I don’t understand the following error:
type Prefix = 'Ms' | 'Mrs' | 'Mr' const broken = <T extends Prefix>(prefix: T): T => { // do something return 'Ms'; // If I do `return 'Ms' as 'Ms'` then it works } const works = <T extends Prefix>(prefix: T): T => { // do something return p; } const alsoWorks = (): Prefix => { // do something return 'Ms'; }
The method broken
is giving me
Type ‘”Ms”‘ is not assignable to type ‘T’. ‘”Ms”‘ is assignable to the constraint of type ‘T’, but ‘T’ could be instantiated with a different subtype of constraint ‘Prefix’.
Not sure why this method doesn’t work, but the other two do?
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Answer
TypeScript is complaining that the following call will not work:
const res: 'Mr' = broken<'Mr'>('Mr');
If T
is instantiated to something else than Ms
, your return 'Ms'
is violating the the return type T
.