# Decorators

Both [event handlers](EventHandlers.md)
and [command handlers](CommandHandlers.md) work
with [TypeScript decorators][decorators].  These identify values that
will be injected into a fresh instance each time the handler runs.  If
you're familiar with Spring, this will be particularly intuitive.

[decorators]: https://www.typescriptlang.org/docs/handbook/decorators.html (TypeScript Decorators)

Consider the following code within a command handler:

```typescript
@Parameter({
    displayName: "Desired Spring Boot version",
    description: "The desired Spring Boot version across these repos",
    pattern: /^.+$/,
    validInput: "Semantic version",
    required: false,
})
public desiredBootVersion: string = "1.5.6.RELEASE";
```

The value of the `desiredBootVersion` parameter (if provided) will be
injected into an instance before the `handle` function is called.

The decorated variable names the parameter.  If you assign a value to
the variable, as in the example, it becomes the parameter’s default
value.