Async constructors for JavaScript

app.js

// In JavaScript, constructors can only be synchronous right now. This makes sense
// from the point of view that a constructor is a function that returns a newly
// initialized object.
//
// On the other hand, it would sometimes be very handy, if one could have async
// constructors, e.g. when a class represents a database connection, and it is
// desired to establish the connection when you create a new instance.
//
// I know that there have been discussions on this on StackOverflow & co., but
// the so-far mentioned counter arguments (such as: doesn't work as expected
// with new) didn't convince me. Actually, it works as expected (see below),
// as long as you use await – but this is true for any async function ;-)
//
// Could this be an idea for TC39?

'use strict';

const { promisify } = require('util');

const sleep = promisify(setTimeout);

class Example {
  // This is what I would like to have.
  // async constructor () {
  //   await sleep(1000);
  //   this.value = 23;
  // }
  
  // This is how it could be implemented.
  constructor () {
    return new Promise(async (resolve, reject) => {
      try {
        await sleep(1000);
        this.value = 23;
      } catch (ex) {
        return reject(ex);
      }
      resolve(this);
    });
  }
}

(async () => {
  // It works as expected, as long as you use the await keyword.
  const example = await new Example();

  console.log(example instanceof Example);
  console.log(example.value);
})();

Here's something I've been playing with the past couple days

function Post(db, post) {
  if (!new.target) {
    return new Post(db, post);
  }
  this.db = db;
  this.post = post;
  //
  // create new post
  //
  return new Promise(async (resolve, reject) => {
    try {
      //
      // if we have an id then the post is already in the
      // database and we have already loaded it!
      //
      if (!post.id) {
        let id = await this._insert(post);
        this.post.id = id[0];
      }
      resolve(this);
    } catch (err) {
      reject(err);
    }
  });
}

  let post = await Posts.create({
    title: 'title',
    body: 'body',
    author_id: author.id
  });

Posts.create = async function(params) {
  return Post(state.db, params);
}

good one!

There is one more way of doing an async constructor - Proxy

Typescript:

class A {

    private data: any
    private info: any

    public constructor (data: any, info: string) {
        this.data = data
        this.info = info
    }

}

interface InterfaceA {
    new(info: string): A;
}

class AProxy {

    public static init (): InterfaceA {
        const proxyHandler = {
            construct: async function (args: any) {
                // Some async operation
                const data = await retrieve()
                return new A(data, args)
            }
        }

        return new Proxy(A, proxyHandler)
    }

}

export default AProxy.init()

import A from './a.ts'

(async () => {
    const a = await new A('Some iNFO')
})()

Javascript:

class A {
    constructor (data, info) {
        this.data = data
        this.info = info
    }
}

class AProxy {

    static init () {
        const proxyHandler = {
            construct: async function (args) {
                // Some async operation
                const data = await retrieve()
                return new A(data, args)
            }
        }

        return new Proxy(A, proxyHandler)
    }

}

module.exports = AProxy.init()

But to be honest, if you need an async constructor, it is always better to use the build pattern as it is more readable and understandable