FAQ: JavaScript Promises - Constructing a Promise Object

FAQ JavaScript FAQ
#23

Hi,
I would like to understand why we don’t invoke the myExecutor function here:
function orderSunglasses() { return new Promise(myExecutor); };

and so why this is actually running and returning a value :
function myExecutor(resolve, reject){ if (inventory.sunglasses > 0) { resolve('Sunglasses order processed.'); } else { reject('That item is sold out.'); } };

thanks for your help

#24

Because we want the Promise to invoke it. It is passed as a reference only the same way we would pass a callback.

1 Like
#25

Why is this a = b = c, not just a=c? What purpose do the function and variable perform?

console.log(new Promise(myExecutor));

Works exactly the same, so why bother with the extra steps???

#26

The way you type still works. The exercise just probably tried to show how arguments were passed down from myExecutor to new Promise(), I guess?

#27

4 posts were split to a new topic: Why do we use Bash all of a sudden?

#29

asking myself the same!

it was recommended in a previous lesson to avoid using the Arrow function, as in some specific cases (that I forgot) it can create error.

but in most of the following exercices, we are forced to use the arrow function.
I don’t understand.

const myExecutor(resolve, reject) {
  if (inventory.sunglasses > 0 ) {
    resolve('Sunglasses order processed.');
  } else { 
    reject('That item is sold out.');
  }
 };
#30

From example:

const myFirstPromise = new Promise(executorFunction);

Is this wrong?, I followed this example but didn’t work… step 2.

#31

13 months later and still the same. Thanks for the insights.

#32

mine is not working too

#33

What the ■■■■? That fixed my problem. There’s nothing wrong with the code at all.

#34

I understand your frustration as I’ve experienced the same thing. However I think when you want a future in programming you have to be very precise in the work you do. Being strict in these assignments actually helps us develope this skill!

1 Like
#35

What is the point of the bash terminal?

#36

So I skipped the step 3 to create another variable with the value of the Promise function and just log my promise function variable as it was in the example
const myExecutor = (resolve, reject) => {

if(inventory.sunglasses  > 0) {

    resolve('Sunglasses order processed.');

  } else {

    reject('That item is sold out.');

  }

};

const orderSunglasses = new Promise(myExecutor);

console.log(orderSunglasses);

This is working also fine, so what the purpose of created another variable ? I understood in one the first lessons that the less we have variable in the global scope, cleaner is our code and here it’s the opposite we are requested… :face_with_monocle:

#37

There’s actually two reasons for that:

  1. this usually (will explain later) scopes to the parent object of the module (function inside an object) in which it’s written. For example, if you had written the function as a module of inventory like so:
const inventory = {
  sunglasses: 1,
  pants: 1088,
  bags: 1344,
  
  myExecutor(resolve, reject) {
    if(this.sunglasses > 0) {
      resolve('Sunglasses order processed.');
    } else {
      reject('That item is sold out.');
    };
  }
};

the this.sunglasses, would reference as intended by you, because this refers to inventory, the parent of the module (function) myExecutor.
Important: notice that I did NOT use an arrow function in declaring my module, as that would not work. I COULD have used a normal function declaration though function myExecutor () {}; , and that would have worked just aswell.

Which brings us to point two:

  1. this does not work the same way inside arrow functions.

In an arrow function the keyword this isn’t bound to it’s functions parent (object in which function is inserted), and I recomend reading documentation on it.

Disclaimer: don’t trust my every word as absolute true, as I’m also still learning.