Javascript equivalent of VBS redim preserve

Question

I'm trying to rewrite some old VBScript as Javascript for an ASP.NET application and there is one line I'm not sure how to translate, and I'm not even entirely positive what it's doing.

The application essentially allows the user to enter in a new employee number to add to the database and then assign it user permissions. Don't ask me why the code is such a mess, I didn't write it originally, I'm just trying to make it work in Chrome

here's the relevant code that i've managed to translate so far:

if(form1.txtEmpno.value != ""){
var oOption;
oOption = document.createElement("OPTION");
oOption.text=form1.txtEmpno.value;
oOption.value=form1.txtEmpno.value;
form1.lstActive.add (oOption);
oOption = document.createElement("OPTION");
oOption.text="";
oOption.value="";
form1.lstPerms.add (oOption);
redim preserve arrUsers(1,ubound(arrUsers,2)+1);
arrUsers(0,ubound(arrUsers,2)) = form1.txtEmpno.value;
arrUsers(1,ubound(arrUsers,2)) = "";
form1.txtEmpno.value = "";
oOption = null;
}

here's the line in question:

redim preserve arrUsers(1,ubound(arrUsers,2)+1);

Answer 1

MSDN defines ReDim [Preserve] varname(subscripts) as:

The ReDim statement is used to size or resize a dynamic array that has already been formally declared using a Private, Public, or Dim statement with empty parentheses (without dimension subscripts). You can use the ReDim statement repeatedly to change the number of elements and dimensions in an array.

If you use the Preserve keyword, you can resize only the last array dimension, and you can't change the number of dimensions at all. For example, if your array has only one dimension, you can resize that dimension because it is the last and only dimension. However, if your array has two or more dimensions, you can change the size of only the last dimension and still preserve the contents of the array.

Arrays in JavaScript have different semantics to VBScript's arrays, especially in that they're actually closer to a vector than a true array, furthermore JavaScript does not provide for true N-dimensional arrays: instead you use staggered-arrays (arrays-within-arrays). Which means your VBScript cannot be syntactically converted to JavaScript.

Here's your relevant code in VBScript:

ReDim Preserve arrUsers(1,ubound(arrUsers,2)+1)
arrUsers(0,ubound(arrUsers,2)) = form1.txtEmpno.value
arrUsers(1,ubound(arrUsers,2)) = ""

We see that arrUsers is a 2-dimensional array. This will need to be converted into a staggered array, but you haven't posted the code that defines and initializes arrUsers, nor how it is used later on, so I can only work from making assumptions.

It looks to be adding 1 element to the last dimension, but the code only seems to use the extra space in the [1] subscript (i.e. it only wants the extra dimensional space for certain values of the 0th dimension instead of all values), which makes this simpler as you don't need to iterate over every 0th-dimension subscript.

JavaScript arrays have numerous function-properties that we'll use, in particular push: which appends an element to the end of an array (internally growing the buffer if necessary), and pop which removes the last (highest-indexed) element from an array (if an array is empty, it's a NOOP):

var arrUsers = [ [], [] ]; // empty, staggered 2-dimensional array
...
arrUsers[0].push( form1.txtEmpno.value );
arrUsers[1].pop();

Much simpler.

However, if this array is just part of some internal model to store and represent data then you should take advantage of JavaScript object-prototypes instead of using array indexes, as that makes the code self-describing, for example:

var User = function(empNo, name) {
    this.employeeNumber = empNo;
    this.name = name;
};

var users = [];
users.push( new User(1, "user 1") );
users.push( new User(23, "user 23") );

...

for(var i = 0; i < users.length; i++ ) {
    alert( users[i].name );
}