Inference Control, Satisfies, and Utility Types

Control inference in large codebases with satisfies, const assertions, utility types, and deliberate generic APIs.

50 min

4 sections

satisfies

utility-types

inference

as-const

01. Satisfies Preserves Specific Values

Section 1 of 4

Type annotations often widen useful literal information. The satisfies operator checks that a value conforms to a target type while preserving the value's specific inferred type. This is ideal for route maps, feature matrices, analytics events, and permission registries.

typescript

type FeatureConfig = {
  enabled: boolean;
  rollout: "internal" | "beta" | "general";
  owner: string;
};

const features = {
  auditTimeline: { enabled: true, rollout: "general", owner: "compliance" },
  aiTicketSummary: { enabled: true, rollout: "beta", owner: "support-ai" },
  invoiceExports: { enabled: false, rollout: "internal", owner: "billing" },
} satisfies Record<string, FeatureConfig>;

type FeatureName = keyof typeof features;
type AiTicketSummaryRollout = typeof features.aiTicketSummary.rollout;

Exercise

Create a checked route registry

Practice

Build a route registry where every route has method, path, and requiredPermission, while preserving exact route names and permission strings.

React and Next.js TypeScript Patterns

TSConfig, Modules, and Monorepos

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