JSFiddle - React, Tailwind, and code Playground

HTML

<div id="text"></div>

JavaScript

var TheArray = ["ABCD","EFGH","IJKL"];
//amount of rows
var RowLength = TheArray.length;
//amount of colums
var ColumnLength = TheArray[0].length;

var text = ''
for (i = 0; i <= (RowLength+ColumnLength-2); i++){
    for (x = 0; x<=i; x++){
    if (TheArray[i-x] && TheArray[i-x][x]){
        text += TheArray[i-x][x];
    }
  }
  text += "<br/>";
}

document.getElementById('text').innerHTML = text;