Enhanced Type Definitions for Decorated Classes in TypeScript

Check out this TypeScript Docs example showcasing decorators:

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

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

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

If you attempt to utilize the newProperty, a transpiler error will be generated:

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

To resolve this, how can you type it so the transpiler recognizes that newProperty is indeed present?

Answer №1

Decorators unfortunately do not have the ability to alter the underlying structure of the type they are applied to. However, you can treat the decorator as a simple function and then use the output as if it were a 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);

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