Cannot define string enum typescript as string is not assignable

Question

enum Cat {
    cat1 = 'cat1',
    cat2 = 'cat2',
    cat3 = 'cat3',
    cat4 = 'cat4',
    cat5 = 'cat5'
}
const category: Cat = 'cat' + cat

Why do I get a typescript error for that? cat is a number, so category is a string. But I want to define some specific string values for this variable.

Type 'string' is not assignable to type 'Cat'.

Answer 1

I assume you want category to be one of Cat, not the enum itself. so

const cat = 4;
const category = Cat[`cat${cat}`] // category: Cat.cat4

This also gives you type safety if trying to access a number out of range. playground

enum Cat {
    cat1 = 'cat1',
    cat2 = 'cat2',
    cat3 = 'cat3',
    cat4 = 'cat4',
    cat5 = 'cat5'
}

const cat = 4;
const category = Cat[`cat${cat}`]

const cat6 = 6;
const category6 = Cat[`cat${cat6}`] // Property 'cat6' does not exist on type 'typeof Cat'.

Answer 2

Typescript cant ensure that cat is 1,2,3,4, or 5 since it could also be someThingElse . Therefore you have to tell typescript that you are sure its gonna be of type Cat

The issue here is that the compiler wont allow 'cat' + someVar to be used as type Cat .

Please be cautious when using this, since you essentially overwrite the compilers error. You really need to make sure that whatever you're doing before will always be a valid cat.

enum Cat {
  cat1 = 'cat1',
  cat2 = 'cat2',
  cat3 = 'cat3',
  cat4 = 'cat4',
  cat5 = 'cat5',
}
const category: Cat = (('cat' + cat) as Cat);

enum Cat {
  cat1 = 'cat1',
  cat2 = 'cat2',
  cat3 = 'cat3',
  cat4 = 'cat4',
  cat5 = 'cat5',
}

// this would be of however [Cat.cat1, Cat.cat2 ...] would be a lot safer. 
// I would generally suggest not tot use dynamic enum values like this.
for (const i of [1,2,3,4,5]) {
  const category: Cat = (('cat' + i) as Cat);
}

// the compiler would allow this as well, since you ensure that you know the type is going to be if type Cat
for (const i of ['dog']) {
  const category: Cat = (('cat' + i) as Cat);
}