I currently have both an array of strings and a string literal union type containing the same strings:
const furniture = ['chair', 'table', 'lamp']; type Furniture = 'chair' | 'table' | 'lamp';
I need both in my application, but I am trying to keep my code DRY. So is there any way to infer one from the other?
I basically want to say something like
type Furniture = [any string in furniture array], so there are no duplicate strings.
TypeScript version 3.4 has introduced so-called **const contexts**, which is a way to declare a tuple type as immutable and get the narrow literal type directly (without the need to call a function like shown above).
With this new syntax, we get this nice concise solution:
const furniture = ['chair', 'table', 'lamp'] as const; type Furniture = typeof furniture[number];
More about the new const contexts is found in this PR as well as in the release notes.
With the use of generic rest parameters, there is a way to correctly infer
string as a literal tuple type and then get the union type of the literals.
It goes like this:
const tuple = <T extends string>(...args: T) => args; const furniture = tuple('chair', 'table', 'lamp'); type Furniture = typeof furniture[number];
Answered By – ggradnig
Answer Checked By – Robin (BugsFixing Admin)