Typescript derive union type from tuple/array values
UPDATE Feb 2019
In TypeScript 3.4, which should be released in March 2019 it will be possible to tell the compiler to infer the type of a tuple of literals as a tuple of literals, instead of as, say, string[], by using the as const syntax. This type of assertion causes the compiler to infer the narrowest type possible for a value, including making everything readonly. It should look like this:
const list = ['a', 'b', 'c'] as const; // TS3.4 syntax
type NeededUnionType = typeof list[number]; // 'a'|'b'|'c';
This will obviate the need for a helper function of any kind. Good luck again to all!
UPDATE July 2018
It looks like, starting with TypeScript 3.0, it will be possible for TypeScript to automatically infer tuple types. Once is released, the tuple() function you need can be succinctly written as:
export type Lit = string | number | boolean | undefined | null | void | {};
export const tuple = <T extends Lit[]>(...args: T) => args;
And then you can use it like this:
const list = tuple('a','b','c'); // type is ['a','b','c']
type NeededUnionType = typeof list[number]; // 'a'|'b'|'c'
Hope that works for people!
UPDATE December 2017
Since I posted this answer, I found a way to infer tuple types if you're willing to add a function to your library. Check out the function tuple() in tuple.ts. Using it, you are able to write the following and not repeat yourself:
const list = tuple('a','b','c'); // type is ['a','b','c']
type NeededUnionType = typeof list[number]; // 'a'|'b'|'c'
Good luck!
ORIGINAL July 2017
One problem is the literal ['a','b','c'] will be inferred as type string[], so the type system will forget about the specific values. You can force the type system to remember each value as a literal string:
const list = ['a' as 'a','b' as 'b','c' as 'c']; // infers as ('a'|'b'|'c')[]
Or, maybe better, interpret the list as a tuple type:
const list: ['a','b','c'] = ['a','b','c']; // tuple
This is annoying repetition, but at least it doesn't introduce an extraneous object at runtime.
Now you can get your union like this:
type NeededUnionType = typeof list[number]; // 'a'|'b'|'c'.
Hope that helps.
Update for TypeScript 3.4:
A new syntax called "const contexts" that will arrive in TypeScript 3.4 will allow for an even simpler solution that doesn't require a function call as demonstrated. This feature is currently in review as seen in this PR.
In short, this syntax allows for the creation of immutable arrays that have a narrow type (i.e. the type ['a', 'b', 'c'] instead of ('a' | 'b' | 'c')[] or string[]). This way, we can create union types from literals as easy as shown below:
const MY_VALUES = <const> ['a', 'b', 'c']
type MyType = typeof MY_VALUES[number]
In alternative syntax:
const MY_VALUES = ['a', 'b', 'c'] as const
type MyType = typeof MY_VALUES[number]