How to check if the 1st number is '0' in a string using jQuery?

I'm trying to create a simple calculator using jQuery.

I have a paragraph tag that I'm using as a display. After pressing 'AC' button that clears my display the paragraph tag contains '0'.

What I wanna do is when I press a button with a number I need this '0' to be replaced with the number I pressed because now it goes like '07' or '05'.

I tried:

    $('#7').on('click', function() {
     if ($('#mainline') == '0')) {
       $('#mainline').remove('0');
    }
     $('#mainline').append('7');
    })

But I guess I'm doing something wrong. How should I check if #mainline has '0'?

If #mainline is a DIV, get its text, then remove leading zeroes with a regexp, then append 7.

$('#7').click(function() {
  $("#mainline").text(function(i, oldtext) {
    return oldtext.replace(/^0+/, '') + '7';
  });
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="mainline">0</div>
<button id="7">7</button>

If it's an input, use .val() instead of .text().