[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]

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"));

Call the toISOString() method:

var dt = new Date("30 July 2010 15:05 UTC");
document.write(dt.toISOString());

// Output:
//  2010-07-30T15:05:00.000Z

As new Date().toISOString() will return current UTC time, to get local time in ISO String format we have to get time from new Date() function like the following method

document.write(new Date(new Date().toString().split('GMT')[0]+' UTC').toISOString().split('.')[0]);