What are scalar types in GraphQL

Question

From their docs, it says:

The leaf values of any request and input values to arguments are Scalars (or Enums) and are defined with a name and a series of serialization functions used to ensure validity

but I am unable to comprehend that definition. From the answer to my previous question, I think scalars are associated with enums. Can someone break down in simpler terms what do GraphQL scalar types do and when to use scalar types as opposed to enum types?

Daniel Rearden Daniel Rearden · Accepted Answer · 2019-07-29T13:03:49

Scalars are equivalent to primitive data types in a programming language. In GraphQL, there are five built-in scalar types:

Boolean, true or false
Int, a signed 32‐bit numeric non‐fractional value
Float, a signed double‐precision fractional values
String, a sequence of UTF‐8 characters
ID, a unique identifier

A scalar simply represents a single value and are the basic building blocks of your schema. This is in comparison to object types, which represent a collection of values. An object type has fields, and each field has a type which may be a scalar or an object type itself. If the field's type is an object, that object will have fields that are also scalars or other objects, and so on. In this way we end up with a tree-like structure in both our schema and the queries made against it.

query {       # <- The "root" of the tree
  movies {
    actors {
      name    # <- A "leaf" on the tree
    }
    crew {
      name    # <- Another "leaf"
    }
  }  
}

Enums are similar to scalars, in that an enum represents a single, concrete value. However, each enum type is defined explicitly in the schema (there are no "built-in" enums) and its definition must include a set of values that the enum type can be. For example, we might create an enum like:

enum Animal {
  Cat
  Dog
  Bird
}

A field that returns an Animal will still have a single value, like a scalar, but that value will be either "Cat", "Dog" or "Bird" -- no other values are allowed.

A leaf type is a term that encompasses both scalars and enums. Leaf types represent the leaves or terminating points in our tree-like queries and their responses.

When a field returns an object type (like movies, actors, or crew in the above example), we have to tell GraphQL which of the object type's fields we want to query. This selection of fields is called a selection set and is wrapped by curly brackets. In our example, name is the selection set for the crew field, crew and actors are the selection set for the movies field, and even the movies field is part of the selection set for the query root type.

The important thing to notice here is that leaf types do not have fields, so any field that returns a leaf type will not have a selection set.

For more information, you can check out the official spec.

What are scalar types in GraphQL

1 Answers