CODEWITHSUNDEEP
typescripttypescript-decorator
Matthew James Davis
Matthew James Davis

Reputation: 12295

Extended Typings on Decorated Class in TypeScript

Here's some code on decorators in the TypeScript Docs:

function classDecorator<T extends {new(...args:any[]):{}}>(constructor:T) {
    return class extends constructor {
        newProperty = "new property";
        hello = "override";
    }
}


@classDecorator
class Greeter {
    property = "property";
    hello: string;
    constructor(m: string) {
        this.hello = m;
    }
}

console.log(new Greeter("world"));

However, if you try to use newProperty you get a transpiler error:

Property 'newProperty' does not exist on type 'Greeter'.

How do you type this so the transpiler knows that newProperty in fact exists?

Upvotes: 0

Views: 105

Answers (1)

Titian Cernicova-Dragomir
Titian Cernicova-Dragomir

Reputation: 249646

Unfortunately decorators can't impact the structure of the type it is used on. You can use the decorator as a simple function and use the result as the class:

const Greeter = classDecorator(class {
    property = "property";
    hello: string;
    constructor(m: string) {
        this.hello = m;
    }
    static test ="";
})
var d = new Greeter("world");
console.log(d.newProperty);

Upvotes: 1

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