How do I deep copy in javascript?

Question

How do I deep copy in javascript?

I have a situation similar to what you see below. The variable id is set to '03' before the first asynchronous callback is returned. Is there a way to deep copy, or "close" around the variable like you can with blocks in Objective-C? Is there a best practice?

var ids = ['01', '02', '03'];

for(var i=0, i < ids.length; i++){
  var id = ids[i];
  collection.find({id: ids} function () {
    console.log(id);
  });
}

The console output is:

03
03
03

javascript
node.js

Answer 1

There are several ways to do it. One is to iterate with a a method that uses a closure as a callback.

ids.forEach(function (elem) {
    collection.find({id: ids} function () {
        console.log(elem);
    });
});

Answer 2

Creating a closure creates a private scope, which will do what you are expecting.

var ids = ['01', '02', '03'];

for(var i=0, i < ids.length; i++){
  var id = ids[i];
  (function(index){ 
      collection.find({id: ids}, function () {
          console.log(index);
      });
  })(id); 
}

Answer 3

If you use .forEach() instead of a for loop you get the closure for free:

var ids = ['01', '02', '03'];

ids.forEach(function (id) {
  collection.find({id: ids} function () {
    console.log(id);
  });
});

Or if you don't have ES5-isms like .forEach() you can use a self-executing function to close over state. Here we'll just create an innerly-scoped i separate from the parent i:

var ids = ['01', '02', '03'];

for (var i=0; i < ids.length; i++) (function (i) {
    var id = ids[i];
    collection.find({id: ids} function () {
        console.log(id);
    });
})(i)

Answer 4

If you are using jQuery. It's easy

var newObject = jQuery.extend(true, {}, oldObject);

That's all you have to do.