Edit in JSFiddle

var gCnt = 0;

function add_global() {
  gCnt += 1;
  document.getElementById("global").innerHTML = gCnt.toString();
}

function add_local() {
  var lCnt = 0;
  lCnt += 1;
  document.getElementById("local").innerHTML = lCnt;
}

<p>전역변수 카운터</p>
<div id="global">0</div>
<button type="button" onclick="add_global()">UP</button>
<br>
<hr>
<p>지역변수 카운터</p>
<div id="local">0</div>
<button type="button" onclick="add_local()">UP</button>