Default

The Default macro generates a static defaultValue() factory method that creates instances with default values. This is analogous to Rust’s Default trait, providing a standard way to create “zero” or “empty” instances of types.

Generated Output

Type	Generated Code	Description
Class	`static defaultValue(): ClassName`	Static factory method
Enum	`defaultValueEnumName(): EnumName`	Standalone function returning marked variant
Interface	`defaultValueInterfaceName(): InterfaceName`	Standalone function returning object literal
Type Alias	`defaultValueTypeName(): TypeName`	Standalone function with type-appropriate default

Default Values by Type

The macro uses Rust-like default semantics:

Type	Default Value
`string`	`""` (empty string)
`number`	`0`
`boolean`	`false`
`bigint`	`0n`
`T[]`	`[]` (empty array)
`Array<T>`	`[]` (empty array)
`Map<K,V>`	`new Map()`
`Set<T>`	`new Set()`
`Date`	`new Date()` (current time)
`T \\| null`	`null`
`CustomType`	`CustomType.defaultValue()` (recursive)

Field-Level Options

The @default decorator allows specifying explicit default values:

@default(42) - Use 42 as the default
@default("hello") - Use “hello” as the default
@default([]) - Use empty array as the default
@default({ value: "test" }) - Named form for complex values

Example

Before (Your Code)

/** @derive(Default) */
class UserSettings {
    /** @default("light") */
    theme: string;

    /** @default(10) */
    pageSize: number;

    notifications: boolean; // Uses type default: false
}

After (Generated)

class UserSettings {
    theme: string;

    pageSize: number;

    notifications: boolean; // Uses type default: false

    static defaultValue(): UserSettings {
        const instance = new UserSettings();
        instance.theme = 'light';
        instance.pageSize = 10;
        instance.notifications = false;
        return instance;
    }
}

Generated output:

class UserSettings {
    theme: string;

    pageSize: number;

    notifications: boolean; // Uses type default: false

    static defaultValue(): UserSettings {
        const instance = new UserSettings();
        instance.theme = 'light';
        instance.pageSize = 10;
        instance.notifications = false;
        return instance;
    }
}

Enum Defaults

For enums, mark one variant with @default:

Before (Your Code)

/** @derive(Default) */
enum Status {
    /** @default */
    Pending,
    Active,
    Completed
}

After (Generated)

enum Status {
    /** @default */
    Pending,
    Active,
    Completed
}

export function statusDefaultValue(): Status {
    return Status.Pending;
}

namespace Status {
    export const defaultValue = statusDefaultValue;
}

Generated output:

enum Status {
    /** @default */
    Pending,
    Active,
    Completed
}

export function statusDefaultValue(): Status {
    return Status.Pending;
}

namespace Status {
    export const defaultValue = statusDefaultValue;
}

Error Handling

The macro will return an error if:

A non-primitive field lacks @default and has no known default
An enum has no variant marked with @default
A union type has no @default on a variant