javascript/loopback: add property which value is the result of a function

Question

EDIT: This is NOT a duplicate. While the answer is great, the question applies to the context of loopbackjs. There, most properties are accessed asynchronously. I tried the proposed solution and it doesn't work because the property count is accessed asynchronously:

node.__count__children(function(err, count) {
   if (err) {
     console.log(err);
     return;
   }

   return count;
});

This means I can't rely on the language features directly, but must account for asynchronicity. EDIT END

In django/python, I could define a property like so:

@property
def count_children(self):
    return len(self.children)

Is this possible in javascript as well? Actually I need it in the context of loopback, but maybe it's a feature of the language?

I have tried manipulating the prototype:

Node.prototype.count_children = function() {
     return this.children.length;
}

But this just adds a function count_children() to the object, it doesn't put the value of the function as property. Is that possible in javascript?

All methods are just properties with function values in JavaScript. Do you want one that is not inherited, or what do you mean? — Bergi, Dec 15 '15 at 02:41
If it's async, you need to provide a callback function and as a property, there's no way to pass a callback. — , Dec 16 '15 at 15:29
I think it's fair to say then that it can't be done, thus maybe you want to provide your comment as an answer? Just to close the question @squint — transient_loop, Dec 17 '15 at 22:34
@faboolous: I updated the answer, and also added some info about returning a Promise, which may satisfy your needs. — , Dec 17 '15 at 23:14

score 2 · Accepted Answer · 2015-12-17T23:13:34.247

I think you're saying you want a "getter" behavior on a property. Basically you want to call a function without the parens. If so, you can define it using Object.defineProperty():

Object.defineProperty(Node.prototype, "count_children", {
  get: function() {
    return this.children.length;
  }
});

document.body.appendChild(document.createElement("pre")).textContent = "Total <li> count: " + 
  document.querySelector("ul").count_children;

<ul>
  <li>foo
  <li>bar
  <li>baz
</ul>

If the behavior you need to wrap is asynchronous, it would seem to require a callback, and therefore a getter property wouldn't provide the chance to assign one.

If the API you're wrapping returns something like a "Promise" object, you can still return that. You could also return your own Promise, whether as part of the new ECMAScript 6, as an external library, or as your own simpler implementation.

thanks for this answer, which would be great in a pure javascript environment, but I am accessing the property in loopbackjs, where the count_children property can only be accessed asynchronously — transient_loop, Dec 15 '15 at 02:56

javascript/loopback: add property which value is the result of a function

1 Answers1