The basics

In What is a schema?, we described what a schema is, and hopefully justified the need for schema languages. Here, we proceed to write a simple JSON Schema.

Hello, World!

When learning any new language, it's often helpful to start with the simplest thing possible. In JSON Schema, an empty object is a completely valid schema that will accept any valid JSON.

schema

{}

This accepts anything, as long as it's valid JSON

data

42

compliant to schema

data

"I'm a string"

compliant to schema

data

{ "an": [ "arbitrarily", "nested" ], "data": "structure" }

compliant to schema

New in draft 6

You can also use true in place of the empty object to represent a schema that matches anything, or false for a schema that matches nothing.

This accepts anything, as long as it's valid JSON:

schema

true

data

42

compliant to schema

data

"I'm a string"

compliant to schema

data

{ "an": [ "arbitrarily", "nested" ], "data": "structure" }

compliant to schema

Documents for this schema will always be invalid:

schema

false

data

"Resistance is futile... This will always fail!!!"

not compliant to schema

The type keyword

Of course, we wouldn't be using JSON Schema if we wanted to just accept any JSON document. The most common thing to do in a JSON Schema is to restrict to a specific type. The type keyword is used for that.

For example, in the following, only strings are accepted:

schema

{ "type": "string" }

data

"I'm a string"

compliant to schema

data

42

not compliant to schema

The type keyword is described in more detail in here.

Declaring a JSON Schema

It's not always easy to tell which draft a JSON Schema is using. You can use the $schema keyword to declare which version of the JSON Schema specification the schema is written to. See $schema for more information. It's generally good practice to include it, though it is not required.

schema

{ "$schema": "https://json-schema.org/draft/2020-12/schema" }

Draft-specific info

In Draft 4, a $schema value of http://json-schema.org/schema# referred to the latest version of JSON Schema. This usage has since been deprecated and the use of specific version URIs is required.

Declaring a unique identifier

New in draft 6

It is also best practice to include an $id property as a unique identifier for each schema. For now, just set it to a URL at a domain you control, for example:

schema

{ "$id": "http://yourdomain.com/schemas/myschema.json" }

The details of $id become more apparent when you start structuring a complex schema.

Draft-specific info

In Draft 4, $id is just id (without the dollar-sign).