What is the difference between scalar types and object types in GraphQL?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Identify the leaf values

   Any field that returns a single primitive — a number, string, boolean or ID — is a scalar and cannot have a sub-selection.

2. Step 2

   Identify the structured entities

   Fields that model a thing with its own named sub-fields (User, Order) are object types and require a selection set.

3. Step 3

   Traverse the type tree

   Every valid query is a path from a root object down through nested objects until it reaches scalar leaves.

4. Step 4

   Add custom scalars where needed

   Define custom scalars like DateTime or Email with serialize/parseValue/parseLiteral to validate non-standard primitives.

5. Step 5

   Enforce selection rules

   The server rejects queries that select sub-fields on a scalar or that leave an object field without a selection set.

Code Example

scalar DateTime

type User {
  id: ID!
  name: String!
  age: Int
  isActive: Boolean!
  createdAt: DateTime!
}

type Query {
  user(id: ID!): User
}

# A query must select scalar leaves on the User object
query {
  user(id: "42") {
    id
    name
    age
    isActive
    createdAt
  }
}

Follow-up Questions

• How do you implement a custom scalar like DateTime?
• What is the difference between the ID scalar and String?
• Why must object fields include a selection set?
• How do enums relate to scalar types?
• What happens if a non-null scalar resolves to null?