How to pick properties from deep object? It seems T[key]['default'] is not allowed in Mapped type?
type MyModule = {
title: {
default: string
},
length: {
default: number
}
}
// expected type
type NewMyModule = {
dataModel: string; // type of 'default' prop
length: number; // type of 'default' prop
}
// error message: 'default' can't be used to index type 'T[key]'
type PickDefault<T> = {
[key in keyof T]: T[key]['default']
}
The reason why Typescript yells at you is that it doesn't know whether or not the key default exists on type T[key]. There are two approaches you could take to resolve this issue. The first one is to constraint the input type parameter T that Typescript would know what the type of T[key] is. See below.
type PickDefault<T extends Record<keyof any, { default: unknown }>> = {
[key in keyof T]: T[key]['default'];
}
Something else you could do is to use type inference. See below.
type PickDefault2<T> = {
[key in keyof T]: T[key] extends { default: infer D } ? D : never;
}
A caveat for the second approach is that if default is not found on T[key], it will not raise any errors. Instead, it will just replace the type with never. Whereas the first approach will only allow appropriate types to pass.
