Library API and Declaration Design

Design public TypeScript APIs, generated declarations, and SDK types that enterprise consumers can trust.

40 min

3 sections

declarations

sdk

public-api

libraries

01. Public Types Are Product Surface

Section 1 of 3

If another team imports a type, that type is now a contract. Keep public types small, intentional, and stable. Avoid leaking database rows, vendor SDK details, or internal helper types into package exports.

typescript

// Good public API
export type CreateCustomerInput = {
  legalName: string;
  billingEmail: string;
  plan: "starter" | "business" | "enterprise";
};

export type CustomerSummary = {
  id: string;
  legalName: string;
  plan: "starter" | "business" | "enterprise";
};

export interface CustomerClient {
  create(input: CreateCustomerInput): Promise<CustomerSummary>;
}

TSConfig, Modules, and Monorepos

Migration, Refactoring, and Any Budgets

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