IsTypeOfFn about graphql HOT 4 CLOSED

Is definitely in graphql-js, see this test:

https://github.com/graphql/graphql-js/blob/a941554fc56d607fecebe2188a7ca8848835c2a8/src/execution/__tests__/abstract.js#L44-L52

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oops.. ya, I am not sure how I missed it in the search. We don't use it now, so may keep this open to track it ???

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Thanks @EmergentBehavior for pulling that out from graphql-js.

For reference, here's the ported test:

To elaborate further on IsTypeOf, it's usually would be used to check for the type of the value at runtime.
More specifically, for abstract types (i.e. interfaces and unions).

So a typical use-case would be when you have the following schema (as defined in the TestIsTypeOfUsedToResolveRuntimeTypeForInterface test)

// Using GraphQL type system shorthand notation, refer to: http://graphql.org/docs/typesystem/

// `Pet` is an interface, with a `name` field
interface Pet {
  name: String
}
// `Dog` implements `Pet`
type Dog : Pet {
  name: String
  woofs: Boolean
}
// `Cat` implements `Pet`
type Cat : Pet {
  name: String
  meows: Boolean
}
type Query {
  pets: [Pet] // `pets` returns a list of `Pet` interface
}

So, given the following query:

query Q {
  pets {
    name
  }
}

The Resolve function for pets field may return list of values that are of a Cat type or a Dog type (or any other type that implements Pet interface).

IsTypeOf function then will be executed at runtime to determine it's concrete type, i.e. if the value return is a Cat type or a Dog type.

Regarding it being on the spec, it is there but it's very subtle:

The interface type should have some way of determining which object a given result corresponds to. Once it has done so, the result coercion of the interface is the same as the result coercion of the object.

• http://facebook.github.io/graphql/#sec-Interfaces

The union type should have some way of determining which object a given result corresponds to. Once it has done so, the result coercion of the union is the same as the result coercion of the object.

• http://facebook.github.io/graphql/#sec-Unions

IsTypeOf is just one way of implementing that spec.

Note that IsTypeOf is defined in the concrete types (i.e. in graphql.ObjectConfig)
You can also choose to define ResolveType function in the abstract types (see graphql.InterfaceConfig and graphql.UnionConfig).

So graphql-js/graphql-go provides two ways to satisfy the above specs.

Hope that make sense, cos it's past midnight here lol. Cheers!

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@sogko thanks you so much for your time explaining it. I did a code search and missed the the usage (it was few commits behind). Thanks again.

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