var date = new Date();

alert( date.getFullYear() + ("0" + (date.getMonth() + 1)).slice(-2) + ("0" + date.getDate()).slice(-2) + ("0" + date.getHours() ).slice(-2) + ("0" + date.getMinutes()).slice(-2) + ("0" + date.getSeconds()).slice(-2) );

edit

function pad2(n) { return n < 10 ? '0' + n : n }

var date = new Date();
    
alert( date.getFullYear().toString() + pad2(date.getMonth() + 1) + pad2( date.getDate()) + pad2( date.getHours() ) + pad2( date.getMinutes() ) + pad2( date.getSeconds() ) );

[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.

If you don't need to use moment.js you can use the toISOString() method returns a string in simplified extended ISO format (ISO 8601), which is always 24 or 27 characters long (YYYY-MM-DDTHH:mm:ss.sssZ or ±YYYYYY-MM-DDTHH:mm:ss.sssZ, respectively). The timezone is always zero UTC offset, as denoted by the suffix "Z".

Then after converting the date using the method, use the .replace() method and use regex to format the string then use the .slice() method.

const timeStamp = (new Date()).toISOString().replace(/[^0-9]/g, '').slice(0, -3)

console.log(timeStamp)

Output: 20210928063849