Decorators

Decorators in TypeSpec allow developers to attach metadata to types within a TypeSpec program. They can also be used to compute types based on their inputs. Decorators form the core of TypeSpec’s extensibility, providing the flexibility to describe a wide variety of APIs and associated metadata such as documentation, constraints, samples, and more.

A range of TypeSpec constructs can be decorated, including namespaces, operations and their parameters, and models and their members.

Decorators are defined using JavaScript functions that are exported from a standard ECMAScript module. When a JavaScript file is imported, TypeSpec will look for any exported functions prefixed with $, and make them available as decorators within the TypeSpec syntax. When a decorated declaration is evaluated by TypeSpec, the decorator function is invoked, passing along a reference to the current compilation, an object representing the type it is attached to, and any arguments the user provided to the decorator.

Applying decorators

Decorators are referenced using the @ prefix and must be placed before the entity they are decorating. Arguments can be provided by using parentheses, similar to function calls in many programming languages, e.g., @myDec1("hi", { a: string }).

Here’s an example of declaring and then using a decorator:

@tag("Sample")
model Dog {
  @validate(false)
  name: string;
}

If no arguments are provided, the parentheses can be omitted.

@mark
model Dog {}

Augmenting decorators

Decorators can also be applied from a different location by referring to the type being decorated. For this, you can declare an augment decorator using the @@ prefix. The first argument of an augment decorator is the type reference that should be decorated. As the augment decorator is a statement, it must end with a semicolon (;).

model Dog {}

@@tag(Dog, "Sample");

This is equivalent to:

@tag("Sample")
model Dog {}

Example: decorating a model property to indicate that it is read-only

model Dog {
  name: string;
}

@@visibility(Dog.name, "read");

Creating decorators

For more information on creating decorators, see Creating Decorators.