Generic keywords¶

Metadata¶

JSON Schema includes a few keywords, title, description and default, that aren’t strictly used for validation, but are used to describe parts of a schema.

The title and description keywords must be strings. A “title” will preferably be short, whereas a “description” will provide a more lengthy explanation about the purpose of the data described by the schema. Neither are required, but they are encouraged for good practice.

The default keyword specifies a default value for an item. JSON processing tools may use this information to provide a default value for a missing key/value pair, though many JSON schema validators simply ignore the default keyword. It should validate against the schema in which it resides, but that isn’t required.

{
  "title" : "Match anything",
  "description" : "This is a schema that matches anything.",
  "default" : "Default value"
}

Enumerated values¶

The enum keyword is used to restrict a value to a fixed set of values. It must be an array with at least one element, where each element is unique.

The following is an example for validating street light colors:

{
  "type": "string",
  "enum": ["red", "amber", "green"]
}

"red"

"blue"

You can use enum even without a type, to accept values of different types. Let’s extend the example to use null to indicate “off”, and also add 42, just for fun.

{
  "enum": ["red", "amber", "green", null, 42]
}

"red"

null

42

0

However, in most cases, the elements in the enum array should also be valid against the enclosing schema:

{
  "type": "string",
  "enum": ["red", "amber", "green", null]
}

"red"

This is in the enum, but it’s invalid against { "type": "string" }, so it’s ultimately invalid:

null