type x = 'first' | 'second'
I am looking to create a type y that is similar to this:
type y = 'FIRST' | 'SECOND'
Here is what I attempted:
type x = 'first' | 'second'
type y = {[key in x]: key['toUpperCase']}
Unfortunately, this did not achieve the desired outcome.
Appreciate you taking the time to read :)
With the advent of Template Literal Types, you can now achieve the following:
type A = 'one' | 'two'
type B = Uppercase<A>
let b: B = 'one' // Type '"one"' is not assignable to type '"ONE" | "TWO"'.
In addition to using Uppercase<StringType>, there are other useful helper types available as well:
You can incorporate them into Template Literal Types like so:
type Fruit = 'Apple' | 'Banana'
type FruitField = `fr_${Uncapitalize<Fruit>}`
const fruit: Record<FruitField, boolean> = {
'fr_apple': true,
'fr_banana': false,
'fr_Apple': true, // error
'fr_peach': false // error
}
Click on this link to access TS Playground
Here is the code snippet:
type A = 'one' | 'two'
type B = Uppercase<A>
let b: B = 'one' // This line will throw an error
On a side note, I find it confusing why a more complex method was used in the previous response to convey the same message.
UPDATE: Derek Nguyen (top answer) has revised their solution with a simpler approach.
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