var st = "04/16/2019";

var pattern = /(\d{2})\/(\d{2})\/(\d{4})/;

let formatStr = st.replace(pattern,'$3-$1-$2');
var dt = new Date(formatStr);

console.log("formatStr: ", formatStr);
console.log("dt: ", dt.toDateString());

var st2 = "2018/10/26";
var pattern2 = /(\d{4})\/(\d{1,2})\/(\d{1,2})/;
let formatStr2 = st2.replace(pattern2,'$1-$2-$3');
var dt2 = new Date(formatStr2);
console.log("formatStr2: ", formatStr2);
console.log("dt2: ", dt2.toDateString());