> For the complete documentation index, see [llms.txt](https://docs.novacura.com/flow-connect/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.novacura.com/flow-connect/reference/reference/flowscript/walkthrough/custom-types.md).

# Custom Types

## Type Definitions

FlowScript is a structurally typed language, but it is possible to create named types. Using a named  type allows you to deserialize [JSON](/flow-connect/reference/reference/flowscript/walkthrough/json-module.md), or write groups of functions operating on the same type of data without a lot of repetition.

```typescript
// Example: define a record type called User, then define
// a function which accepts a User as its argument.
type User = {
    name: text,
    email: text,
    gropIds: number*
};

function getEmail(user: User) => user.email;

let u = new User {
    name: "Florian",
    email: "florian@example.com",
    gropIds: [1, 2]
};

return getEmail(u);
```

Note that all values in this program are still structurally typed. In other words, the following permutation is perfectly valid:

```typescript
// assuming the same definitions as above

let u2 = {
    name: "Joe",
    email: "joe@example.com",
    gropIds: [0, 8],
    department: "Accounting" // this member is not in the type.
};
return getEmail(u2);
```

In other words, the getEmail function accepts all records which have *at least* the members defined in the User type. There is no requirement that they be created using the **new User** construct, and they are allowed to have additional members not listed in the type. (For in-depth information, refer to the [appendix on subtyping rules](/flow-connect/reference/reference/flowscript/walkthrough/appendix-subtyping-rules.md).)

## Advanced: Generic type definitions

Like functions, types can be generic. Type variables are written in angle brackets after the type name.

```typescript
type Response<T> = {
    timestamp: datetime,
    data: T
};

function getSomeNumber(): Response<number> {
    return new Response<number> { timestamp: now(), data: random(1, 10) };
}

return getSomeNumber();
```


---

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