javascript Object - access child property using dot notation

Question

I created an object that requires a dictionary object be passed to initialize, and has attributes (like length). I'd like to be able to simply access the dictionary's key : value pair by passing something like:

myNewObj['a'] or myNewObj.a

and not having to instead pass myNewObj.dictionary['a'], but can't figure out how to do so.

Is there a way to set a dynamic attribute that, if no other attribute or method exits, will instead look into the dictionary and find the associated key?

var newObject = function(dictionary) {
    this.dictionary     = dictionary;
    this.length         = dictionary[[Object.keys(dictionary)[0]]].length;

    this.get = function (column) {
        return this.dictionary[[column]];
    }
};

var myNewObj = new newObject({
    a   : [1,2,3],
    b   : [4,5,6]
});

console.log(myNewObj.get('a'));

I've updated this to show a .get() method that works, but it's still not exactly what I'm looking for.

jsFiddle: http://jsfiddle.net/2uMjv/571/

@thebjorn - if that's the only way to do what I'm looking for, I may just stick with the .get() method, but I appreciate the reference. — elPastor
– elPastor, Commented Sep 29, 2018 at 20:47
Since you cannot override operators (e.g., []) there's no clean way to do this. Proxies might work, but you run the risk of removing access to actual properties, particularly if you don't know what will be in the "dictionary". There are ways you could avoid that, but IMO, more trouble than it's worth. — Dave Newton
– Dave Newton, Commented Sep 29, 2018 at 21:03

woat · Accepted Answer · 2018-09-29 21:22:10Z

0

Although this might not exactly fit your use case of dynamic attributes, the closest thing I can think of is extending the prototype. At least by using Object.setPrototypeOf* you can avoid defining each method individually and have a base set of functions to apply to each one of these newObject's.

var newObject = function(dictionary) {
    this.dictionary = dictionary;
    this.length = dictionary[[Object.keys(dictionary)[0]]].length;

    this.get = function(column) {
        return this.dictionary[[column]];
    }
};

// Essentially a wrapper constructor.
var createNewObject = function(dictionary) {
    Object.setPrototypeOf(dictionary, new newObject(dictionary));
    return dictionary;
}

var myNewObj = createNewObject({
    a   : [1,2,3],
    b   : [4,5,6]
});

console.log(myNewObj['a']);
console.log(myNewObj.get('b'));

*You may want to look at the potential drawbacks in using this.

edited Sep 29, 2018 at 21:22

answered Sep 29, 2018 at 21:14

woat

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1 Comment

elPastor Over a year ago

thanks for the code. I'm pretty new to objects in javascript, so I'll need some time to wrap my head around this before I can tell whether it does what I need.

Jonas Wilms · Accepted Answer · 2018-09-29 22:50:38Z

0

Why don't you just set properties? :

 class Dictionary {
  constructor(dictionary) {
   Object.assign(this, dictionary);
  }

  get length() {
    return Object.keys(this).length;
  }

  get(column) {
    return this[column];
  }
}

var dict = new Dictionary({
  a   : [1,2,3],
  b   : [4,5,6]
});

console.log(dict.a, dict["a"], dict.get("a"));

answered Sep 29, 2018 at 22:50

Jonas Wilms

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