is it possible to define an array directly within a JavaScript Object and then use the array.length property on a function?

Question

The reason i am asking is because this code which has an array directly declared into the object will throw the following error: Uncaught ReferenceError: projects is not defined

var client = {
    projects: ['a', 'b', 'c', 'd'],
    test: function() {
        return projects.length;
    }
};
console.log(client.test());

Of course we can just declared the variable and then reference it within the object like this:

 var projects = ['a', 'b', 'c', 'd'];
    var client = {
        projects: projects,
        test: function() {
            return projects.length;
        }
    };
    console.log(client.test());

And that would work. But i really want to know why in the world is the first example giving the error. What is wrong with it??

Because that's not how JS objects work? Some basic JS O.O tutorials will clear this up for you quickly. — Dave Newton, Oct 06 '17 at 13:09
@DaveNewton Poor boy, give him some time to discover this. Then we could mark as duplicate his [next question](https://stackoverflow.com/questions/20279484/how-to-access-the-correct-this-inside-a-callback). — n00dl3, Oct 06 '17 at 13:16
@n00dl3 Fair enough—I’d already deleted my comment in preparation ;) — Dave Newton, Oct 06 '17 at 13:23
@daveNewton i know we can use this to refer to the object. What is not obvious to me, (maybe to you it is cause you are such a genius) is why we have to use the this keyword, when on the first example, it is evident that projects has been defined as a variable (just like on the second example). I thought the question was a bit more profound than it seemed. — escolta13, Oct 06 '17 at 13:38
Ok, maybe my mistake is that i was assuming that key value pairs were variables when they are not. Are they? heheh. Sorry for my existencial JavaScript questions. — escolta13, Oct 06 '17 at 13:41
@escolta13 Me being a genius isn’t related-what I’m saying is that there are thorough tutorials that discuss JS’s OOP nature that will answer this question, and your next one, better than an answer on SO. Key/value pairs are just that-JS objects aren’t classes (which JS doesn’t have despite ES6’s syntactic sugar). — Dave Newton, Oct 06 '17 at 13:52

score 4 · Answer 1 · answered Oct 06 '17 at 13:09

Use this.projects.length.

Within functions on the object, this refers to the current object.

Note that this will not work with arrow functions:

var client = {
    projects: ['a', 'b', 'c', 'd'],
    test: function() {
        return this.projects.length;
    }
};
console.log(client.test());

test: () => { would not work. However, with new object literal declaration syntax you could write test() { and that would work.

is it possible to define an array directly within a JavaScript Object and then use the array.length property on a function?

1 Answers1