TypeScript strict enum checking

Question

Is there any way to enforce strict usage of enum? Some examples:

enum AnimalType {
  Cat,
  Dog,
  Lion
}

// Example 1
function doSomethingWithAnimal(animal: AnimalType) {
  switch (animal) {
    case Animal.Cat: // ...
    case Animal.Dog: // ...
    case 99: // This should be a type error
  }
}

// Example 2
someAnimal.animalType = AnimalType.Cat; // This should be fine
someAnimal.animalType = 1; // This should be a type error
someAnimal.animalType = 15; // This should be a type error

Basically, if I say that something has an enum type, then I want the TypeScript compiler (or alternatively tslint) to make sure that it is used correctly. With the current behavior I don't really understand the point of enums as they are not enforced. What am I missing?

T.J. Crowder · Accepted Answer · 2018-10-24 08:10:52Z

7

It's an intentional design decison on the TypeScript team's part to enable bitflags, see this issue for more details. Reading that issue and the various ones it links to, I get the distinct sense they wish they'd separated enums and bitflags originally but can't get themselves around to the place of making the breaking change / adding a flag.

It works the way you want it to with a string enum rather than a numeric one:

enum AnimalType {
  Cat = "Cat",
  Dog = "Dog",
  Lion = "Lion"
}

// Example 1
function doSomethingWithAnimal(animal: AnimalType) {
  switch (animal) {
    case AnimalType.Cat: // Works
    case AnimalType.Dog: // Works
    case "99": // Error: Type '"99"' is not assignable to type 'AnimalType'. 
  }
}

// Example 2
const someAnimal: { animalType: AnimalType } = {
  animalType: AnimalType.Dog
};
let str: string = "foo";
someAnimal.animalType = AnimalType.Cat; // Works
someAnimal.animalType = "1"; // Type '"1"' is not assignable to type 'AnimalType'.
someAnimal.animalType = str; // Error: Type 'string' is not assignable to type 'AnimalType'.

Live Example in the Playground

edited Oct 24, 2018 at 8:10

answered Oct 18, 2018 at 12:04

T.J. Crowder

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2 Comments

Frigo Over a year ago

Well that's unfortunate... Very strange behavior, in my opinion. I don't like the answer, but I'll accept it! :)

T.J. Crowder Over a year ago

@Frigo - Yeah. :-) I've added a new first paragraph above that may be useful.

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