Consider this example of a function that takes a union type:
const isNull = (value: string | null) => value === null
How can I allow only this kind of usage (where the value is possibly null):
const maybeNull = Math.random() < 0.5 ? 'value' : null
isNull(maybeNull)
While disallowing this (where the value is definitely not null):
const definitelyNotNull = 'value'
isNull(definitelyNotNull) // should error
CodePudding user response：
To do this cleanly you'd need some kind of negated type (see for example issue 4183), which is not possible in TypeScript, but as a workaround you can make isNull generic and validate that null extends the parameter type:
type MaybeNull<T> = null extends T ? T : never
const isNull = <T>(value: MaybeNull<T>) => value === null
This works for the example applications, though the error isn't amazing.
const maybeNull = Math.random() < 0.5 ? 'value' : null
isNull(maybeNull) // Allowed
const definitelyNotNull = 'value'
isNull(definitelyNotNull)
// Error: Argument of type 'string' is not assignable to parameter of type 'never'.