TypeScript - Async Constructor Workaround Example

index.ts

/**
 * Original code from [comment in microsoft/TypeScript issue #13355](https://github.com/microsoft/TypeScript/issues/13355#issuecomment-313847099)
 */

function loooooongTask() {
    return new Promise<string>(resolve => {
        setTimeout(() => {
            resolve("world");
        }, 3000);
    })
}

class Foo {
    name: string;
    constructor() { // I want add type assertion to here. Promise<Foo>. 
        return loooooongTask().then(v => {
            this.name = v;
            return this;
        })
    }

    hello() {
        console.log(`Hello, ${this.name}`);
    }
}

(async () => {
    const obj = await new Foo();
    obj.hello();
})();

/**
 * Modified form 
 */

class ModFoo
{
    name!: string;
    static new(): Promise<ModFoo>
    {
        return (async (instance: ModFoo) => 
        {
            instance.name = await loooooongTask();
            return instance;
        })(new ModFoo())
    }
    private constructor() { }

    hello() {
        console.log(`Hello, ${this.name}`);
    }
}

(async () => {
    const obj = await ModFoo.new();
    obj.hello();
})();