I want to initialise an array with values based on a type:
type Pets = 'dog' | 'cat';
const PETS: Array<Pets> = []; // ['dog', 'cat'];
Pets is an ever-changing type, which I need to be represented in PETS.
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2 Answers
You could initialize the constant and then create the type based on this constant:
const PETS = ['dog', 'cat'] as const
type Pets = typeof PETS[number];
It's the other way around, but still you only need to write it once.
Comments
It's a bit trickier with types.
There's the option A_A proposed which is somewhat common.
If it doesn't necessarily have to be a type, you can replace it with an enum which will allow you to iterate over it's values.
Example:
enum Pets {
Cat = 'cat',
Dog = 'dog',
}
const pets: Pets[] = (Object.keys(Pets) as Array<keyof typeof Pets>).map(x => Pets[x])
// pets = ['cat', 'dog']
To access values in an enum you can either use their string value or the lefthand property.
For example, both of these should work:
const pets: Pets[]
// First option:
if (pets[0] === 'cat') {
...
}
// Second option:
if (pets[0] === Pets.Cat) {
...
}
Notes:
- Unlike
const enum(which is not iterable),enumtypes add some JS code to the bundle (which TS normally shouldn't do), keep this in mind going forward.
For instance, if you have a great number of regular enums in your project it might add quite a bit of code to your bundle, whether or not that's ok depends on you.
2 Comments
Jake Jackson
From what I can tell, I can just use an
enum. Thanks for you post.
tomleb
Sure thing! edited the post and added some more info. Note the drawback of enums I've mentioned under
Notes.