Actually, you don't need to perform any such conversion.

The string you have is already in very nearly an acceptable format for new Date(string) and in fact even as is will be accepted by most (if not all) modern browsers.

To make it fully compliant with the ISO8601 variant that ES5 mandates, just replace the space between the date and time with a literal T character, and add a time-zone specifier at the end (e.g. either Z for UTC, or +01:00, for example).

See http://www.ecma-international.org/ecma-262/5.1/#sec-15.9.1.15 for more.

[Addendum 12/2022]: Here's a library to format dates using Intl.DateTimeFormat.

[Addendum 01/2024]: And here is a (ES-)Date manipulation library

Try something like this

var d = new Date,
    dformat = [d.getMonth()+1,
               d.getDate(),
               d.getFullYear()].join('/')+' '+
              [d.getHours(),
               d.getMinutes(),
               d.getSeconds()].join(':');

If you want leading zero's for values < 10, use this number extension

Number.prototype.padLeft = function(base,chr){
    var  len = (String(base || 10).length - String(this).length)+1;
    return len > 0? new Array(len).join(chr || '0')+this : this;
}
// usage
//=> 3..padLeft() => '03'
//=> 3..padLeft(100,'-') => '--3' 

Applied to the previous code:

var d = new Date,
    dformat = [(d.getMonth()+1).padLeft(),
               d.getDate().padLeft(),
               d.getFullYear()].join('/') +' ' +
              [d.getHours().padLeft(),
               d.getMinutes().padLeft(),
               d.getSeconds().padLeft()].join(':');
//=> dformat => '05/17/2012 10:52:21'

See this code in [jsfiddle][1]

[edit 2019] Using ES20xx, you can use a template literal and the new padStart string extension.

const dt = new Date();
const padL = (nr, len = 2, chr = `0`) => `${nr}`.padStart(2, chr);

console.log(`${
    padL(dt.getMonth()+1)}/${
    padL(dt.getDate())}/${
    dt.getFullYear()} ${
    padL(dt.getHours())}:${
    padL(dt.getMinutes())}:${
    padL(dt.getSeconds())}`
);

Just leverage the built-in toISOString method that brings your date to the ISO 8601 format:

let yourDate = new Date()
yourDate.toISOString().split('T')[0]

Where yourDate is your date object.

Edit: @exbuddha wrote this to handle time zone in the comments:

const offset = yourDate.getTimezoneOffset()
yourDate = new Date(yourDate.getTime() - (offset*60*1000))
return yourDate.toISOString().split('T')[0]

We should use in-built toISOString function to covert it to ISO date format and remove not required data using string manipulation.

let datenow = new Date();

console.log(datenow); // "2021-07-28T18:11:11.282Z"
console.log(generateDatabaseDateTime(datenow)); // "2021-07-28 14:11:33"

function generateDatabaseDateTime(date) {
  return date.toISOString().replace("T"," ").substring(0, 19);
}

Ideally solution should be to use momentjs or dayjs library.

You can fully format the string as mentioned in other posts. But I think your better off using the locale functions in the date object?

var d = new Date("2017-03-16T17:46:53.677"); 
console.log( d.toLocaleString() ); 

edit :

ISO 8601 ( the format you are constructing with ) states the time zone is appended at the end with a [{+|-}hh][:mm] at the end of the string.

so you could do this :

var tzOffset = "+07:00" 
var d = new Date("2017-03-16T17:46:53.677"+ tzOffset);
console.log(d.toLocaleString());
var d = new Date("2017-03-16T17:46:53.677"); //  assumes local time. 
console.log(d.toLocaleString());
var d = new Date("2017-03-16T17:46:53.677Z"); // UTC time
console.log(d.toLocaleString());

edit :

Just so you know the locale function displays the date and time in the manner of the users language and location. European date is dd/mm/yyyy and US is mm/dd/yyyy.

var d = new Date("2017-03-16T17:46:53.677");
console.log(d.toLocaleString("en-US"));
console.log(d.toLocaleString("en-GB"));

If you are looking for alternatives in jquery or Javascript , then you can go with Moment.js,where you can Parse, Validate, Manipulate, and Display dates in JavaScript.

example:

  var date= moment("06/06/2015 11:11:11").format('DD-MMM-YYYY');