Question

I have a loop goes through an array of objects MyArrayOfObjects and then pushes the objects to a new array like this:

var NewArray = new Array();

for (i = 0; i < MyArrayOfObjects.length; i++) {

  TempObject = null;
  TempObject = new Object();

 // I have logic that copies certain properties but not others
 // but overall it looks like this:

  TempObject.prop1 = MyArrayOfObjects[i].prop1;
  TempObject.prop2 = MyArrayOfObjects[i].prop2;

  NewArray.push(TempObject);
}

As I loop through MyArrayOfObjects, I clear the TempObject and create a new one each time. Does NewArray contain the objects that I’m copying or just a reference to the objects copied and that then become deleted as the loop iterates?

Thanks.

Answer 1

> var NewArray = new Array();

It is generally considered better to use an array literal to create an array. Variable names starting with a capital letter are, but convention, used for constructors. Using “new” at the start of a variable name can easily slip to become “new Array”, and the name should reflect its purpose, so something like the following might be better:

var objectArray = [];

.

> for (i = 0; i < MyArrayOfObjects.length; i++) {

You should always declare variables, especially counters as undeclared variables are made properties of the global object (effectively global variables) when they are first assigned a value. Also, it is considered better to store the length of the array than get it in each iteration:

for (var i = 0, iLen = MyArrayOfObjects.length; i < iLen; i++) {

.

>   TempObject = null;
>   TempObject = new Object();

Again, declare variables. Assigning a value of null serves no useful purpose when you’re going to assign some other value immediately afterward. Just do the second assignment (and use a literal):

  var TempObject = {};

.

>  // I have logic that copies certain properties but not others
>  // but overall it looks like this:
> 
>   TempObject.prop1 = MyArrayOfObjects[i].prop1;
>   TempObject.prop2 = MyArrayOfObjects[i].prop2;
> 
>   NewArray.push(TempObject);

At this point, TempObject and NewArray[NewArray.length – 1] both reference the same object.

> }

As I loop through MyArrayOfObjects, I clear the TempObject and create
a new one each time.

There is no need to “clear” the object, just assign a new value to the variable. In javascript, all variables have a value that might be a primitive (e.g. string, number) or a reference to an object (e.g. Object, Array, Number, String)

Does NewArray contain the objects that I’m
copying or just a reference to the objects copied and that then become
deleted as the loop iterates?

It contains references to the new objects created on each iteration.

As variables hold references to objects, assigning a new value to the variable doesn’t do anything to the object. When an object is no longer referenced by any variable or object property, it is made available for garbage collection and may be removed automatically at some later time when garbage collection runs.