Guides

Generate TypeScript types from a real payload

Types written by hand from documentation describe the API someone meant to build. Types generated from a response describe the API that is actually deployed.

How the shape is inferred

The generator walks the indexed document and records, for every object slot, which types were seen and how many of the containing objects had that key at all. A key present in 40 of 40 sampled objects is required; a key present in 31 is emitted as optional. Arrays are unified across their elements, so an array of objects produces one interface rather than a union of near-duplicates.

[
  { "id": 1, "name": "Ada",    "status": "active",   "team": "core" },
  { "id": 2, "name": "Grace",  "status": "inactive" },
  { "id": 3, "name": "Alan",   "status": "active",   "team": "core" },
  { "id": 4, "name": "Edsger", "status": "inactive" }
]

produces

export type Root = RootItem[]

export interface RootItem {
  id: number
  name: string
  status: "active" | "inactive"
  team?: string
}

The bar for a literal union is deliberately high: at least two distinct values, each seen more than once, none of them long, and none of them looking like a formatted value such as a timestamp or a UUID. A wrong enum is worse than no enum, so two records would have produced status: string.

Options

Where it is honest about its limits

Inference describes the sample, not the contract. A field that is null in every record in your sample is typed null, because nothing in the data says otherwise. A field that happens to be absent from every sampled object cannot be invented. Treat generated types as a fast, accurate starting point for a document you have, and review them against the API you are coding against.

Generated output carries a provenance header naming the source document, so a file that ends up committed can be traced back to what produced it.

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Last updated 2026-09-14.