# Configuration in tests

A test never reads your machine's environment. Each configuration field
gets its value in a test from `config.jadpo`: its `test:` value, or else
its `default:` ([Defaults and test values](settings-config-jadpo.md#defaults-and-test-values)).
So a test gives the same answer everywhere. A test changes a field for
itself, and the code it calls, with its `config:` setting:

```jadpo
type FromAddress = Email {}

config {
    mail_api_key: Text {
        secret: true
    }
    mail_sender: FromAddress {
        test: "reminders@example.test"
    }
}

function address() -> FromAddress {
    return config.mail_sender
}

test "reminders come from the test sender" {
    assert address() == "reminders@example.test"
}

test "a test can change the sender" {
    config: {
        mail_sender: "planning@example.test"
    }
    assert address() == "planning@example.test"
}
```

## Secrets

Secrets never appear in test source. Each test run gets a generated
throwaway value for each one. That is enough, because a secret can only
reach places the compiler owns, such as signing and integration
credentials, and integrations are faked in tests. A secret in a test's
`config:` is `configuration.secret_default`.

## What the compiler checks

A field that tested code reads with no `test:`, `default:` or `config:`
value is a compile error, `configuration.test_value_missing`. The compiler
follows the functions, actions and fixtures a test calls to find these
reads. A name in a test's `config:` that is no field is
`configuration.field_unknown`. A value in `config:` is checked as a
`default:` is.