Javascript objects: get parent [duplicate]

Question

A nested object (child) inside another object (parent) cannot get data directly from its parent.

Have a look on this:

var main = {
    name : "main object",
    child : {
        name : "child object"
    }
};

If you ask the main object what its child name is (main.child.name) you will get it.
Instead you cannot do it vice versa because the child doesn't know who its parent is.
(You can get main.name but you won't get main.child.parent.name).

By the way, a function could be useful to solve this clue.
Let's extend the code above:

var main = {
    name : "main object",
    child : {
        name : "child object"
    },
    init : function() {
        this.child.parent = this;
        delete this.init;
        return this;
    }
}.init();

Inside the init function you can get the parent object simply calling this.
So we define the parent property directly inside the child object.
Then (optionally) we can remove the init method.
Finally we give the main object back as output from the init function.

If you try to get main.child.parent.name now you will get it right.
It is a little bit tricky but it works fine.

No. There is no way of knowing which object it came from.

s and obj.subObj both simply have references to the same object.

You could also do:

var obj = { subObj: {foo: 'hello world'} };
var obj2 = {};
obj2.subObj = obj.subObj;
var s = obj.subObj;

You now have three references, obj.subObj, obj2.subObj, and s, to the same object. None of them is special.

This is an old question but as I came across it looking for an answer I thought I will add my answer to this to help others as soon as they got the same problem.

I have a structure like this:

var structure = {
    "root":{
        "name":"Main Level",
        nodes:{
            "node1":{
                "name":"Node 1"  
            },
            "node2":{
                "name":"Node 2"  
            },
            "node3":{
                "name":"Node 3"  
            }
        }
    }
}

Currently, by referencing one of the sub nodes I don't know how to get the parent node with it's name value "Main Level".

Now I introduce a recursive function that travels the structure and adds a parent attribute to each node object and fills it with its parent like so.

var setParent = function(o){
     if(o.nodes != undefined){
          for(n in o.nodes){
              o.nodes[n].parent = o;
              setParent(o.nodes[n]);
          }
     }
}

Then I just call that function and can now get the parent of the current node in this object tree.

setParent(structure.root);

If I now have a reference to the seconds sub node of root, I can just call.

var node2 = structure.root.nodes["node2"];
console.log(node2.parent.name);

and it will output "Main Level".

Hope this helps..

Many of the answers here involve looping through an object and "manually" (albeit programmatically) creating a parent property that stores the reference to the parent. The two ways of implementing this seem to be...

1. Use an init function to loop through at the time the nested object is created, or...
2. Supply the nested object to a function that fills out the parent property

Both approaches have the same issue...

How do you maintain parents as the nested object grows/changes??

If I add a new sub-sub-object, how does it get its parent property filled? If you're (1) using an init function, the initialization is already done and over, so you'd have to (2) pass the object through a function to search for new children and add the appropriate parent property.

Using ES6 Proxy to add parent whenever an object/sub-object is set

The approach below is to create a handler for a proxy always adds a parent property each time an object is set. I've called this handler the parenter handler. The parenter responsibilities are to recognize when an object is being set and then to...

1. Create a dummy proxy with the appropriate parent and the parenter handler

   var p = new Proxy({parent: target}, parenter);
   

2. Copy in the supplied objects properties-- Because you're setting the proxy properties in this loop the parenter handler is working recursively; nested objects are given parents at each level

   for(key in value){
       p[key] = value[key];
     }
   

3. Set the proxy not the supplied object

   return target[prop] = p;
   

Full code

var parenter = {
  set: function(target, prop, value){
    if(typeof value === "object"){
      var p = new Proxy({parent: target}, parenter);
      for(key in value){
        p[key] = value[key];
      }
      return target[prop] = p;
    }else{
      target[prop] = value;
    }
  }
}

var root = new Proxy({}, parenter);

// some examples
root.child1 = {
    color: "red", 
    value: 10, 
    otherObj: { 
       otherColor: "blue", 
       otherValue: 20
    }
}

// parents exist/behave as expected
console.log(root.child1.color)                 // "red"
console.log(root.child1.otherObj.parent.color) // "red"

// new children automatically have correct parent
root.child2 = {color: "green", value3: 50};
console.log(root.child2.parent.child1.color)   // "red"

// changes are detected throughout
root.child1.color = "yellow"
console.log(root.child2.parent.child1.color)   // "yellow"

Notice that all root children always have parent properties, even children that are added later.