> For the complete documentation index, see [llms.txt](https://modelico.javiercejudo.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://modelico.javiercejudo.com/quickstart/metadata.md).

# Metadata

Each of the types above has an associated metadata function in `M.metadata()`, with some useful additions:

```javascript
const {
  _,
  base,
  asIs,
  any,
  anyOf,
  maybe,
  withDefault,

  number,
  string,
  boolean,
  date,

  wrappedNumber,
  map,
  enumMap,
  stringMap,
  list,
  set
} = M.metadata()
```

## \_(Type, innerMetadata = \[])

eg. `_(Animal)`, `_(M.List, [string()])`, `_(M.Map, [date(), number()])`

To retrieve the metadata of Modélico types

## base(Type)

Retrieve the metadata of standard models for extension. See [JSON validation (Overridingthe reviver)](/advanced/json_validation.md#overriding-the-reviver) for an example.

## asIs(transform = identity)

Useful for simple transformations, eg.: `asIs(x => 2 * x)` to double numbers. It serves as base for primitive metadata and can be used to fix a value for debugging purposes, eg.: `asIs(() => 0)`. It is up to you to implement `toJSON()` on the instance to reverse the transformation if you need to.

## any()

Leaves the JSON input untouched (same as `asIs(identity)`).

## anyOf(\[\[metadata1, enumValue1], ..., \[metadataN, enumValueN]] = \[], field = 'type')

Useful to revive polymorphic JSON based on an enumerated field. The field defaults to `type` if not provided. See [Reviving polymorphic JSON: (example 1)](/advanced/reviving_polymorphic_json.md#example-1-revive-based-on-an-enumerated-field) for an example. Note that `anyOf` does not return metadata, but a metadata-returning function. This is something you can use as well to revive some cases of polymorphic JSON.

## number()

For native numbers

## wrappedNumber()

Wraps numbers with `ModelicoNumber` to support `-0`, `NaN` and ±`Infinity`.

## string()

For native numbers

## boolean()

For native booleans

## date()

To parse `date-time` strings, as defined by [RFC 3339, section 5.6](https://tools.ietf.org/html/rfc3339#section-5.6), eg. `1978-12-06T00:00:00.000Z`.

## map(keyMetadata = any(), valueMetadata = any())

eg. `map(date(), string())`

For ordered maps with arbitrary keys and values with JSON representation

```
[[<key1>, <value1>], [<key2>, <value2>]]
```

## enumMap(enumMetadata, valueMetadata = any())

eg. `enumMap(_(MyEnum), number())`

For ordered maps with enum keys and arbitrary values with JSON representation

```
{"<enum1>": <value1>, "<enum2>": <value2>}
```

## stringMap(valueMetadata = any())

eg. `stringMap(date())`

For ordered maps with string keys and arbitrary values with JSON representation

```
{"<string1>": <value1>, "<string2>": <value2>}
```

## list(itemMetadata = any()), list(\[...itemMetadata])

eg. `list(string())`; tuples: `list([number(), string()])`

For arrays. It has support for tuples in the sense of fixed-length arrays with per-item metadata.

## set(itemMetadata)

eg. `set(number())`

For ordered collections with no repeated values

## maybe(metadata = any())

eg. `maybe(string())`

To declare a value as optional. Unless a value is a `Maybe`, it cannot be missing from the JSON or be `null`. See [Optional / null values: Maybes](/basics/optional_values.md#maybes) for more details.

## withDefault(metadata, default)

eg. `withDefault(number(), 100)`

To set a default if the value is `null` or missing. See [Optional / null values: using with default](/basics/optional_values.md#using-withdefault) for more details.