What happens to arrays of objects?

They're merged into one interface describing a single element, and the property is typed as an array of it. Any key that appears in some array members but not others becomes optional. This matches how real paginated data behaves, where records come from one table and some columns are nullable. The one case it handles badly is a genuinely heterogeneous array of different event types, which you should hand-convert into a discriminated union.

Should null become an optional key or a union with null?

It depends on whether your API omits absent fields or sends them as null. If it omits them, key?: T is accurate. If the key is always present and sometimes null, key: T | null is accurate, and using ? would wrongly allow the key to be missing. The converter defaults to optional and lets you switch, because the two serialise differently — JSON.stringify drops undefined properties but emits null ones.

Can it infer accurate types from a single JSON sample?

It infers accurate types for that sample, which isn't the same thing. A field that's a number in your one record might be null in others; a field present in all three records you pasted might be optional across the full dataset. Use a representative payload with several records rather than one hand-picked object, and treat the output as a reviewed draft rather than a finished contract.

What's the difference between JSON to TypeScript and JSON Schema to TypeScript?

This tool infers types from an example value; JSON Schema to TypeScript translates a formal schema that already declares types, required fields, and enums. The schema is authoritative and inference is a guess, so if you have a JSON Schema, an OpenAPI spec, or a GraphQL schema, use it. Inference is for the very common case where no schema exists and all you have is a response body.

How does it handle keys that aren't valid identifiers?

Keys with dashes, dots, spaces, or leading digits are quoted in the output, so "content-type" becomes "content-type": string. That's valid TypeScript, accessed with bracket notation. Reserved words like class don't need quoting as property names. Interface names derived from such keys are PascalCased and prefixed if they'd start with a digit, and colliding names get a numeric suffix so the output always compiles.

Should I generate interfaces or type aliases?

Match whatever your codebase already does — this is mostly a consistency question. Interfaces support declaration merging and extends, and give marginally clearer error messages. Type aliases can't merge, which is usually desirable, and are required for anything that isn't an object shape. A root that's an array or a primitive is emitted as an alias either way, since there's no object to declare an interface for.

Is my JSON uploaded to a server?

No. The entire inference engine runs as JavaScript in your browser, with no network calls, no logging, and no storage. This matters here more than for most tools, because the JSON you'd paste into a type generator is usually a real API response containing tokens, customer records, or internal IDs. Open your network tab while generating, or disconnect from the internet — it keeps working.

JSON to TypeScript: Generate Accurate Interfaces From Real API Data

Toolz Team

|Jul 21, 2026|4 min read

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Related tools: JSON Formatter for inspecting the payload first, JSON to YAML and JSON to XML for format conversion, and JSON Diff for spotting what changed between two responses. Further reading: the ultimate guide to JSON tools and the developer's coding tools guide.

Frequently Asked Questions

Paste your JSON into the converter, set the root type name to whatever the resource is called, and press Generate. It infers the type of every key, pulls nested objects out into their own named interfaces, merges arrays of objects into a single element type, and outputs code you can copy straight into a .ts file. There's no signup and no upload — the inference runs in your browser.

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JSON to TypeScript: Generate Accurate Interfaces From Real API Data

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