How do I traverse an array diagonally in javascript

I have an array with strings that I would like to traverse diagonally.
Assumptions:

• Each string is the same length.
• Arrays could be square or rectangular, horizontally or vertically.

The matrix looks like this:

A B C D
E F G H
I J K L

I Would like to get (from top left to bottom right):

A
EB
IFC
JGD
KH
L

and (from the bottom left to top right):

I
JE
KFA
LGB
HC
D

I already have a piece of code that works 3/4 of the way, but i cant seem to figure out what I am doing (wrong).

//the array
var TheArray = ['ABCD','EFGH','IJKL'];

//amount of rows
var RowLength = TheArray.length;
//amount of colums
var ColumnLength = TheArray[0].length;

The code I have chops up the diagonals into 4 of these loops to get all the diagonals. It looks as 2 for loops with an if to not loop over unbound values. The pseudo code looks a bit like this:

for(loop rows){
 var outputarray = [];
   for(loop columns){
      if(delimit for out of bound){
       var temprow = TheArray[something?];
       var tempvalue = temprow[something?];
       outputarray.push(tempvalue);
       }
   }
 //use values
document.getElementById("theDiv").innerHTML += outputarray.join("")+"<br>";
}

I hope somebody can help me with this.