Typescript generic interface parameter assignment

Question

I’m struggling to understand why this example isn’t considered valid by the typescript compiler:

interface IExample<T> {
  param: T
}

function testFunc<U, I extends IExample<U>>(myParam: U): I {
  return { param: myParam };
}

The error produced is:

Type 'U' is not assignable to type 'I["param"]'.

My (assumedly incorrect) reading of this snippet is:

IExample<T> expects param to have type T.
I is a subtype of IExample<U>, meaning param has type U.
myParam has type U from the parameter annotation.
Therefore myParam should be a valid value for param of I.

Prefixing the return value with <I> clears the error, so why does the error appear in the first place?

I would have expected a different error message: something like { param: U } isn't assignable to generic type I: I is an unknown type, which could have many more attributes and methods than just param. So { param: myParam } is almost guaranteed to not be of type I. — JB Nizet
– JB Nizet, Commented Nov 12, 2018 at 17:38
Found a duplicate which I missed before posting: stackoverflow.com/q/40690797/1813169 — MTCoster
– MTCoster, Commented Nov 12, 2018 at 17:54

Titian Cernicova-Dragomir · Accepted Answer · 2018-11-12 17:38:47Z

1

The problem is that I might have other required properties in addition to param. Typescript will force your generic function implementation to return a valid value for any I that satisfies the constraint of extending IExample<U>, and it does not.

For example:

interface DerivedIExample extends IExample<number> {
  other : string
} 
let o = testFunc<number, DerivedIExample>(0)
o.other // required by the DerivedIExample but not assigned

In your simple example, it would be best to do away with I completely:

function testFunc<U>(myParam: U): IExample<U> {
  return { param: myParam };
}

You can force the compiler to accept your code using a type assertion but as outlined above that is not type-safe:

function testFunc<U, I extends IExample<U>>(myParam: U): I {
  return { param: myParam } as I; // NOT TYPE SAFE! USE WITH CARE !
}

answered Nov 12, 2018 at 17:38

Titian Cernicova-Dragomir

253k37 gold badges464 silver badges394 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

MTCoster Over a year ago

I don’t know why I didn’t just eliminate I in the first place, thanks! Is it worth opening an issue to note the unhelpful(/misleading?) error message?

MTCoster Over a year ago

It turns out the error has already been fixed in the nightly builds, it’s now: Type '{ param: U; }' is not assignable to type 'I'.

Collectives™ on Stack Overflow

Typescript generic interface parameter assignment

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related