Set div's width equal to another div [duplicate]

Answer 1

Missing jQuery library in your fiddle, there is no error in your code . Just include jQuery library in your code

$(document).ready(function() {
  $(".second_div").css({
    'width': ($(".first_div").width() + 'px')
  });
});

.second_div,
.first_div {
  border: 1px solid black;
  padding: 2px 4px;
  display: inline-block;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<div class="first_div">First Div</div>
<div class="second_div">Second.width == First.width</div>