TypeScript Enumerations Are Numbers

Whilst answering a question on Stack Overflow I realised that TypeScript enumerations may well catch a few people out. In particular, this:

“…Enum types are assignable to the Number primitive type, and vice versa, but different enum types are not assignable to each other…” – TypeScript Language Specification, 3.2.7

This means that you can use any arbitrary number in place of an enum, which means you might choose a number that doesn’t actually exist. For example:

enum BallType {

var ballType: BallType = 100;

Clearly, there are not enough ball types for 100 to be a valid choice as we have enough items to cover 0 – 4 only. Rather than treating the assignment of a number to the variable typed with the BallType annotation, the compiler allows this.

This is the area of hazard for TypeScript enums, but you can add in safety checks…

var ballType: BallType = 100;

if (typeof BallType[ballType] === 'undefined') {
    alert('No ball type for ' + ballType);

You might be a little upset about this implementation detail – but don’t throw the baby out with the bathwater because enums are still useful… not just because you get auto-completion for them and using enums instead of numbers prevents sync issues, but because they prevent this…

var ballType: BallType = SportType.pingPong;