Why write code like that, instead of extending the class with a mandatory property? The above code is going to be inscrutable to a lot of engineers, and this isn't something like an ORM where there's a good reason for that.
The other thing is that types are more often used than read. You don't need to read the `MandatoryFields` type definition often, because your IDE/typechecker will automatically enforce the contract and tell you when you're missing properties.
Another reason could be a generic interface. If you have a lifecycle where a type is mutable at one point but immutable at later points, you could use mapped types to enforce those constraints on the class methods generically.
[0]: https://www.typescriptlang.org/play?#code/MYGwhgzhAEAKCmAnCB...
Even though the code may seem inscrutable, note that the resulting type is fairly easy to understand in your IDE. That is, if you hover over the "B" to see what the type definition is, you see:
type B = A & {
foo: number;
baz: number;
}
> Type 'number' is not assignable to type 'never'.(2322)
Here typescript is saying that a prop cannot have a value (type never) because the base class defines it as "number?" but the extended one defines it as "string", and the intersection between them is empty. This is hard to understand when it pops out where you don't expect it. Harder than ignoring the weird "Ensure" thing, seeing what it does (the resulting type B definition) and moving on.
Defining advanced types may be cumbersome, but dealing with code that uses them is still approachable. This allows the more experienced team members to "shape the ground" and less experienced members still reap the benefits even if they don't fully understand how the thing works.
The advanced programmer simply writes "type Ensure<T, K extends keyof T> = T & { [U in keyof Pick<T, K>]-?: T[U] };", thus removing the need to copy the property.
The master programmer copies the property definition.
The beginner programmer copy/pastes the code.
The advanced programmer writes a function, thus removing the need to maintain copies of the code.
The master programmer copy/pastes the code.
It is not like "Ensure" would be a single-use thing. "Ensure" here is a utility type definition, which is what a "function in the world of types" would be.
> Without arguments, your comment has as much substance as me replying:
Yes, what you substituted is equivalent, and they likely could have written that to the exact same effect.
> It is not like "Ensure" would be a single-use thing. "Ensure" here is a utility type definition, which is what a "function in the world of types" would be.
There's a few ways to interpret the stanza. The way I interpreted it is that the beginner uses a library to provide the definitions, the advanced programmer just writes their own definitions inline as needed, and the master programmer uses a library for the definitions they need (whether written by themself or someone else).
In that respect, I think you're both in agreement.
For what it's worth, I think comments like these are generally beneficial, if maybe I prefer at least a line of context. That they can be interpreted differently and may require a bit of thought to map onto the current context can sometimes allow people to view their beliefs from a slight remove where more introspection is possible, or spur interesting tangents to explore. Both of those are generally beneficial in a forum like this, IMO.
I'm not sure how often you would need type helpers in application code, for frameworks and libraries they are a godsend.
Your argument is a bit like saying we don't need parameterized types like Array<T> because you can just copy paste the code for each type.
So: FormElement<string|undefined> needs to have a "disabled" function, indicating conditions under which it becomes disabled (and absent from the model), while FormElement<string> must not have a disabled function, as it will always be present in the model.
One pitfall of this approach is it requires a lot of trial and error to find the incantation that both works and doesn't swallow error messages.