JSON to IO-TS Converter

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Generate Runtime Type Validators from JSON with io-ts

TypeScript checks your types at compile time, but once your code is running, all bets are off. Data from APIs, user inputs, and external files can be anything - and TypeScript cannot protect you from malformed runtime data. That is exactly the problem io-ts solves: it lets you define codecs that validate data at runtime against a schema. The JSON to io-ts Converter generates these codec definitions automatically from sample JSON data, giving you runtime safety without writing validators by hand.

If you have ever written a t.type({ name: t.string, age: t.number }) definition manually for a large, nested API response, you know how tedious it gets. Twenty properties deep with optional fields, arrays of objects, and nullable values - the io-ts codec definition can easily span a hundred lines. This converter produces that entire definition from a single JSON sample in under a second.

Understanding io-ts and Why It Matters

The io-ts library, created by Giulio Canti, bridges the gap between TypeScript's static type system and JavaScript's dynamic runtime. Each io-ts codec serves double duty: it defines a TypeScript type via t.TypeOf and provides a runtime validator that produces structured error results. When you validate incoming data with an io-ts codec, you get either a decoded value you can trust or a detailed error report explaining what went wrong.

This approach is especially valuable at application boundaries - the points where your code meets the outside world. API responses, WebSocket messages, form submissions, file contents, and query parameters are all untrusted data that should be validated before your business logic touches them. The JSON to io-ts Converter gives you the codecs for that validation layer.

What the Converter Produces

Paste a JSON object into the tool and you get a complete io-ts codec definition. Primitive values map to their corresponding io-ts types: t.string, t.number, t.boolean, t.null. Objects become t.type({...}) with each property typed. Arrays produce t.array(elementCodec). Nullable fields generate t.union([t.string, t.null]) patterns.

Nested objects get their own named codec definitions, keeping the output organized and readable. The converter also generates the corresponding TypeScript type aliases using t.TypeOf, so you get both the runtime validator and the compile-time type from a single definition.

Practical Scenarios Where This Shines

Imagine you are integrating a third-party payment API. The webhook payload has a specific structure, but the API documentation might be incomplete or outdated. You capture a real webhook payload, paste it into the JSON to io-ts Converter, and instantly have a codec that validates incoming webhooks. If the API provider changes their payload structure without warning, your codec catches the mismatch before it corrupts your data.

Another common scenario is configuration file validation. Your application reads a JSON config file at startup. Without runtime validation, a typo in the config silently produces undefined values that cause errors later. With an io-ts codec generated from a known-good config sample, you catch configuration errors at startup with clear error messages.

Functional Programming Meets Practical Tooling

The io-ts library is rooted in functional programming concepts - it uses fp-ts under the hood with Either types and pipe operations. For developers who embrace FP patterns in TypeScript, io-ts is a natural fit. This converter lowers the barrier to adoption by handling the boilerplate codec definitions automatically, letting you focus on the validation logic and error handling that make your application robust.

Fast, Private, and Always Available

The JSON to io-ts Converter runs entirely in your browser. Your JSON data stays on your machine - nothing is transmitted to any server. The conversion is instantaneous regardless of payload size, and you can use the tool as many times as you need without creating an account or hitting rate limits. Generate codecs for your entire API surface in one sitting if you want to. Your data, your codecs, your privacy.