new object with async routines

Question

I want to instantiate an object where the constructor performs async calls before returning. The purpose is to do async currying. I am using co. The below example fails. What am I doing wrong?

var co = require('co')

function asyncFunction() { 
  return new Promise(function (resolve) {
    resolve()    
  })
}

function MyObject () {
  co(function * () {
    yield asyncFunction()
  }).then(()=> {
    this.runSomething = function() {
      return 'something'
    }
  })
}

new MyObject().runSomething()
// TypeError: (intermediate value).runSomething is not a function

When you see a pure dot notation sequence of properties without any callback (lke `new MyObject().runSomething()`), it is synchronous. You need a callback in the main code. — trincot, Aug 28 '16 at 08:50
Don't do it! Allow `new MyObject()` to be wholly synchronous and relegate anything async to method(s) of MyObject(). — Roamer-1888, Aug 28 '16 at 10:53

score 2 · Accepted Answer · answered Aug 28 '16 at 08:47

A new operator will always return an object immediately, and synchronously. You can't delay that operation.

You could instead create a function that will return a Promise for your object.

function makeObjectAsync() {
  const asyncResult = asyncOperation();
  return asyncResult.then(result => ({
    runSomething() { /* ... */ }
  }));
}

myObjectAsync()
  .then(obj => {
    // obj is ready to use.
    return obj.runSomething();
  });

You can combine that with co fairly easily to get read of some of the .then()s.

Thank you. I was suspecting this was the only path when using new. — Pål Thingbø, Aug 28 '16 at 08:50

new object with async routines

1 Answers1