Default value

You can specify a default value for an item using the default keyword. When a data doesn’t have a corresponding value, the value of this keyword will be used instead to do the validation checks. This keyword is not mandatory, and the value of this keyword can be anything.

This keyword can alter the input data, defining the missing properties on objects, as shown in this PHP example. You can disable it by setting the allowDefaults option to false.

{
    "type": "object",
    "properties": {
        "prop1": {
            "type": "string",
            "default": "test"
        }
    }
}

PHP Example

Consider the following schema

{
  "$id": "http://example.com/schema.json",
  
  "type": "object",
  "properties": {
    "name": {
      "type": "string"
    },
    "age": {
      "$comment": "value is null <=> age not disclosed",
      "type": ["integer", "null"],
      "default": null
    }
  },
  "required": ["name"]
}

As you can see, the name is required, but the age isn’t. If not present, the age should be considered not disclosed (value is null).

Our PHP code is something like this:

$data = (object)['name' => 'Opis', 'age' => 18];

$result = $validator->validate($data, "http://example.com/schema.json");
if ($result->isValid()) {
    inset_into_db($data->name, $data->age);
}

What happens when age property is missing?

$data = (object)['name' => 'Opis'];

The age property is automatically added to the object with the default value. In this way you avoid the double check, so you don’t have to do:

$data->age ?? null

You might think that this isn’t something that saved precious time. Well… what if we had another property, let’s say address, that contains an object?

Instead of editing the same object in two or more files, you edit just the value of the default keyword.

{
  "type": "object",
  "properties": {
    "address": {
      "type": "object",
      "properties": {
        "country": {},
        "county": {},
        "city": {},
        "street": {},
        "street2": {}
      },
      "default": {
        "country": null,
        "county": null,
        "city": null,
        "street": "",
        "street2": ""
      }
    }
  }
}

Now in php you have to do something like this

$data->address ?? (object) [
    'country' => null,
    'county' => null,
    'city' => null,
    'street' => '',
    'street2' => '',
];

The fun begins when you have to add the zip property to the address: add it into the schema document, then find where you have to change in PHP files just to add the same object. We hate that, that’s way we set the object’s missing properties to default values (if default keyword is present).

If you don’t want this behaviour just disable it by setting the allowDefaults option to false.