TypeScript array to string literal type
This answer is out of date, see answer above.
The best available workaround:
const furnitureObj = { chair: 1, table: 1, lamp: 1 };
type Furniture = keyof typeof furnitureObj;
const furniture = Object.keys(furnitureObj) as Furniture[];
Ideally we could do this:
const furniture = ['chair', 'table', 'lamp'];
type Furniture = typeof furniture[number];
Unfortunately, today furniture
is inferred as string[]
, which means Furniture
is now also a string
.
We can enforce the typing as a literal with a manual annotation, but it brings back the duplication:
const furniture = ["chair", "table", "lamp"] as ["chair", "table", "lamp"];
type Furniture = typeof furniture[number];
TypeScript issue #10195 tracks the ability to hint to TypeScript that the list should be inferred as a static tuple and not string[]
, so maybe in the future this will be possible.
TypeScript 3.4+
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.
TypeScript 3.0+
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];
More about generic rest parameters
easiest in typescript 3.4: (note TypeScript 3.4 added const assertions)
const furniture = ["chair", "table", "lamp"] as const;
type Furniture = typeof furniture[number]; // "chair" | "table" | "lamp"
also see https://stackoverflow.com/a/55505556/4481226
or if you have these as keys in an object, you can also convert it to a union:
const furniture = {chair:{}, table:{}, lamp:{}} as const;
type Furniture = keyof typeof furniture; // "chair" | "table" | "lamp"