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It’s often useful to ask a GraphQL schema for information about what queries it supports. GraphQL allows us to do so using the introspection system!

For our Star Wars example, the file starWarsIntrospection-test.ts contains a number of queries demonstrating the introspection system, and is a test file that can be run to exercise the reference implementation’s introspection system.

We designed the type system, so we know what types are available, but if we didn’t, we can ask GraphQL, by querying the __schema field, always available on the root type of a Query. Let’s do so now, and ask what types are available.

Wow, that’s a lot of types! What are they? Let’s group them:

  • Query, Character, Human, Episode, Droid - These are the ones that we defined in our type system.
  • String, Boolean - These are built-in scalars that the type system provided.
  • __Schema, __Type, __TypeKind, __Field, __InputValue, __EnumValue, __Directive - These all are preceded with a double underscore, indicating that they are part of the introspection system.

Now, let’s try and figure out a good place to start exploring what queries are available. When we designed our type system, we specified what type all queries would start at; let’s ask the introspection system about that!

And that matches what we said in the type system section, that the Query type is where we will start! Note that the naming here was just by convention; we could have named our Query type anything else, and it still would have been returned here had we specified it was the starting type for queries. Naming it Query, though, is a useful convention.

It is often useful to examine one specific type. Let’s take a look at the Droid type:

What if we want to know more about Droid, though? For example, is it an interface or an object?

kind returns a __TypeKind enum, one of whose values is OBJECT. If we asked about Character instead we’d find that it is an interface:

It’s useful for an object to know what fields are available, so let’s ask the introspection system about Droid:

Those are our fields that we defined on Droid!

id looks a bit weird there, it has no name for the type. That’s because it’s a “wrapper” type of kind NON_NULL. If we queried for ofType on that field’s type, we would find the ID type there, telling us that this is a non-null ID.

Similarly, both friends and appearsIn have no name, since they are the LIST wrapper type. We can query for ofType on those types, which will tell us what these are lists of.

Let’s end with a feature of the introspection system particularly useful for tooling; let’s ask the system for documentation!

So we can access the documentation about the type system using introspection, and create documentation browsers, or rich IDE experiences.

This has just scratched the surface of the introspection system; we can query for enum values, what interfaces a type implements, and more. We can even introspect on the introspection system itself. The specification goes into more detail about this topic in the “Introspection” section, and the introspection file in GraphQL.js contains code implementing a specification-compliant GraphQL query introspection system.