Literal String Union Autocomplete

[MrTarantula]

Autocomplete works for literal string unions, but adding a union of string negates autocomplete entirely. This has been brought up before but I believe there is enough value in this feature to be reconsidered.

My use case is to have a union of string literals for several colors, but also allow hex codes without having to add 16.7 million string literals.

TypeScript Version: 3.4.0-dev.20190202

Search Terms: Literal string union autocomplete

Code

interface Options {
 borderColor: 'black' | 'red' | 'green' | 'yellow' | 'blue' | string
};

const opts: Options = {borderColor: 'red'};

Expected behavior:

Actual behavior:

Playground Link: https://stackblitz.com/edit/typescript-bwyyab

Related Issues: #12687 #13614

[RyanCavanaugh]

'black' | 'red' | 'green' | 'yellow' | 'blue' | string

From the compiler's point of view, this is just a very fancy way of writing string. By the time we're looking for suggestions at borderColor, all the literals are lost.

You could write something like this:

Naturally this doesn't stop you from writing "bluck". You might want to track #6579

[sindresorhus]

By the time we're looking for suggestions at borderColor, all the literals are lost.

Why not improve the compiler to keep more metadata around?

[MrTarantula]

You could write something like this:

It may accomplish the same behavior, but that's not intuitive to me at all. I doubt I could have gotten there on my own, and I know I would have trouble explaining it to someone newly coming to TS from JS.

[spcfran]

It would be great to have this built in, although understand it may be difficult to implement on the compiler.

In the meantime, a generic workaround based on Ryan Cavanaugh (@RyanCavanaugh)'s solution might help:

type LiteralUnion<T extends U, U = string> = T | (U & { zz_IGNORE_ME?: never })

type Color = LiteralUnion<'red' | 'black'>

var c: Color = 'red' // Has intellisense
var d: Color = 'any-string' // Any string is OK
var d: Color = { zz_IGNORE_ME: '' } // { zz_IGNORE_ME } placeholder is at the bottom of intellisense list and errors because of never 

type N = LiteralUnion<1 | 2, number> // Works with numbers too

See it in action here

[BendingBender]

Would be great if this could be implemented. It has the potential to improve even core Node.js APIs. The hash.digest encondig param, for example should accept plain strings but there are values available on all platforms that could be enumerated for ease of use.

[neurolag]

Hey Guys
Though the issue still isn't solved but I found out that in TS 3.5.1 you don't even have to create some weird property in order to get the workaround done:

type LiteralUnion<T extends U, U = string> = T | (U & {});
let x: LiteralUnion<"hello" | "world">;
x = "hey";

While this code is perfectly valid, "hello", and "world" are still showing up in the autocompletion.
Thank you for this great fix, Ryan Cavanaugh (@RyanCavanaugh) and Fran Hodl (@spcfran)!