The correct way to bind object to Promise.then() argument

Question

I found out the hard way that one can't simply just pass in a object's function into the Bluebird then. I'm assuming that Bluebird's then is doing some magic and wrapping the passed in function in an anonymous function. So I attached a .bind to the function and it worked. Is this the correct way to do this with bluebird? Or is there some better way?

var Promise = require("bluebird")

var Chair = function(){
  this.color = "red"
  return this
}


Chair.prototype.build = function(wood){
  return this.color + " " + wood
}

var chair = new Chair()

//var x = chair.build("cherry")

Promise.resolve("cherry")
  .then(chair.build.bind(chair)) // color is undefined without bind
  .then(console.log)

I know none of this is async so please bare with the sync example, my usage is async.

Haven't heard of Bluebird, but if its Promises follow the Promise pattern as implemented by jQuery, then this is the way to go, as long as you're targeting modern browsers which support `Function.bind` (otherwise you'd need to roll your own, or use Underscore for a `bind` shim) — Lambart, Jun 23 '15 at 21:20
Sure. If @thefourtheye is right and this `Promise` has its own `bind` feature, I'd probably use that instead. — Lambart, Jun 23 '15 at 21:27

thefourtheye · Accepted Answer · 2015-06-23T21:47:04.317

So I attached a .bind to the function and it worked. Is this the correct way to do this with bluebird?

Yes, that is one way to retain the context. You can also pass an anonymous function (you might already know this).

Promise.resolve("cherry")
    .then(function (value) {
        return chair.build(value);
    })
    .then(console.log);

Or is there some better way?

You can actually use bluebird's Promise.bind method, like this

Promise.resolve("cherry")
  .bind(chair)
  .then(chair.build)
  .then(console.log)

Now, whenever the Promise handlers (fulfill handlers or rejection handlers) are called, inside the function, this will refer the chair object only.

_{Note 1: In this specific case, console.log also gets this as chair object, but it still works fine, because, in Node.js, console.log function is defined not only on the prototype bu also on the object itself, bound to the console object. The corresponding code is here.}

_{Note 2: If different handlers need different contexts, then better write anonymous functions. In that case, Promise.bind will not help much. But if you choose to use it, then you have to use different contexts for each of the handlers and your code might look something like this}

var chair1 = new Chair("red")
var chair2 = new Chair("green")

Promise.resolve("cherry")
    .bind(chair1)            // Changing the binding to `chair1`
    .then(chair1.build)
    .tap(console.log)
    .bind(chair2)            // Changing the binding to `chair2`
    .then(chair2.build)
    .tap(console.log);

The correct way to bind object to Promise.then() argument

1 Answers1