Why should you care?

Shared interfaces keep Angular templates and Express handlers agreeing on JSON field names.

See it live — copy this example

Paste into your MeanVerse project (Angular + Express + MongoDB), then run with ng serve / node / mongosh as noted.

export interface TransferDto {
  fromAccountId: string;
  toAccountId: string;
  amountCents: number;
  memo?: string;
}

export interface ApiError {
  code: string;
  message: string;
  field?: string;
}

What happened?

• TransferDto documents POST body.
• Optional memo?
• may be absent.
• ApiError standardizes error JSON from Express error middleware.

Practice next

1. Create shared/interfaces/transfer.dto.ts.
2. Import TransferDto in Angular service and Express route typing.
3. Extend interface for internal fields vs public DTO.
4. Add readonly createdAt: string to audit interfaces.
5. Create PaginatedResponse generic interface.

Remember

Interfaces describe data contracts. Optional fields use ? suffix. Share DTO interfaces across MEAN tiers.

OpenAPI alignment

MeanVerse publishes Swagger generated from same DTO interfaces.

Outcome: Mobile and web clients share one typed contract.