Support property decorators for shorthand constructor-parameter syntax

TypeScript Version: 2.0.2

Code I’m taking advantage of shorthand property declarations syntax via constructor. The code below contains a problem, it won’t work as expected for class City, it such case typescript will treat PropertyDecorator as ParameterDecorator, there is no way to supply both PropertyDecorator and ParameterDecorator simultaneously for such shorthand syntax:

function PropertyDecorator(target: Object, propertyKey: string | symbol) { }
function ParameterDecorator(target: Object, propertyKey: string | symbol, index: number) {}

class Country {
    @PropertyDecorator public name: string;

    public constructor(name: string) {
        this.name = name;
    }
}

class City {
    public constructor(
        @PropertyDecorator public name: string,
        @PropertyDecorator public country: Country) {
    }
}

It looks better to use the following syntax that would allow distinguishing different decorator types:

function PropertyDecorator(target: Object, propertyKey: string | symbol) { }
function ParameterDecorator(target: Object, propertyKey: string | symbol, index: number) {}

class Country {
    public name: string;

    public constructor(name: string) {
        this.name = name;
    }
}

class City {
    public constructor(
        @PropertyDecorator public @ParameterDecorator name: string,
        @PropertyDecorator public @ParameterDecorator country: Country) {
    }
}

[aeoncleanse]

This would be really handy, and I don’t understand why this issue seems to have deadended at “But it would be ambiguous still”. Well, fine, come up with a special decorator syntax alias for use explicitly in this situation.

Since parameter decoration is the more common within a function declaration, even a constructor, we leave that the default.

To allow the use of decorators for the shorthand declaration, how about !@Prop or @@Prop? Just… Use a unique, special-case prefix that does the same thing as @, with the addition of telling the compiler “This particular decorator is a property decorator within a class constructor”.

Voila!

[sbuzonas]

I’m trying to make a private readonly property not enumerable and end up redefining the property in the constructor to get the value initialized.

export class Character {
    @enumerable(false)
    private readonly attributes: AttributeBag;

    constructor(attributes: AttributeBag = new AttributeBag()) {
        const descriptor = Object.getOwnPropertyDescriptor(Object.getPrototypeOf(this), 'attributes'));
        delete descriptor.set;
        delete descriptor.get;
        descriptor.value = attributes;
        Object.defineProperty(this, 'attributes', descriptor);
    }
}

This could be solved by supporting decorators with the shorthand syntax. I disagree that the proposed syntax is confusing. Decorators preceding a parameter name are parameter decorators, and any decorators preceding a token that makes it a property are property decorators.

Another solution could be to support declaration in the constructor even when a property is declared.