Why does the function return the value of the local JSON variable instead the global?

Question

Hello,

I have a global JSON variable where I store some parameters and then each time I run the function I want to be able to modify them but just locally inside that function.

So every time I run the function I want a fresh copy of the global variable inside the local one.

The problem is that I copy the global variable to a local one defined in the function, and I make changes to the local variable, but the next time I run the function, instead having an intact copy of the global variable, I am having the one where I have already modified things.

Thanks! :)

var test = {"name":"me"};
function bla() {
  var t=test;
  t.name="you";
  t.age=55;
  alert(test.name); // Returns "you" that have been set locally instead of "me" that was global value.
}
bla();

Answer 1

Objects are assigned by reference so after

var t=test;

variables t and test refer to the same object. You need to make a copy of the object to get the behavior you are looking for. Since you are working with JSON, you could just serialize the original object, then re-parse into a new variable

var t = JSON.parse(JSON.stringify(test));

Then modify the new object as you please

Answer 2

var t = test does not create a copy of the object. It simply references the same object with another name. If you wish to copy the object, consider using something like jQuery's $.extend() function:

var t = $.extend({}, test);

While on that topic, the equivalent code for arrays is the following:

var t = test.slice(0);

Answer 3

on your line

var t=test;

you're not copying the contents of test, you're creating a new reference to the same content. therefore, any modification to t's content, modifies test as well.

Answer 4

To copy an arbitrary object to another variable, use a prototype function like this one:

Object.prototype.clone = function() {
  var newObj = (this instanceof Array) ? [] : {};
  for (i in this) {
    if (i == 'clone') continue;
    if (this[i] && typeof this[i] == "object") {
      newObj[i] = this[i].clone();
    } else newObj[i] = this[i]
  } return newObj;
};

Usage:

var bar = foo.clone();

http://my.opera.com/GreyWyvern/blog/show.dml/1725165

Answer 5

You need to clone the object, so that you can modify the clone and leave the original intact... http://stackoverflow.com/questions/122102/what-is-the-most-efficient-way-to-clone-a-javascript-object