Typescript peculiarities in mapped types

Question

I recently stumbled upon the following snippet:

type NamedProperty<Name extends string, T> 
  = { [_ in 0 as Name]: T }

This is later used like below:

type TargetPropertyGroup<Name extends string> 
  = NamedProperty<`has${Capitalize<Name>}Target`, boolean> 
  & NamedProperty<`${Name}Target`, Element> 
  & NamedProperty<`${Name}Targets`, Element[]>

Can anybody help me figure out what [_ in 0 as Name] means?

I also found this bit in the playground but still...

to me the `in 0 as` is useless, `{[_ in Name]: T}` achieves the same here. But am I missing something ? — Matthieu Riegler, Feb 17 '22 at 14:05
Here's the relevant section in the [mapped types documentation](https://www.typescriptlang.org/docs/handbook/2/mapped-types.html#key-remapping-via-as) for key remapping with `as`. Is there some specific question that the documentation doesn't already answer? — Emma, Feb 17 '22 at 14:06
@MatthieuRiegler What `{[_ in Name]: T}` does even mean? Why can't we just do `{[Name]: T}`? Also, `in 0` should have spit out zero iterations, right? — stratis, Feb 17 '22 at 14:25
I also noticed that rewriting this as `type NamedProperty = { [Name: string]: T }` doesn't work the same way. — stratis, Feb 17 '22 at 14:50

Matthieu Riegler · Accepted Answer · 2022-02-17T15:30:23.637

{ [_ in 0 as Name]: T } means the same thing as { [_ in Name]: T } as the key remapping changes nothing here.

In mapped type, the left part defines the keys, very ofter we see it as [k in MyType] or [key in MyType]. Since the key won't be reused, its replace by an underscore (like it's ofter the case in JS/TS with unused function parameters).

So [_ in Name] means every key in the type name. Since Name is just a string, the mapped type will have only one key, the string.

{ [_ in 0 as 'myKey']: number }  ===  { myKey: number }

In the example you give { [Name: string]: T }, Name is not a type but a "variable". As you can see in the playground, the generic parameter is unused. In mapped types, [Name: string] every key that is of type string.

About the key remapping,

Per the documentation, the key remapping can be used to infer the keys from template literal types.

Typescript peculiarities in mapped types

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