Something is amiss with this, or it's not been released yet. Still receiving the 'ol error TS1337: An index signature parameter type cannot be a union type. Consider using a mapped object type instead. errors. Ryan Cavanaugh (@RyanCavanaugh) can you confirm it's been released as of 4.3.5?

Something is amiss with this, or it's not been released yet. Still receiving the 'ol error TS1337: An index signature parameter type cannot be a union type. Consider using a mapped object type instead. errors. Ryan Cavanaugh (@RyanCavanaugh) can you confirm it's been released as of 4.3.5?

It's currently in the 4.4 release which is in beta scheduled to be released by the end of August.

Anders Hejlsberg (@ahejlsberg) Could you please clarify why those cases work differently in terms of assignability (playground)? AFAIU for a type alias and anonymous type it uses an empty literal type from getTypeFromTypeLiteralOrFunctionOrConstructorTypeNode without resolving its members, but is it the expected behavior?

const sym = Symbol();

function gg3(x: { [key: string]: string }, z: { [sym]: number }) {
  x = z;  // Ok
}

interface GG4 {
    [sym]: number;
}

function gg4(x: { [key: string]: string }, z: GG4) {
  x = z; // Error
}

type GG5 = {
    [sym]: number;
}

function gg5(x: { [key: string]: string }, z: GG5) {
  x = z; // Ok ?!
}

Is this in and working? I'm sitting on v4.4.2 and still receiving:

An index signature parameter type cannot be a union type. Consider using a mapped object type instead

type Test = 'one' | 'two';

export interface Foo {
    [key: Test]: any;
}

Douglas Gaskell (@douglasg14b)

You can do it using type and in (although not with interface).

type Test = 'one' | 'two';
type Foo= {
  [key in Test]: any;
}
//type Foo = {
//    one: any;
//    two: any;
//}

type Foo2= {
  [key in `${Test}${Test}`]: any;
}
//type Foo2 = {
//    oneone: any;
//    onetwo: any;
//    twoone: any;
//    twotwo: any;
//}

playground

I believe that because there is no "free" string or number template, this is old hat, and not related to this new feature.

Anders Hejlsberg (@ahejlsberg) - This is really a great feature, thanks!

I noticed that spaces seem to count as digits with the number type. How about not allowing that?
Of course with unquoted literal labels it doesn't matter, but with quoted string indices it does.

type TP4 = {[k:`${number}`]:null}
const tp4_1:TP4 = {'1':null}
const tp4_2:TP4 = {' ':null}   // even though there is no number present, it passes
const tp4_3:TP4 = {' 1':null}
const tp4_4:TP4 = {'1 ':null}
const tp4_5:TP4 = {' 1 ':null}

playground

Index signatures for template literal string with generic?

type Props<T extends string> = {
  renders: {[key in T]: any};
  [key: `render${Capitalize<T>}`]: any;
};

TS 4.5.2 says:

An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.(1337)

1. If there're other keys needing restricting value type, how should I put it?

type Test = 'one' | 'two';
type Foo= {
  stringKey: string; // A mapped type may not declare properties or methods.
  numberKey: number;
  [key in Test]: any;
}

playground

2. Can this feature be used in interfaces?

enum Test {
  ONE = 'one',
  TWO = 'two'
}
interface Foo {
  [key: Test]: any; // An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.
}

playground

1. If there're other keys needing restricting value type, how should I put it?

As two separate mapped objects