Resolution
Instead of using NonNullable, consider utilizing it this way
type MyType = {
func?: (value: number) => void;
other: string;
};
type NewType = {
func: NonNullable<MyType['func']>;
}
function greet(): NewType {
return {
func: value => value * 2,
};
}
const result = greet().func(25);
Practice Field
Cause
Examine the provided code - Practice Field
type MyType = {
func?: (value: number) => void;
other: string;
};
type X = MyType['func'] // ((value: number) => void) | undefined)
type UsingRequired = Required<X> // ((value: number) => void) | undefined
type UsingNonNullable = NonNullable<X> // (value: number) => void
type UsingRequiredAlt = Required<string | undefined> // string | undefined
type UsingNonNullableAlt = NonNullable<string | undefined> // string
The property func within MyType is optional, meaning it could be undefined. Therefore, the type of X is
((value: number) => void) | undefined
The Required<TypePassed> utility type makes properties of TypePassed mandatory by eliminating any occurrences of undefined from them, not affecting the type TypePassed itself.
Hence, the type of UsingRequiredAlt remains as string | undefined
In contrast, NonNullable<TypePassed> eliminates both null and undefined from TypePassed
As a result, the type of UsingNonNullableAlt transforms into just string.