@Arg defaultValue "null" seems not to be recognized

[karladler]

Describe the bug
Setting the defaultValue like explicitly to null in @Arg('id', { defaultValue: null }) seems not to be applied, when omitting the argument id on request.

To Reproduce

  @Query(() => User)
  user(@Arg('id', { defaultValue: null }) id: string): User {
    console.log('id', id);

    if (id === null) {

Logs id undefined;

Expected behavior
Logs id null; when requested without arguments.

Environment (please complete the following information):

• OS: OSX
• Node 12
• Package version: "type-graphql": "^0.17.6",
• TypeScript version: "typescript": "^3.8.3"

[MichalLytek]

@karladler Can you reproduce the default null value using GraphQL SDL?

[eezing]

@MichalLytek Yes, default null is valid in SDL. In addition, providing null as argument in query also produces undefined in type-graphql.

A graphql-js example where null arguments are valid:

https://repl.it/repls/ClearcutRaggedFormat

const { graphql } = require('graphql');
const { makeExecutableSchema } = require('graphql-tools');

const typeDefs = `
  type Query {
    hello(msg: String = null): Boolean
  }
`;

const resolvers = {
  Query: {
    hello: (_, args) => {
      console.log(args); // --> { msg: null }
      return true;
    },
  },
};

const schema = makeExecutableSchema({ typeDefs, resolvers });

const query = `
  query {
    a: hello
    b: hello(msg: null)
  }
`;

graphql(schema, query)

[MichalLytek]

And what is the practical difference between:

hello(msg: String = null)

and

hello(msg: String)

🤔

In both cases if argument is not provided, it gonna be null/undefined, and from client perspective the arg is optional.

[eezing]

Is default null practical? No, not really.

But, an explicit null argument from query does have meaning; for example, delete record if argument is null or leave it untouched if argument is undefined (explicit vs implicit).

[MichalLytek]

But there is no such distinction in GraphQL Specification between null and undefined and nothing about the implicit undefined behavior when value not provided and explicit null when null provided.

It's just a case in JS where we have two empty values representation.
So relying on this behavior means your not using GraphQL but GraphQLJS and when somebody change the language of the implementation of this query (other microservice) event if the schema is the same - it won't work anymore 😕

[eezing]

Ok, I stand corrected. Semantically there is a difference, but like @MichalLytek said, the spec states that we should not treat implicit and explicit null differently.

https://spec.graphql.org/June2018/#sec-Null-Value

[MichalLytek]

More context:
graphql/graphql-js#1298 (comment)

So, if query { hello } gives undefined for hello(msg: String) and null for hello(msg: String = null), maybe just treat undefined and null in the same way, like arg ?? value or if (arg != null) 😉

With hello(msg: String = null) you never have undefined but the null value even if somebody doesn't provide any arg value. So you can't "leave it untouched if argument is undefined".

[karladler]

so { defaultValue: null } is basically doing nothing since it's the same as setting default to undefined.
Since it Maybe there should be a warning generated, bc it's not that logic and if I test later for id !== null the rest of my logic will fail.
A hint in the doc could be also helpful.

[MichalLytek]

Closing as null as default value is not really practical and supporting this would confuse more people than satisfy, e.g. when they think that defaultValue: null means no default value 🔒