Here is a sample for that:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="viewport" content="initial-scale=1, maximum-scale=1,user-scalable=no"/>
<title>Simple Map</title>
<link rel="stylesheet" href="https://js.arcgis.com/3.30/esri/css/esri.css">
<style>
html, body, #map {
height: 100%;
margin: 0;
padding: 0;
}
#scale {
position: absolute;
bottom: 5px;
left: 5px;
background: rgba(255,255,255,0.7);
font-size: 13px;
line-height: 15px;
}
</style>
<script src="https://js.arcgis.com/3.30/"></script>
<script>
var map;
require(["esri/map", "dojo/domReady!"], function(Map) {
map = new Map("map", {
basemap: "topo",
center: [-122.45, 37.75],
zoom: 13
});
map.on('update-end', function(evt){
document.getElementById('scale').innerHTML = '1:' + map.getScale().toFixed(2);
});
});
</script>
</head>
<body>
<div id="map"></div>
<div id="scale"></div>
</body>
</html>