Incorrect error generated when comparing enum values.

[dbaeumer]

TypeScript Version: 2.0.2
Code

enum Direction {
    LEFT,
    RIGHT
}

function bug(direction?: Direction): number {
    if (!direction) {
        return 10;
    } else {
        return (direction === Direction.LEFT) ? 10 : 0;
    }
}

You get a compile error saying the Direction can't be used in ===. This is IMO incorrect.

[mhegazy]

Direction.LEFT is 0, it will be filtered out in !direction check. so the only value direction could have in the else clause is Direction.RIGHT; and Direction.RIGHT will never be Direction.LEFT.

So depending on what is the intent here, either make LEFT non-false:

enum Direction {
    LEFT = 1,
    RIGHT
}

or just trust that this will produce the same result:

    if (!direction) {
        return 10;
    } else {
        return  0;
    }

[dbaeumer]

You are absolutely right and that error actually catches a problem in our code. JS falsy ....

[dbaeumer]

Closing.

[basarat]

JS falsy

I've had this happen to be even before it was an error. Hence : https://basarat.gitbooks.io/typescript/content/docs/enums.html#changing-the-number-associated-with-an-enum I added I quite commonly initialize the first enum with = 1 as it allows me to do a safe truthy check on an enum value. Can help fix that error quickly ;) 🌹

[forthedamn]

I encountered the same situation, and solved in same way.

initialize the first enum with = 1

That will be great!