JSON to TypeScript

What does this tool do?

JSON to TypeScript turns the structure of a JSON document into TypeScript interfaces and type aliases. Paste a JSON object or array and it infers the types you would otherwise write by hand: nested objects become nested interfaces, arrays become T[], and missing fields become optional properties.

How the inference works

The generator follows a small, predictable set of rules:

• Objects become interfaces. Each object — the root, nested objects and objects inside arrays — gets its own named interface.
• Arrays of objects are merged. When array items differ, their shapes are merged and fields missing in some items become optional (name?: string).
• null becomes a union. A field that is null in one item and string in another is inferred as string | null.
• Incompatible types become unions. A field holding both numbers and strings is inferred as number | string.
• Empty arrays become unknown[]. With no items, there is nothing to infer from, so the tool says so explicitly instead of guessing.
• Names are normalized. snake_case, kebab-case and camelCase keys are converted to PascalCase (user_profile → UserProfile), arrays are singularized (users → User), and keys that would not be valid TypeScript identifiers are fixed.

When to use it

• Typing an API response you have as a sample payload.
• Bootstrapping types for a config file or fixture before tightening them.
• Generating a starting point you then refine into a hand-written type.

It is a structural converter: it reads shapes, not runtime semantics. It does not know that 2026-09-10 is a date or that "1" should be a number — it reports what is actually there.

JSON to TypeScript vs the rest of the JSON tools

• JSON Formatter beautifies raw JSON.
• JSON Validator checks syntax and reports errors with position.
• JSON Query extracts data from a document.
• JSON to TypeScript generates static types from the structure.

Each answers a different question about the same document.

Limitations

• Inference is structural and lossy: it cannot recover information that is not in the sample (empty arrays, null-only fields).
• Array item merging is heuristic; genuinely mixed arrays become unions or optional fields rather than a single perfect type.
• Generated names are a best effort for pluralization and casing — rare keys may need manual renaming.

Privacy

Your JSON is parsed and converted locally in your browser. Nothing is uploaded, stored or sent anywhere.