# TypeScript

React Query is now written in **TypeScript** to make sure the library and your projects are type-safe!

Things to keep in mind:

- Types currently require using TypeScript v3.8 or greater
  - for [useQueries](../reference/useQueries), TypeScript v4.1 or greater is required to get the correct return type for each individual query (below v4.1 will mean the type of each returned `data` property is left as `unknown`)
- Changes to types in this repository are considered **non-breaking** and are usually released as **patch** semver changes (otherwise every type enhancement would be a major version!).
- It is **highly recommended that you lock your react-query package version to a specific patch release and upgrade with the expectation that types may be fixed or upgraded between any release**
- The non-type-related public API of React Query still follows semver very strictly.

## Defining Custom Hooks

When defining a custom hook you need to specify the result and error types, for example:

```ts
function useGroups() {
  return useQuery<Group[], Error>('groups', fetchGroups)
}
```

## Further Reading

For tips and tricks around type inference, have a look at [React Query and TypeScript](../community/tkdodos-blog#6-react-query-and-typescript) from
the Community Resources.