[typescript-fetch] Improve code generation for oneOf cases without discriminator

[odiak]

Problem

Previously, generated code from the oneOf schema without a discriminator might cause runtime errors.

schema:

openapi: 3.0.0
info:
  version: 1.0.0
  title: Test
  description: Test
paths:
  /foo:
    get:
      responses:
        200:
          description: foo
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Foo'
components:
  schemas:
    Foo:
      oneOf:
        - $ref: '#/components/schemas/Bar'
        - $ref: '#/components/schemas/Buzz'
    Bar:
      type: object
      required:
        - abc
      properties:
        abc:
          type: array
          items:
            type: object
            properties:
              xyz:
                type: number
    Buzz:
      type: object
      required:
        - def
      properties:
        def:
          type: number

generated model code (partial):

// models/Foo.ts
// ...
export function FooFromJSON(json: any): Foo {
    return FooFromJSONTyped(json, false);
}

export function FooFromJSONTyped(json: any, ignoreDiscriminator: boolean): Foo {
    if (json == null) {
        return json;
    }
    return BarFromJSONTyped(json, true) || BuzzFromJSONTyped(json, true);
}
// ...

// on models/Bar.ts
// ...
export function BarFromJSONTyped(json: any, ignoreDiscriminator: boolean): Bar {
    if (json == null) {
        return json;
    }
    return {
        'abc': ((json['abc'] as Array<any>).map(BarAbcInnerFromJSON)),  // ← this causes error
    };
}
// ...

In BarFromJSONTyped, if the given json doesn't have the abc property, (json['abc'] as Array<any>).map(...) will fail.
The same problem also occurs if abc is a string with a date-time format.

Solution

I modified the template to generate FooFromJSONTyped like the following:

// models/Foo.ts
// ...
export function FooFromJSONTyped(json: any, ignoreDiscriminator: boolean): Foo {
    if (json == null) {
        return json;
    }
    if (instanceOfBar(json)) {
        return BarFromJSONTyped(json, true);
    }
    if (instanceOfBuzz(json)) {
        return BuzzFromJSONTyped(json, true);
    }
}
// ...

Because instanceOfXXXX functions check for the existence of required keys, it makes the generated code safer.

How to check

1. Check out this branch.
2. Build the generator.
3. Generate an API client using the schema above.
4. Check generated model files.

PR checklist

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[odiak]

Dear technical committee members of TypeScript, please check this pull request if you have time. 🙇
@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo @amakhrov @davidgamero @mkusaka

[macjohnny]

LGTM

thanks for your contribution!

An edge case that wouldnt work is if the models dont contain required fields or they are the same, but i guess then its hard to distinguish without a discriminator anyway

[sir4ur0n]

@odiak I just updated our openapi-generator to 7.7.0 which comes with this PR, and now Typescript fails typechecking:

Function lacks ending return statement and return type does not include 'undefined'

And indeed, if the input JSON is not an instance of any of the variants, this function returns undefined but it's not in the signature.

Don't you have this exact same problem?
I feel like I am missing something 🤔

[macjohnny]

@sir4ur0n thanks for reporting that. do you want to try to fix this?

[sir4ur0n]

I don't really know how to fix that:

• should the return type of the function be changed to | undefined?
• throw an error in the default case?
• other?

I think this PR was right to use instanceOf functions, I am not challenging that.
I just don't know how the function xxxFromJSONTyped is supposed to behave when the input JSON is incorrect.

I also don't feel very confident in attempting to fix because I am not great at Typescript 😅

[odiak]

@sir4ur0n I also realized same problem last month. I have workaround for it but I forgot to submit pull request. Sorry!

My solution is just adding return json to the end of the function. I think it's not perfect though.

[sir4ur0n]

@odiak no worries 👍

Would you mind opening a PR with your workaround please? 🙏

[odiak]

@sir4ur0n Sure!

[sir4ur0n]

@odiak did you get a chance to work on that? 🙏

[odiak]

I'll do it tonight!
(Currently 9pm on my timezone)