elementof? Can I accomplish this?

Question

Here's what I want to accomplish:

const names = ["foo", "bar", "baz"];
type NameType = elementof names;  // This is not valid TypeScript

The behavior would be identical to this:

type NameType = "foo" | "bar" | "baz";

A solution will need to accomplish a few things.

Provide an ordered enumerable sequence of strings.
Expose the names as an addressable union of string literal types.
Allow string maintenance without redundant source code edits.

Can TypeScript do this?

Note: This is not the same as Convert string[] literal to string type literal, because that question doesn't require a specifically ordered sequence of strings. Union types don't produce any way to get order information at run time.

Possible duplicate of [Convert string\[\] literal to string type literal](https://stackoverflow.com/questions/44686104/convert-string-literal-to-string-type-literal) — jonrsharpe, Jun 05 '18 at 16:26
Possibly answered in [TypeScript String Union to String Array](https://stackoverflow.com/questions/44480644/typescript-string-union-to-string-array) — jcalz, Jun 05 '18 at 16:30
@SLaks `["foo", "bar", "baz"]` is indeed a type: a [tuple](https://www.typescriptlang.org/docs/handbook/basic-types.html#tuple) of [string literals](https://www.typescriptlang.org/docs/handbook/advanced-types.html#string-literal-types). — jcalz, Jun 05 '18 at 16:33
@jcalz while that version works, the tuple type seems overkill, you don't really need tuple here .. just my 2c — Titian Cernicova-Dragomir, Jun 05 '18 at 16:35
Yeah it depends if the ordering of the elements is important to represent in the type level, which is up to @recursive to decide — jcalz, Jun 05 '18 at 17:17

score 3 · Accepted Answer · edited Oct 02 '19 at 01:36

3

You can get the type of an item using a type query:

type NameType = typeof names[number];

The problem is that the type of names is string[], so the above code just produces string.

You can infer the string literal types for the const using a helper function.

function array<T extends string>(p: T[]) : T[] {
  return p
}

const names = array(["foo", "bar", "baz"]);
type NameType = typeof names[number];   // The same as "foo" | "bar" | "baz"

edited Oct 02 '19 at 01:36

Peter Mortensen

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answered Jun 05 '18 at 16:33

Titian Cernicova-Dragomir

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This is perfect. When did `typeof names[number]` get introduced. The operand isn't a legal expression on its own. I didn't know you could do that. – recursive Jun 05 '18 at 17:28
1

It's parsed as `(typeof names)[number]`, which is a valid type query. It isn't `typeof (names[number])`, which is indeed an error. – jcalz Jun 06 '18 at 13:36

elementof? Can I accomplish this?

1 Answers1