Converting a string to a date in JavaScript


How can I convert a string to a date in JavaScript?

var st = "date in some format"
var dt = new date();

var dt_st= //st in date format same as dt

var a = "13:15"
var b = toDate(a, "h:m")
//alert(b);
document.write(b);

function toDate(dStr, format) {
  var now = new Date();
  if (format == "h:m") {
    now.setHours(dStr.substr(0, dStr.indexOf(":")));
    now.setMinutes(dStr.substr(dStr.indexOf(":") + 1));
    now.setSeconds(0);
    return now;
  } else
    return "Invalid Format";
}


Performance

Today (2020.05.08) I perform tests for chosen solutions - for two cases: input date is ISO8601 string (Ad,Bd,Cd,Dd,Ed) and input date is timestamp (At, Ct, Dt). Solutions Bd,Cd,Ct not return js Date object as results, but I add them because they can be useful but I not compare them with valid solutions. This results can be useful for massive date parsing.

Conclusions

  • Solution new Date (Ad) is 50-100x faster than moment.js (Dd) for all browsers for ISO date and timestamp
  • Solution new Date (Ad) is ~10x faster than parseDate (Ed)
  • Solution Date.parse(Bd) is fastest if wee need to get timestamp from ISO date on all browsers

enter image description here

Details

I perform test on MacOs High Sierra 10.13.6 on Chrome 81.0, Safari 13.1, Firefox 75.0. Solution parseDate (Ed) use new Date(0) and manually set UTC date components.

let ds = '2020-05-14T00:00Z'; // Valid ISO8601 UTC date
let ts = +'1589328000000';    // timestamp

let Ad = new Date(ds);
let Bd = Date.parse(ds);
let Cd = moment(ds);
let Dd = moment(ds).toDate();
let Ed = parseDate(ds);

let At = new Date(ts);
let Ct = moment(ts);
let Dt = moment(ts).toDate();

log = (n,d) => console.log(`${n}: ${+d} ${d}`);

console.log('from date string:', ds)
log('Ad', Ad);
log('Bd', Bd);
log('Cd', Cd);
log('Dd', Dd);
log('Ed', Ed);
console.log('from timestamp:', ts)
log('At', At);
log('Ct', Ct);
log('Dt', Dt);



function parseDate(dateStr) {
  let [year,month,day] = dateStr.split(' ')[0].split('-');
  let d=new Date(0);
  d.setUTCFullYear(year);
  d.setUTCMonth(month-1);
  d.setUTCDate(day)
  return d;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.19.1/moment-with-locales.min.js"></script>

This snippet only presents used soultions  

Results for chrome

enter image description here