One interesting feature of TypeScript is the ability to exhaustively check Enums, such as in switch statements.
Challenge
I am faced with the task of using an Enum as a Generic type to restrict other parameters based on the provided enumVal.
While it may be simpler to overload the function for this basic example, practicality becomes an issue when dealing with numerous possible combinations. For instance, consider the following enum:
Illustration
enum MyEnum {
FOO = 'foo',
BAR = 'bar'
}
Utilizing the Enum directly results in successful compilation:
function exhaustiveSwitch(enumVal: MyEnum) {
switch (enumVal) {
case MyEnum.FOO:
break
case MyEnum.BAR:
break
default:
const _exhaustiveCheck: never = enumVal
}
}
However, incorporating the Enum as a Generic triggers the error message
Type 'EnumVal' is not assignable to type 'never'function exhaustiveSwitchGeneric<EnumVal extends MyEnum> (enumVal: EnumVal) {
switch (enumVal) {
case MyEnum.FOO:
break
case MyEnum.BAR:
break
default:
const _exhaustiveCheck: never = enumVal //Type 'EnumVal' is not assignable to type 'never'
}
}
(view example here)
Inquiry
- Why does using the Enum as a Generic fail in this scenario?
- Is there an alternative method to utilize the exact value provided (e.g.
MyEnum.FOO) for another parameter without employing Generics? (excluding function overloading)
While there might be valid reasons behind this behavior, I believe that employing Generics in this manner should yield similar results to direct usage.