How do I restrict a field to a list of values in JSON Schema?

Use the "enum" keyword to restrict a field to a fixed list of allowed JSON values. Place "enum" inside the property schema: {"type": "object", "properties": {"status": {"type": "string", "enum": ["draft", "published", "archived"]}}, "required": ["status"]}. Only the exact values listed in the enum array will pass validation — any other value, including values that differ only in case like "Draft", will fail. Enum values are compared using strict equality. You can combine "enum" with "type" to be explicit, or omit "type" and let the enum alone restrict the value.

What is the difference between enum and const in JSON Schema?

Both "enum" and "const" restrict a value, but "enum" allows a list of values while "const" allows exactly one value. Use "enum" when multiple values are valid: {"enum": ["small", "medium", "large"]}. Use "const" when exactly one value is valid: {"const": "invoice"}. "const" is equivalent to {"enum": ["invoice"]} but more expressive and more readable in schemas. A common use of "const" is discriminator fields in oneOf/anyOf compositions, where each branch has a "const" type field that identifies which variant the data represents.

JSON Schema Enum: Allowed Values, Examples, and Mistakes

Q: Can a JSON Schema enum contain multiple types?

Yes. The "enum" keyword accepts any JSON values regardless of type, so a single enum array can mix strings, numbers, booleans, and null: {"enum": ["auto", "manual", 0, false, null]}. This means the field accepts any of those exact values. When mixing types, omit the "type" keyword because specifying a single type would restrict the valid values beyond what the enum allows. Mixed-type enums are useful for fields that need to accept both a sentinel value (like null or false) and meaningful string options.

Q: How do I use enum for status fields?

Status fields are one of the most common uses of "enum". Define the property under "properties" with "type": "string" and list your allowed statuses: {"type": "object", "required": ["status"], "properties": {"status": {"type": "string", "enum": ["pending", "active", "suspended", "cancelled"]}}}. This ensures only those four strings are accepted. If you later add a new status like "archived", add it to the enum array in the schema. Keep in mind that enum comparison is case-sensitive — "Active" and "active" are different values, so normalize status strings before validation.

Q: Can enum values be null?

Yes. To allow null as one of the valid values, include null in both the "type" array and the "enum" array: {"type": ["string", "null"], "enum": ["small", "medium", "large", null]}. If you include null in "enum" but not in "type", some validators will reject null because the "type" constraint fails first. If you omit "type" entirely and only use "enum" with null in the list, null is allowed. The safest pattern when mixing null with other values is to explicitly declare "type": ["string", "null"] alongside the enum.

Q: What happens if a value is not in the enum list?

If a value is not in the enum list, the validator reports a validation error and the data fails validation. For example, if the enum is ["active", "inactive"] and the data contains "suspended", the validator will report something like: "value must be one of: active, inactive". The exact error message depends on the validator library. Some libraries like Ajv report the path to the failing field (e.g., "/status") and list the allowed values. No exception is made for close matches or case differences — the comparison is exact and case-sensitive.

Last updated: May 12, 2026

The JSON Schema enum keyword restricts a value to an exact list — validation fails if the input matches none of the allowed values. An enum can mix all 6 JSON types: strings, numbers, booleans, null, arrays, and objects. String-only enums should pair enum with "type": "string" to block accidental numeric coercion. The const keyword is shorthand for a 1-entry enum and produces the same validation result. This guide covers basic string enums with 3–10 values, mixed-type enums, const vs enum tradeoffs, using if/then for conditionally required enum fields, and generating TypeScript union types from an enum schema using json-schema-to-typescript.

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Basic enum example

{
  "type": "object",
  "properties": {
    "status": {
      "type": "string",
      "enum": ["draft", "published", "archived"]
    }
  },
  "required": ["status"]
}

Only those three string values pass. "Draft", "deleted", and null fail.

Enum can contain any JSON value

An enum is not limited to strings. It can include numbers, booleans, null, arrays, and objects, though string enums are the easiest to maintain.

{
  "enum": ["auto", "manual", 0, false, null]
}

Nullable enum

If null is allowed, include it in both the type list and the enum values.

{
  "type": ["string", "null"],
  "enum": ["small", "medium", "large", null]
}

Enum inside arrays

Put enum under items when every array value must come from the same allowed list.

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": ["read", "write", "delete"]
  },
  "uniqueItems": true
}

Enum vs const

Use enum for multiple allowed values. Use const when exactly one value is allowed.

{
  "type": "object",
  "properties": {
    "type": { "const": "invoice" }
  },
  "required": ["type"]
}

For broader schema examples, see the JSON Schema examples guide.

Common mistakes

Mistake	Fix
Forgetting enum is case-sensitive	Add every accepted casing or normalize input first
Allowing null in type but not enum	Add `null` to the enum array
Using enum for large dynamic lists	Validate dynamic IDs in application logic
Putting enum beside an array schema	Put it inside `items` for array elements

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Frequently asked questions

How do I restrict a field to a list of values?

Use the enum keyword inside the property schema: {"type": "string", "enum": ["draft", "published", "archived"]}. Only those exact values pass — comparison is strict and case-sensitive.

Can a JSON Schema enum contain multiple types?

Yes. The enum array can mix strings, numbers, booleans, and null: {"enum": ["auto", 0, false, null]}. When mixing types, omit the type keyword to avoid conflicting constraints.

What is the difference between enum and const?

enum allows a list of values; const allows exactly one. Use const for discriminator fields in oneOf/anyOf schemas where each branch has a unique type identifier.

How do I use enum for status fields?

Define the property with type: "string" and list all valid statuses in enum. Keep statuses lowercase and normalize input before validation, since enum comparison is case-sensitive.

Can enum values be null?

Yes. Include null in both the type array (["string", "null"]) and the enum array. If null is in enum but not in type, some validators will reject it.

What happens if a value is not in the enum list?

The validator reports a validation error listing the allowed values. No exception is made for close matches or case differences — the comparison is exact. The error path points to the failing field.