What does this JSON Schema validate?

## Handling Null Values in JSON Schema JSON has a dedicated null type represented by the literal keyword null. In JSON Schema, you can allow a field to accept null alongside other types using a type array. ### Nullable Fields with Type Arrays json { "type": ["string", "null"] } This schema accepts both string values like "hello" and the literal null. It rejects numbers, booleans, arrays, and objects. ### Null vs. Missing vs. Empty These three states are fundamentally different in JSON S

Null Type and Nullable Fields in JSON Schema

Learn how to handle null values and nullable fields in JSON Schema. Understand the difference between null type, missing properties, and empty strings.

Basic Types

JSON Schema

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "firstName": {
      "type": "string"
    },
    "middleName": {
      "type": ["string", "null"]
    },
    "deletedAt": {
      "type": ["string", "null"],
      "format": "date-time"
    }
  },
  "required": ["firstName", "middleName"]
}

Test Data

{
  "firstName": "Alice",
  "middleName": null,
  "deletedAt": null
}

Detailed Explanation

Handling Null Values in JSON Schema

JSON has a dedicated null type represented by the literal keyword null. In JSON Schema, you can allow a field to accept null alongside other types using a type array.

Nullable Fields with Type Arrays

{
  "type": ["string", "null"]
}

This schema accepts both string values like "hello" and the literal null. It rejects numbers, booleans, arrays, and objects.

Null vs. Missing vs. Empty

These three states are fundamentally different in JSON Schema:

State	JSON	Passes `required`?	Passes `type: "string"`?
Present with value	`"Alice"`	Yes	Yes
Present as null	`null`	Yes	No (unless nullable)
Missing entirely	(key absent)	No	N/A (not checked)
Empty string	`""`	Yes	Yes

How the Example Schema Works

The schema models a user record with soft-delete support:

firstName is a required string that must always have a value — null is not allowed.
middleName is required but nullable. The user must explicitly send this field, but its value can be null (for people without a middle name) or a string.
deletedAt is an optional, nullable date-time string. When the record is active, this is null. When soft-deleted, it holds an ISO 8601 timestamp.

The test data passes validation: "Alice" is a string, null satisfies ["string", "null"], and deletedAt (also null) satisfies the same union type.

Draft Differences

In Draft 2020-12 and Draft 7, nullable fields use type arrays: "type": ["string", "null"]. The OpenAPI 3.0 ecosystem introduced a separate nullable: true keyword, but this is not part of the JSON Schema specification — it is an OpenAPI extension. OpenAPI 3.1 aligns with JSON Schema Draft 2020-12 and drops nullable in favor of type arrays.

Best Practices

Prefer null over missing keys when you want to distinguish "not provided" from "explicitly empty." This makes your API contract clearer.
Combine with required: Making a nullable field required forces clients to acknowledge the field exists, even if the value is null.
Document intent: Use the description keyword to explain why a field is nullable, so API consumers understand the semantics.

Use Case

Use nullable types for database-backed APIs where columns allow NULL values, soft-delete timestamps, optional relationship fields, and any scenario where the absence of a meaningful value must be explicitly represented rather than omitted.

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