Don't declare a new str variable inside the loop with var str. Reuse the one you declare outside the loop. Also do +=

var divLength = $('div').length;

var str = '';
for(var i = 0; i < divLength; i++) {
  str += "Div #" + i + ", ";
  console.log(str);
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div></div>
<div></div>
<div></div>
<div></div>

The string c is not modified in place, but returns a new string.

Use c = c.concat(key + ': ' + jsObj2[key]);

Your code has 1++ where it should be i++.

Try creating a variable len set to char.length - 1, decrement len within for loop, set data-alphabet to char[len]

$("body").append('<div class="grid"><div class="grid__col--12 lesson blacktxt"></div></div>');

var char = "ابتثجحخدذرزسشصضطظعغفقكلمنهوي";

for (i = 0, len = char.length - 1; i < 28; i++, len--) {
  var $sound = '<nav class="cell"><a class="pointer blacktxt" data-asound="'+ i +'"><i class="fa fa-volume-up"></i></a></nav>'
  var $csound = '<nav class="cell"><input type="text" class="txtcenter" data-csound="'+ i +'" placeholder="'+ i +'" /></nav>'
  var $newDiv = $('<header class="table lessonSec" data-alphabet="'+ char[len] +'"/>').html($sound + $csound)
  $(".lesson").append( $newDiv )
}

body {
  background: #7fb8ff;
}

/* Center DIVs Vertically and Horizontally */
.table {
  display: table;
  width: 100%;
  height: 100%;
  text-align: center;
  margin: 1em 0;
}
.cell {
  display: table-cell;
  vertical-align: middle;
}

/* Global Misc Classes */
.pointer {
  cursor: pointer;
}
.txtcenter {
  text-align: center;
}

<link href="http://treehouse-code-samples.s3.amazonaws.com/poly/css/application.css" rel="stylesheet"/>
<link href="https://fortawesome.github.io/Font-Awesome/assets/font-awesome/css/font-awesome.css" rel="stylesheet"/>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

You set

  examplestring = ""

inside the loop, so you empty it, then you add "foo", then you empty it, add another foo, empty it add a foo and get only one foo back. You want to declare the variable outside off the loop, before the loop runs.