TS: Use both input and output types in scalars parse/serialize

[honzajerabek]

Hi, currently the parseValue and serialize functions in GraphQLScalarTypeConfig are receiving unknown type in the value arg, but the type is known - it's the inverted type of the function to each other.

...
serialize: (val: TInternal) => TExternal
parseValue: (val: TExternal) => TInternal
...

With current unknown typing there are TS errors when trying to have it strongly typed.

This minimal custom scalar

export const Date = new GraphQLScalarType({
  name: 'Date',
  serialize: (value: Date): number => value.valueOf(),
  parseValue: (value: number): Date => new Date(value),
});

causes these errors:

serialize:

TS2322: Type '(value: Date) => number' is not assignable to type 'GraphQLScalarSerializer<number>'.   Types of parameters 'value' and 'outputValue' are incompatible.     Type 'unknown' is not assignable to type 'Date'

parseValue

TS2322: Type '(value: number) => Date' is not assignable to type 'GraphQLScalarValueParser<Date>'.   Types of parameters 'value' and 'inputValue' are incompatible.     Type 'unknown' is not assignable to type 'number'. 

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Honza Jerabek (28b7a0d)

[honzajerabek]

Let me pls know if this makes sense at all, I'll then fix tests and everything

[IvanGoncharov]

@honzajerabek Thanks for reporting.
It's unknown on purpose since you need to do validation inside both functions.
parseValue should validate query variable and serialize should validate return of resolvers.
graphql-js do all necessary validation for all types except for custom scalars so for custom scalars, you need to do all validation yourself that's why we need to have unknown there.

[honzajerabek]

Okay thanks for explanation

[yaacovCR]

See #3451 (comment) for some follow-on thoughts, perhaps the serializer could be made type-safe....