Typescript types for required and optional attributes are not generated correctly

[rehos]

Checkboxes for prior research

• I've gone through Developer Guide and API reference
• I've checked AWS Forums and StackOverflow.
• I've searched for previous similar issues and didn't find any solution.

Describe the bug

Typescript types are for required and optional attributes are not generated corrrectly. I.e. required attributes can accept undefined. And optional attributes cannot accept undefined when setting the typescript compilation option exactOptionalPropertyTypes to true.

Fixing this will probably also fix this issue

SDK version number

3.540.0

Which JavaScript Runtime is this issue in?

Node.js

Details of the browser/Node.js/ReactNative version

v20.11.0

Reproduction Steps

Just look at the generated typescript types for all clients

Observed Behavior

Typescript types of required attributes are all generated as a union with their type and undefined. And optional attributes are not generated as a union with undefined which gives problems when setting the typescript compilation option exactOptionalPropertyTypes to true.

See for example the in the DynamoDB client the TableName and IndexName attributes in the type QueryInput:

export interface QueryInput {
  TableName: string | undefined;
  IndexName?: string;

 // omitted other attributes

Expected Behavior

I expect required attributes cannot be set to undefined while optional attributes can accept undefined. Also when setting the typescript compilation option exactOptionalPropertyTypes to true.

So I expected that the generated code for the example above is generated as:

export interface QueryInput {
  TableName: string;
  IndexName?: string | undefined;

  // omitted other attributes

Possible Solution

Probably the following line in DocumentClientCommandGenerator.java:

String typeSuffix = isRequiredMember(member) ? " | undefined" : "";

should be changed to:

String typeSuffix = isRequiredMember(member) ? "" : " | undefined";

Additional Information/Context

No response

[RanVaknin]

Hi @rehos ,

Thanks for reaching out. The reason why we generate " | undefined" for our types, is intentionally designed for forward compatibility reasons. This allows AWS services to change their APIs by making previously required parameters optional, ensuring that existing client applications will not break when the API becomes less strict.

To improve the typescript experience, you can cast your client to be an UncheckedClient or AssertiveClient from Smithy-Typescript package. Just be mindful that using UncheckedClient eliminates compiler-generated nullability warnings for output fields by treating them as non-nullable. You should still perform manual type checks as necessary for your application's specific needs.

Thanks,
Ran~

[rehos]

@RanVaknin, thank you for the clarification for required types. For optional types I think the generated code should also generate the " | undefined" to avoid compiler errors when setting the typescript compilation option exactOptionalPropertyTypes to true. We set this to true for our code base to get the most benefits when using @effect/schema.

[kuhe]

TS playground demonstrating the secondary issue: TSP

[cbudau]

I understand the forward compatibility reasoning behind allowing | undefined for required parameters. However, this approach creates an issue when using TypeScript’s exactOptionalPropertyTypes setting.

For example, in QueryInput:

export interface QueryInput {
  TableName: string | undefined; // Required but allows undefined (not ideal)
  IndexName?: string; // Optional but doesn't allow undefined (causes issues)
}

Problems:
TableName is required but still allows undefined, which doesn’t align with TypeScript’s typical handling of required properties.
IndexName is optional but doesn’t explicitly allow undefined, causing a TypeScript error when exactOptionalPropertyTypes is enabled.

Possible solution:
Would it make sense for the Smithy TypeScript Codegen project to refine type generation when exactOptionalPropertyTypes is enabled? For instance:

• Required properties (TableName) should not allow undefined.
• Optional properties (IndexName) should explicitly allow undefined.

Temporary workaround:
Until a fix is available, one workaround is to extend the type locally:

type CorrectedQueryInput = Omit<QueryInput, "TableName" | "IndexName"> & {
  TableName: string;
  IndexName?: string | undefined;
};

[kuhe]

All fields contain | undefined in the latest version of the AWS SDK.

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