transform-custom-date-format.js

var date = new Date("01/21/2017");
var yyyy = date.getFullYear().toString();
var mm = (date.getMonth()+1).toString();
var dd  = date.getDate().toString();
 
// CONVERT mm AND dd INTO chars
var mmChars = mm.split('');
var ddChars = dd.split('');
 
// CONCAT THE STRINGS IN YYYY-MM-DD FORMAT
var datestring = yyyy + '-' + (mmChars[1]?mm:"0"+mmChars[0]) + '-' + (ddChars[1]?dd:"0"+ddChars[0]);
console.log("the result is: "); //2017-01-21
console.log(datestring);