Cannot assign a string literal type in TypeScript to a type parameter that is extending a string literal type

Question

Cannot assign a string literal type in TypeScript to a type parameter that is extending a string literal type

Here is some code snippet that exhibits a specific issue:

type FooType = 'Bar';

abstract class Meow<T extends FooType> {
  baz: T = 'Bar';
}

An error is triggered stating

Type '"Bar"' is not assignable to type 'T'

.

The confusion arises because if baz has the specified type T, why wouldn't it allow any value from FooType?

What adjustments are needed to enable the class property baz to accept any value in line with FooType, as well as other string literals that potential sub-classes might desire?

typescript

Answer 1

Answer №1

When defining a generic type constraint, you are specifying the basic requirements that the type parameters must meet. However, it is important to note that the actual type used could have additional properties not covered by your constraint. This is why TypeScript typically does not allow assignment of literals to generic types.

Let's look at an example:

type FooType = 'Bar';

class Meow<T extends FooType> {
  baz: T = 'Bar';
}

let m = new Meow<'Bar' & { extra: number }>()
m.baz.extra // why is extra not assigned as it's type suggests it should be ?

While this example may seem unusual with string literal types, it showcases how the type system works. There has been discussion about allowing this behavior, but it's uncertain if it will happen.

To address this issue, a simple solution is to use a type assertion:

class Meow<T extends FooType> {
  baz: T = 'Bar' as T;
}

Answer 2

When defining a generic type constraint, you are specifying the basic requirements that the type parameters must meet. However, it is important to note that the actual type used could have additional properties not covered by your constraint. This is why TypeScript typically does not allow assignment of literals to generic types.

Let's look at an example:

type FooType = 'Bar';

class Meow<T extends FooType> {
  baz: T = 'Bar';
}

let m = new Meow<'Bar' & { extra: number }>()
m.baz.extra // why is extra not assigned as it's type suggests it should be ?

While this example may seem unusual with string literal types, it showcases how the type system works. There has been discussion about allowing this behavior, but it's uncertain if it will happen.

To address this issue, a simple solution is to use a type assertion:

class Meow<T extends FooType> {
  baz: T = 'Bar' as T;
}

Cannot assign a string literal type in TypeScript to a type parameter that is extending a string literal type

Answer №1

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