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.
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.
This accepts anything, as long as it's valid JSON
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
This accepts anything, as long as it's valid JSON:
Documents for this schema will always be invalid:
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.
When this book refers to JSON Schema "keywords", it means the "key" part of the key/value pair in an object. Most of the work of writing a JSON Schema involves mapping a special "keyword" to a value within an object.
For example, in the following, only strings are accepted:
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.
For brevity, the
$schema keyword isn't included in most of the
examples in this book, but it should always be used in the real world.
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¶
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:
The details of
$id become more apparent when you start structuring a complex schema.
id(without the dollar-sign).