# Object Types, Scalars and Nullability — GraphQL

Source: https://www.skillbyai.com/en/graphql/s-types

> Building blocks of SDL.

## Fields, lists and non-null

An **object type** has fields, each with a type. Built-in **scalars** are `Int`, `Float`, `String`, `Boolean` and `ID`; custom scalars such as `DateTime` can be added. `!` marks a field as **non-null** and `[Type]` is a list, so `[Order!]!` is a non-null list of non-null orders. Fields can take **arguments**, and descriptions in triple quotes become documentation. Design types around the client's domain language, not your database tables.

## The contract between client and server

The Schema Definition Language (SDL) describes types, fields and operations clients can use.

![Three ideas: object types and scalars, root operation types, abstract and input types.](assets/figures/graphql/section-2-map.svg) — Figure 2.1 — Types, operations and abstract types.

## A shop schema

GraphQL SDL.

```graphql
"""A registered customer."""
type User {
  id: ID!
  name: String!
  email: String!
  orders(first: Int = 10, after: String): OrderConnection!
}

type Order {
  id: ID!
  placedAt: DateTime!
  status: OrderStatus!
  total: Money!
  items: [OrderItem!]!
}

type OrderItem {
  product: Product!
  quantity: Int!
}

scalar DateTime
```

## Be deliberate with non-null

Non-null fields are a promise; if a resolver fails, the null bubbles up to the nearest nullable parent, possibly wiping out more data than expected.

**Quiz:** What does [Order!]! mean?

- [ ] An optional list of optional orders
- [x] A non-null list whose items are non-null orders
- [ ] Exactly one order
- [ ] A map of orders

*Answer:* A non-null list whose items are non-null orders. Both the list and its items are non-null.