GraphQL specification defines 5 built-in scalar types - String, Int, Float, Boolean and ID (a string with a unique identifier). These types are JSON serializable and available in every programming language.

Be careful with the built-in scalar Int type:

• This is the signed 32-bit integer.
• If the integer interval value represents a value less than -2^31 or greater than or equal to 2^31, a field error should be raised.
• In numbers: -2'147'483'648 / 2'147'483'647.

These numbers in real life:

• Time (in milliseconds): ~24.86 days
• Data Volume (in bytes): ~2.15 Gb

If you return from the backend or pass via client an argument whose value is not within the 32-bit Int interval - you'll get an error: Int cannot represent non-32-bit signed integer value.

However, when a scalar type is not representable in JSON by default (e.g. Date) the backend has to figure out a data format that can be serializable and transmittable via JSON. The backend also needs to deserialize the field received from a client.

In such cases, GraphQL allows you to create your own custom scalar types.