red */).
Because there is no JavaScript object that associates with const enum is generated at run time, it is not possible to loop over the const enum values.

TypeScript will throw an error when we try to iterate over the Light enum:
```
// ERROR
for (let i in Light) {
    console.log(i);
}
```

Good to know

If you do not want TypeScript to erase the generated code for const enums, you can use the preserveConstEnums compiler flag.

See also

literal union type vs string enums

concat vs push const vs let vs var