Converter Data aaaammdd
Converter a data para padrao BR
HTML
<!DOCTYPE html>
<html>
<body>
<p id="demo" >Click the button to change the text in this paragraph.</p>
<input type="text" class="form-control" name="proximoVenc" id="proximoVenc" value="20181205">
<label>resultado</label>
<input type="text" class="form-control" name="proximoVenc" id="result">
<button onclick="convertData()">Try it</button>
<script>
function convertData() {
var dataprotheus = document.getElementById("proximoVenc").value;
var str = document.getElementById("proximoVenc").value;
var ano = Number(str.substring(0, 4));
var mes = Number(str.substring(4, 6));
var dia = Number(str.substring(6));
var output = new Date(ano, mes - 1, dia).toLocaleDateString('pt-BR');
document.getElementById("result").value = output;
}
</script>
</body>
</html>