Generate TypeScript Types from JSON (and where the auto-generators trip up)

You've got a JSON API response and you want TypeScript interfaces for it. Here's how to generate them fast — and where the auto-generators quietly get it wrong. The fast path Paste your JSON, get interfaces: { "id" : 1 , "name" : "Ada" , "roles" : [ "admin" ], "profile" : { "active" : true } } → interface Root { id : number ; name : string ; roles : string []; profile : Profile ; } interface Profile { active : boolean ; } jsonviewertool.com/json-to-typescript does this in the browser (client-side), nesting objects into their own interfaces. Where generators trip up A generator only sees the ONE sample you give it, which causes predictable gaps: Nullable fields. If your sample has "avatar": null , the generator infers null — but the real type is probably string | null . Feed it a populated sample, or fix it by hand. Empty arrays. "tags": [] infers any[] — the element type is unknowable from an empty array. Optional fields. A field missing from your sample won't appear at all. If the API sometimes omits middleName , mark it middleName?: string . Unions. A status that's "active" in your sample becomes string , not the literal union "active" | "banned" | "pending" . Narrow it manually for the safety. Numbers that are really enums or IDs. "currency": 840 types as number ; you may want an enum or branded type. When to use a schema instead If the JSON has a JSON Schema or OpenAPI spec, generate types from that ( json-schema-to-typescript , openapi-typescript ) — it encodes nullability, optionality, and unions the raw sample can't. Sample-based generation is for quick throwaway typing; schema-based is for anything you'll maintain. Rule of thumb Generate from a sample to skip the boilerplate, then read every field — the generator gives you a draft, not a contract. Nullability and optional fields are where the runtime bugs hide.