Turn JSON into TypeScript types

Typing an API response by hand means reading every field of a long sample and guessing which ones can be missing. This reads the sample for you and writes TypeScript that matches it: one interface per object, nested types named after the keys that hold them, and lists merged into a single item type.

How the types are worked out

Every value in the sample is read. Strings, numbers and booleans keep their type, null stays null, and a field that holds more than one kind of value across the sample becomes a union. Objects inside a list are merged, so a key that appears in only some of them is marked optional, which is usually the thing that catches an application out when a field is missing.

Keys keep the order they were written in, including keys that are numbers, so the interface reads like the sample. A key that is not a valid name in TypeScript, such as first-name or a year, is written in quotes.

Naming

The top-level type is called Root unless you name it. A nested object is named after its key in PascalCase, a list of objects gives its items the singular form of the key, and two keys that would give the same name are told apart with a number. Choose type instead of interface if your code base prefers type aliases.

What a sample cannot tell

A type written from one sample describes that sample. A field missing from every item in it, or a number that is a string in other responses, will not show up here. Paste a longer sample, or several responses as a list, to make the types more complete.