I learned something new about GraphQL today

I’ve been working with GraphQL for three years. I thought I knew what the following definition meant:

type FooInput {
    field: [ValueInput!]!
}

type ValueInput {...}

mutation {
    addFoo(foo: FooInput) : Foo
}

But I was surprised when, in a mutation, GraphQL Playground allowed me to do this:

addFoo(foos: {field: []})

I had always though that [ValueInput!]! meant a non-empty array was required. Yesterday, someone pointed out to me that I was wrong.

So what’s the difference between [ValueInput!]! and [ValueInput]!, then? A brief Googling turned up this StackOverflow post: https://stackoverflow.com/questions/46770501/graphql-non-nullable-array-list

The answer by Daniel Reardon included this great table:

declaration accepts: | null | []   | [null] | [{foo: 'BAR'}]
-------------------------------------------------------------
[Vote!]!             | no   | yes  | no     | yes
[Vote]!              | no   | yes  | yes    | yes
[Vote!]              | yes  | yes  | no     | yes
[Vote]               | yes  | yes  | yes    | yes

Aha! It’s all about what is nullable! Whereas [ValueInput]! allows both [] and [null], [ValueInput!]! only allows []. The extra exclamation mark ensures that if a non-empty array is passed as a parameter, it cannot contain nulls, but it doesn’t require any array elements.

You can’t know everything.