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.
{ }
This accepts anything, as long as it’s valid JSON
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"I'm a string"
{ "an": [ "arbitrarily", "nested" ], "data": "structure" }
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.
true
This accepts anything, as long as it’s valid JSON
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"I'm a string"
{ "an": [ "arbitrarily", "nested" ], "data": "structure" }
false
"Resistance is futile... This will always fail!!!"
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.
Note
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": "string" }
"I'm a string"
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The type
keyword is described in more detail in Type-specific keywords.
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.
Note
For brevity, the $schema
keyword isn’t included in most of the
examples in this book, but it should always be used in the real
world.
{ "$schema": "https://json-schema.org/draft/2020-12/schema" }
$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¶
It is also best practice to include an $id
property as a unique
identifier for each schema. For now, just set it to a URL at a domain
you control, for example:
{ "$id": "http://yourdomain.com/schemas/myschema.json" }
The details of $id become more apparent when you start Structuring a complex schema.
New in draft 6
$id
is just id
(without the dollar-sign).