JSON to TypeScript — Interface & Type Generator

Generate interfaces or type aliases from the values in a JSON sample. The result is a starting point: it cannot infer unseen values or business rules, and it does not validate or convert data at runtime. Review the declarations before using them in your project.

Generation and formatting run in your browser. This tool does not send your input JSON to a server. Page resources, analytics and advertising may still use the network.

Inference examines all array elements. Different observed types become a union, and a property absent from some objects becomes optional. Nested objects are combined in the same way. A property whose only observed value is null stays null. Making null-containing properties optional allows them to be absent; it does not remove null from their type.

Empty arrays become unknown[]. An empty object by itself becomes Record<string, unknown>. Date strings remain string, and numbers become number. A number annotation does not guarantee JavaScript numeric precision or range. If an object has duplicate keys, inference uses the last value.

Formatting is independent of the root type name and declaration limits. It checks JSON syntax and length limits and preserves the original number notation, including large numbers.

Choose interface or type, export, readonly, optional null-containing properties, and inlining for child objects with at most two properties. Array and primitive roots always use type. readonly prevents property reassignment; it does not make array elements immutable.

The root type name accepts up to 64 ASCII letters, digits, underscores or dollar signs, with no leading digit. Reserved names and Record are rejected. Child type names are adjusted to safe identifiers and numbered to avoid collisions; property names are preserved.

The output is a set of declarations. With export turned off, use it inside a module that already contains an import or export, or add export {} to a standalone file. In a global script, names such as Array and Node may conflict with existing declarations. Check other names in the same scope too.

Limits are 100,000 input UTF-16 code units, nesting depth 64 with the root at zero, 10,000 values, 500,000 output characters and 1,000 declarations. Invalid or over-limit input produces no partial type output. Formatting leaves the input unchanged if the formatted text would exceed the input limit.