I have so many modules and I am showing border to each module.

Below is what I have

div.ja-moduletable-inner,
div.moduletable-inner {
  background: none;
  padding: 1.5em;
  box-shadow: 0px 0px 3px 3px rgba(0,0,0,.25);
}
<div id="Mod143">
    <div class="moduletable-inner clearfix">

    </div>
</div>
<div id="Mod148">
    <div class="moduletable-inner clearfix">

    </div>
</div>
<div id="Mod149">
    <div class="moduletable-inner clearfix">

    </div>
</div>

Note : These modules are added by-default by Joomla, so I can't handle this. What I want is using Javascript, I want to add class in Mod149 so that I will have it as

<div id="Mod149">
    <div class="moduletable-inner clearfix newMyOwnClass">`
                                           ^^^^^^^^^^^^^^
</div>

and I will have in css as

div.newMyOwnClass {
  box-shadow: 0px 0px 0px 0px rgba(0,0,0,.25);
                      ^^^^^^^
}

Any idea how to get this done in Javascript?

No jQuery... Only Javascript AND only with div id Mod149