---
title: "Make a field optional"
description: "Suffix a key with `?` so a field that never receives a value is dropped instead of erroring."
source: "https://aontu.dev/how-to/make-a-field-optional/"
---

# Make a field optional

Suffix a key with \`?\` so a field that never receives a value is dropped instead of erroring.

Rendered from [`docs/how-to/make-a-field-optional.md`](https://github.com/aontu-lang/aontu/blob/main/docs/how-to/make-a-field-optional.md) in the engine repository, where a correction belongs, and where the test suite executes every example on this page.

Some fields are genuinely sometimes-absent, and a schema that demands them anyway teaches producers to send empty strings. Suffix the key with `?` instead. An optional field that never receives a concrete value is dropped from the output:

```aontu
record: { id: integer, note?: string }
record: { id: 1 }
```

```json
{
  "record": {
    "id": 1
  }
}
```

Dropped means absent: no `note`, not `note: null`. Supplying a value keeps the field, checked against its constraint as usual:

```aontu
record: { id: integer, note?: string }
record: { id: 1, note: hi }
```

```json
{
  "record": {
    "id": 1,
    "note": "hi"
  }
}
```

An optional key with a [ranked default](https://aontu.dev/docs/reference-language#preference--default-) is filled rather than dropped, because the default is a concrete value arriving:

```aontu
record: { id: integer, retries?: *3 | integer }
record: { id: 1 }
```

```json
{
  "record": {
    "id": 1,
    "retries": 3
  }
}
```

Use `note?:` for “may not exist” and `retries?: *3 | integer` for “always exists, sender may omit”. The two read almost the same and generate differently.

An optional key is still a declared key, so it coexists with [`close`](https://aontu.dev/how-to/forbid-unexpected-keys): the sealed map admits the key when it arrives and drops it when it does not:

```aontu
config: close({ id:integer note?:string })
config: id: 1
```

```json
{
  "config": {
    "id": 1
  }
}
```

The full rules, including how optionality survives references, are in [Optional keys `?`](https://aontu.dev/docs/reference-language#optional-keys-). For defaults on their own, see [provide defaults](https://aontu.dev/how-to/provide-defaults); on a JSON Schema export an optional key is simply absent from `required`: see [export JSON Schema](https://aontu.dev/how-to/export-json-schema).