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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

42
"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

42
"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"
42

The type keyword is described in more detail in Type-specific keywords.

Declaring a JSON Schema

Since JSON Schema is itself JSON, it’s not always easy to tell when something is JSON Schema or just an arbitrary chunk of JSON. The $schema keyword is used to declare that something is JSON Schema. 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": "http://json-schema.org/schema#" }

You can also use this keyword to declare which version of the JSON Schema specification that the schema is written to. See The $schema keyword for more information.

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 The $id property become more apparent when you start Structuring a complex schema.

New in draft 6

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