allof

allOf is a keyword in JSON Schema that requires the instance to be valid against every subschema listed in its array. It expresses a logical conjunction: the overall schema is the intersection of the constraints defined by the subschemas. The instance must satisfy all constraints from all subschemas to be valid.

Usage of allOf is common for composing schemas from reusable parts or modeling inheritance and mixins by merging base properties with more specific constraints. Subschemas can describe objects, arrays, or primitive constraints; the validator applies them all.

Validation semantics: the validator evaluates each subschema against the instance; if any subschema fails, the instance is invalid. When subschemas define overlapping or conflicting constraints (for example, one requires a property to be a string while another requires it to be a number), the combination is unsatisfiable and no instance will validate.

Example: to require an object with an integer id and a string name, you can use a schema such as:

{ "allOf": [

{ "type": "object", "properties": { "id": { "type": "integer" } }, "required": ["id"] },

{ "type": "object", "properties": { "name": { "type": "string" } }, "required": ["name"] }

] }

Interaction with other keywords: allOf works with $ref, definitions, and other composition keywords. It is distinct from anyOf and oneOf, which specify alternatives, whereas allOf enforces all constraints.

Practical notes: with objects, the property definitions from subschemas are effectively combined, and their constraints must be mutually compatible. Validators may differ in how they merge metadata or handle defaults, so be mindful of required arrays and overlapping definitions.