JSON树和内存分配

Question

In JavaScript, if I create an Object, which is my understanding is that this is essentially represented as JSON, and if this object has a handle on its parent object, and the parent has a handle on the child object, how is this held in memory (is it a pointer? Or duplication of JSON).

For example, if I construct a Person Object, like the following, where each Person may have multiple children, and also has a handle on its parent.

var Person = function(json, parent) {
  this.name = json.name;
  this.parent = parent;
  this.children = new Array();
  if(json.children) {
    for(var i=0; i < json.children.length; i++) {
      this.children.push(new Person (json. children[i], this));
  }
}

My understanding is that if I change a property of a person, then this will be reflected in the Person, but also the the Parent of any of its children, so this would indicate the actual object is held once in memory and JavaScript uses pointers? Am I correct in thinking this?

My fear is this type of structure is expensive to hold in memory, as I may encounter the scenario where I have a few thousand of this type of hierarchical construct, which can go a few levels deep.

Answer 1

JSON is a text format for representing javascript objects in a plain text format.

Javascript objects themselves in Javascript are not JSON at all. They are some sort of internal format that is up to the Javascript implementation and is a balance between good runtime performance and efficient size. When one object contains a reference to another object in Javascript, that is not a copy of the object. You can think of it like a pointer to the other object and its likely that it is some sort of pointer internal to the implementation, but that is up to the implementation. The crux is that it works like a pointer.

A simple Javascript experiment shows that there's only one copy of the data (thus pointers to a single set of data are used) when you assign an object to multiple variables:

var x = {counter: 2};
var y = x;
x.counter = 10;
console.log(x.counter);   // 10
console.log(y.counter);   // 10

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JSON was invented long after Javascript itself as a means of exchanging Javascript data structures between different processes or applications or computers. Since then it has become a common data interchange structure used by many different languages, not just Javascript. You can read more about it here: http://www.json.org/ .