By using the type system, it can be predetermined whether a GraphQL query is valid or not. This allows servers and clients to effectively inform developers when an invalid query has been created, without having to rely on runtime checks.

For our Star Wars example, the file starWarsValidation-test.js contains a number of queries demonstrating various invalidities, and is a test file that can be run to exercise the reference implementation's validator.

To start, let's take a complex valid query. This is a nested query, similar to an example from the previous section, but with the duplicated fields factored out into a fragment:

And this query is valid. Let's take a look at some invalid queries...

A fragment cannot refer to itself or create a cycle, as this could result in an unbounded result! Here's the same query above but without the explicit three levels of nesting:

When we query for fields, we have to query for a field that exists on the given type. So as hero returns a Character, we have to query for a field on Character. That type does not have a favoriteSpaceship field, so this query is invalid:

Whenever we query for a field and it returns something other than a scalar or an enum, we need to specify what data we want to get back from the field. Hero returns a Character, and we've been requesting fields like name and appearsIn on it; if we omit that, the query will not be valid:

Similarly, if a field is a scalar, it doesn't make sense to query for additional fields on it, and doing so will make the query invalid:

Earlier, it was noted that a query can only query for fields on the type in question; when we query for hero which returns a Character, we can only query for fields that exist on Character. What happens if we want to query for R2-D2s primary function, though?

That query is invalid, because primaryFunction is not a field on Character. We want some way of indicating that we wish to fetch primaryFunction if the Character is a Droid, and to ignore that field otherwise. We can use the fragments we introduced earlier to do this. By setting up a fragment defined on Droid and including it, we ensure that we only query for primaryFunction where it is defined.

This query is valid, but it's a bit verbose; named fragments were valuable above when we used them multiple times, but we're only using this one once. Instead of using a named fragment, we can use an inline fragment; this still allows us to indicate the type we are querying on, but without naming a separate fragment:

This has just scratched the surface of the validation system; there are a number of validation rules in place to ensure that a GraphQL query is semantically meaningful. The specification goes into more detail about this topic in the "Validation" section, and the validation directory in GraphQL.js contains code implementing a specification-compliant GraphQL validator.

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