Use the parseInt Function with a Radix

function using_parseint_radix(maybe_an_int, radix = 2) {
  var result_html = document.getElementById("result");
  var parseint_result = parseInt(maybe_an_int, radix);
  result_html.innerHTML = parseint_result;
  return parseint_result;
}

HTML

<div id='result' class='mb-3 alert alert-primary'>..</div>
<button onclick='using_parseint_radix("0000", "2");' class='btn btn-primary mb-1'>Binary Conversion of 0000</button>
<button onclick='using_parseint_radix("1235", "2");' class='btn btn-primary mb-1'>Binary Conversion of 1235</button>
<button onclick='using_parseint_radix("00001235", "10");' class='btn btn-primary mb-1'>Base 10 Conversion of 00001235</button>

Result