jns42-generator, TypeScript edition

Generate types from a JsonSchema document.

Mapping

Mapping between the intermediate model and TypeScript types.

Compounds

Compounds are combinations of types. They may appear on any schema.

Type

If a schema has compound types then the containing type is merged with the compound types via intersection (&). It is possible that there are many

Example:

type ContainingType = unknown;
type CompoundType1 = Type1 | Type2;
type CompoundType2 = Type3 & Type4;
type Type = ContainingType & CompoundType1 & CompoundType2;

These are the compound types:

• reference
• oneOf
• anyOf
• allOf
• ifThenElse
• not

The reference compound refers to a schema that is then merge via intersection. Example:

type Type = ContainingType & ReferenceType;

The oneOf compound is a one to one match with TypeScript's union merge. Example:

type Type = OneOf1 | OneOf2;

In a anyOf compound, one or more types should be valid. We can achieve this by figuring out all possible combinations (https://stackoverflow.com/questions/64414816/can-you-return-n-choose-k-combinations-in-javascript-using-array-flatmap/64414875#64414875) and then merging those combinations via an intersection and then merging the result via a union.

Example

type Type =
  | AnyOf1
  | AnyOf2
  | AnyOf3
  | (AnyOf1 & AnyOf2)
  | (AnyOf1 & AnyOf3)
  | (AnyOf2 & AnyOf3)
  | (AnyOf1 & AnyOf2 & AnyOf3);

The allOf compound is the same as a TypeScript intersection merge. Example:

type Type = AllOf1 & AllOf2;

The ifThenElse compound is represented in TypeScript as an intersection join between if and then and then a union join with the else type. Example

type Type = ThenType | ElseType;

If the if schema is valid then we want to validate the then schema. If the if schema is not valid, then we want the else schema.

The not compound will only affect validation.

Validation

Validation is always done on the containing type, but, in addition any compound needs to be valid as well.

Validating the reference compound means simply validating the schema it points to.

When we want to validate oneOf then we need to walk through every type and see if it is valid. If one type is valid, then we continue and expect all other types top be invalid. In a oneOf exactly one type must be valid, not 0, not 2, not 10.

In case we validate anyOf we simply walk all types and if one is valid, the compound is valid. There might be more valid types, but we need at least one to be valid.

Validating ifThenElse can be done by first validating the against the if schema, if that is valid, we validate the then schema and that is the result of the compound validation. If the if schema is not valid, then we validate the else schema and then that is the result of the validation.

When validating the not schema we simply invert the result if the validation of the not schema.

Map

The intermediate document's type map (object in JsonSchema) may be mapped to a TypeScript interface. It is possible that the interface has an index signature.

Type

Properties that influence what the interface looks like are:

• required
• patternProperties
• mapProperties
• objectProperties

Interfaces can be created with an index signature example:

interface ExampleWithIndex {
  [key: ExampleKey]:
    | ExampleMapProperty
    | ExamplePatternProperty
    | ExampleOtherPatternProperty;
}

This is the case if mapProperties or patternProperties is set. If both are set, or multiple patternProperties are specified, then the types they represent are joined via union (|).

Interfaces may also be created without the index signature. Example:

interface ExampleNoIndex {
  id: ExampleId;
  name?: ExampleName;
}

In this case the required property will be used to make a property optional.

And then there is a case where an interface will have an index signature and normal properties. This happens if mapProperties or patternProperties is set and objectProperties is also set.

Example:

interface ExampleBoth {
  [key: ExampleKey]:
    | ExampleId
    | ExampleName
    | ExampleMapProperty
    | ExamplePatternProperty
    | ExampleOtherPatternProperty;
  id: ExampleId;
  name?: ExampleName;
}

Something special is happening here, all normal properties are added to the union of the index type. This is necessary to make the interface work in TypeScript, see https://www.typescriptlang.org/docs/handbook/2/objects.html#index-signatures.

Validation

Validation of map types is influences by the following properties:

• required
• minimumProperties
• maximumProperties
• objectProperties
• patternProperties
• propertyNames
• mapProperties

The properties defined in required must not be undefined for the validation to succeed. Properties are counted and if they are less than minimumProperties or more than maximumProperties validation fails.

If a property name is in the key of the objectProperties map then the property value is validated by the schema that is the value of the objectProperties map.

If a property name is not in the key of objectProperties then the property name is matched against the regular expressions in the keys of patternProperties, if there is a match the property value is matched with the schema in the corresponding property value.

If property name is not in objectProperties and does not match a key of patternProperties then the property name is validated via the schema provided by propertyNames. All property names need to be valid for the validation to succeed. Also the values of these properties are all validated via the schema in mapProperties.

Array

The array type may be rendered in TypeScript as a tuple or an array/.

Types

TODO

Validation

TODO