How to pass default GraphQL arguments to AWS AppSync resolver

Question

AWS AppSync allow to define default values in schema like that

type Foo {
  bar(
    arg: Int = 20
  ): Bar!
}

or that

type Foo {
  bar(
    arg: Int! = 20
  ): Bar!
}

But either way when the value is not explicitly defined, the default value is not passed to the resolver.

Is there something I should opt-in to activate the default value to be passed? Is that an AWS bug? If so, is there a workaround?

PS: In the GraphQL specs

If no value is provided for a defined input object field and that field definition provides a default value, the default value should be used. If no default value is provided and the input object field’s type is non‐null, an error should be thrown. Otherwise, if the field is not required, then no entry is added to the coerced unordered map.

Answer 1

Default arguments are currently a known issue and the best way to get around them is to use the $util.defaultIfNull() velocity helper function. For example, you can default arguments for limit and next token doing something like this:

{
  "version": "2017-02-28",
  "operation": "Scan",
  "limit": $util.defaultIfNull($ctx.args.first, 20),
  "nextToken": $util.toJson($util.defaultIfNullOrEmpty($ctx.args.after, null)),
}