String enum too verbose to use

[Dominator008]

TypeScript Version: 2.1.1 / nightly (2.2.0-dev.201xxxxx)

Code

type Axes = "x" | "y" | "z";
const Axes = {
  X: "x", // Not X: "x" as Axes
  Y: "y",
  Z: "z"
}

function test(axes: Axes) {}

test(Axes.X); // Argument of type 'string' is not assignable to parameter of type 'Axes'.

Expected behavior:
No error
Actual behavior:
Assignability error

Since Axes is declared as a const, I expect the type inference to resolve Axes.X to "x" without the need for "as Axes".

[jesseschalken]

Axes is const, so someone can't do Axes = .., but the properties of the object referred to be Axes are still mutable, so someone could do Axes.X = "foo", which is allowed since TypeScript inferred the X property to be of type string, not "x".

There's probably a few ways to do what you want. Here's one:

type Axes = "x" | "y" | "z";
namespace Axes {
  export const
    X = "x",
    Y = "y",
    Z = "z"
}

function test(axes: Axes) {}

test(Axes.X);

You could also use an enum, but the values have to be numbers then.

[Dominator008]

Thanks. Hmm. I think I used "x" as Axes when I tried to reassign a random string to Axes.X, so it is indeed working as intended.

Using namespace is not always ideal as it compiles to an IIFE and bloats code size. I came up with the following:

type Axes = {
  readonly X: "x",
  readonly Y: "y",
  readonly Z: "z",

}
const Axes: Axes = {
  X: "x",
  Y: "y",
  Z: "z",
}

function test(axes: Axes[keyof Axes]) {}

test(Axes.X);

Still verbose but you get the real const check.

[mhegazy]

String enums is tracked by #1206