What do 3 dots/periods/ellipsis in a relay/graphql query mean?

Question

The relay docs contain this fragment:

query RebelsRefetchQuery {   node(id: "RmFjdGlvbjox") {     id     ... on Faction {       name     }   } } 

What does this ... on Faction on syntax mean?

Answer 1

There are two uses of ... related to fragments.

Incorporating a fragment by reference

query Foo {   user(id: 4) {     ...userFields   } }  fragment userFields on User {   name } 

Has the effect of composing the fields from the fragment into the embedding query:

query Foo {   user(id: 4) {     name   } } 

Note that fragments may compose other fragments.

Inline fragments

From the docs:

If you are querying a field that returns an interface or a union type, you will need to use inline fragments to access data on the underlying concrete type.

https://graphql.org/learn/queries/#inline-fragments

These can be used to compose fields in a type-dependent way. For example:

query Foo {   profile(id: $id) {     url     ... on User {       homeAddress     }     ... on Business {       address     }   } } 

In this example, the server will determine whether to return the homeAddress or address field at runtime, based on whether the requested object is a User or a Business.