What do double brackets mean in javascript and how to access them

Question

Situation

I have the following function which uses Promise.

var getDefinitions = function() {     return new Promise(function(resolve) {         resolve(ContactManager.request("definition:entities"));     }); }  var definitions = getDefinitions() 

The contents of definitions is:

Promise {     [[PromiseStatus]]: "resolved",     [[PromiseValue]]: child } 

Accessing the PromiseValue property directly returns undefined

var value = definitions.PromiseValue; // undefined 

Question

What do the double brackets [[ ]] mean, and how do I retrieve the value of [[PromiseValue]].

Answer 1

What's the stuff inside [[]]

My question is what do the double brackets [[ ]] mean, and how do I retrieve the value of [[PromiseValue]].

It's an internal property. You cannot access it directly. Native promises may only be unwrapped in then with promises or asynchronously in generally - see How to return the response from an asynchronous call. Quoting the specification:

They are defined by this specification purely for expository purposes. An implementation of ECMAScript must behave as if it produced and operated upon internal properties in the manner described here. The names of internal properties are enclosed in double square brackets [[ ]]. When an algorithm uses an internal property of an object and the object does not implement the indicated internal property, a TypeError exception is thrown.

You cannot

Seriously though - what are they?

Very nice! As the above quote says they're just used in the spec - so there is no reason for them to really appear in your console.

Don't tell anyone but these are really private symbols. The reason they exist is for other internal methods to be able to access [[PromiseValue]]. For example when io.js decides to return promises instead of taking callbacks - these would allow it to access these properties fast in cases it is guaranteed. They are not exposed to the outside.

Can I access them?

Not unless you make your own Chrome or V8 build. Maybe in ES7 with access modifiers. As of right now, there is no way as they are not a part of the specification and will break across browsers - sorry.

So how do I get my value?

getDefinitions().then(function(defs){     //access them here }); 

But what if it returns an error? In prevision towards these cases, add the following at the end (and outside of) of your .then().

.catch(function(defs){     //access them here }); 

Although if I had to guess - you're not converting the API correctly to begin with since this conversion would only work in case the method is synchronous (in that case don't return a promise) or it returns a promise already which will make it resolved (which means you don't need the conversion at all - just return.