# Server `variables`

## Server variable object

Server variables are used when you need to make a substitution into the server URL such as when the subdomain is unique per tenant.

| Field Name | Type | Description |
| --- | --- | --- |
| default | string | **REQUIRED**. The default value to use for substitution, which SHALL be sent if an alternate value is _not_ supplied. Note this behavior is different than the Schema Object's treatment of default values, because in those cases parameter values are optional. If the `enum` is defined, the value MUST exist in the enum's values. |
| enum | [string] | An enumeration of string values to be used if the substitution options are from a limited set. The array MUST NOT be empty. If defined, the array MUST contain the default value. |
| description | string | An optional description for the server variable. |

## Visuals

The following example defines a server variable `project` with a default value of `app`.

```yaml
servers:
  - url: https://{project}.redoc.ly
    description: Hosted docs
    variables:
      project:
        default: app
        description: The project name
```

The default server variable value displays in the documentation when `expandDefaultServerVariables: true`.

![default server variable substituted into server url](/content-assets/server-variables-01-2688569ea872fc77.png)

The placeholder server variable value displays in the documentation when `expandDefaultServerVariables: false`.

![server variable substituted into server url](/content-assets/server-variables-02-632314ef4acefbec.png)

The try it console has server variable fields regardless of that setting.

![try it console server variable fields](/content-assets/server-variables-03-46141c80e174080c.png)

## Types

- `ServerVariable`

```ts
const ServerVariable: NodeType = {
  properties: {
    enum: {
      type: 'array',
      items: { type: 'string' },
    },
    default: { type: 'string' },
    description: null,
  },
  required: ['default'],
};
```