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convert date string into the actual date format?

I have called an API and then get a date which is string format like '15/07/21-23:59:59'. But I want to convert this string into the actual date format like this:
**15/07/21** OR **2009-06-01T10:00:00.000**.
so how can I achieve this?

It's strange to see a response returning a formatted date expression. By the way, your task would be easily done with momentjs. Here is my snippet:
// since your date format is not a standard one, you would have to pass an
// instruction of your date format as a second parameter to the moment constructor
const momentDate = moment('15/07/21-23:59:59', 'DD/MM/YY-HH:mm:ss');
momentDate.format('DD/MM/YYYY'); // => "15/07/2021"
momentDate.format(`yyyy-MM-dd'T'HH:mm:ss.SSSZ`); // => "2021-07-Th'T'23:59:59.000+07:00"

you can pass this string to a Date object as follow:
var date = new Date(YOUR STRING);
or you can use Date.parse() method if it does not work:
Date.parse('04 Dec 1995 00:12:00 GMT');

JavaScript input date convertion

Is there any way to convert any date string (not necessarily current date) (could be any format) to specific date format in Javascript. Like converting "MM-DD-YYYY" or "ddMMYYYY" to "DD-MMM-YYYY"?
I know that from current date as var date = new Date(), we can get time and hours but what to do in case of existing date string like "31/01/1999" to "31-JAN-1999".
Given the input date string can be of any format.

This is a common problem.
You should be able to do it with moment.js.
Ex.
moment("31/01/1999").formatWithJDF("dd - MM - yyyy");
Have a look here, https://momentjs.com/docs/ for more details.

Using DateFormatter.js
var date = new Date('2020-03-25 10:30:25');
var formatter = new DateFormatter();
displayFormat = 'D M d Y h:i:s';
var dateString = formatter.formatDate(date, displayFormat); // Wed Mar 25 2020 10:30:25

This can be done by first checking the input date format with the arrays or Map of regex provided, then need to convert that format to JS accepted format ISO format.
Once converted to ISO format this date now can be converted to any form with logic written for it in separate functions.

What is the right way to pass a date as a string in a specific format and return a date object with momentjs?

I have been looking at the momentjs momenjs documentation because I want to use it to return a date object from a date that is a string in a particular format. My code looks like this.
const date = moment(stringDate, 'YYYY-MM-DD HH:MM:SS');
console.log(date);
But it creates an invalid date as can be seen here.
What am I doing wrong. How can I get a date object from a date string that is in a particular format?

You're using MM:SS for minutes:seconds, but it should be mm:ss; details here.
Example:
const stringDate = "2018-05-11 14:25:37";
// Parsing
const m = moment(stringDate, 'YYYY-MM-DD HH:mm:ss');
// Show it in a different format to demonstrate parsing worked
console.log(m.format("DD/MM/YYYY HH:mm"));
// Accessing the underlying Date object
const dt = m.toDate();
// Log that dateobject
console.log(dt);
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.22.1/moment.min.js"></script>

In order to parse a string then return a different format you can:
> moment(dateString).format('DD.MM.YYYY')
Further examples:
Official docs: https://momentjs.com/docs/
Other sources: https://coderwall.com/p/vc3msa/a-very-short-introduction-to-moment-js

How to convert "date(1494343074100)" that into formatted datetime in javascript

I am passing a date time from controller to view. In view i can get the date as date(1494343074100). How can i convert the date into formatted datetime. please any one help

Below is a way to convert to date.
var time = 1494343074100;
var date = new Date(time);
console.log(date.toString());

Given string format to date

I have given a very unusual date format in string. I need to convert it to a JS date object and add up a few days. The last step is clear, but I don't know how to convert the string into the JS date object. Take a look at the string date: October 02, 2016

You should use moment.js
Syntax:
moment(dateString, format, locale)
var dateStr = "Oktober 02, 2016"
var d = moment(dateStr, "MMM DD, YYYY", 'de');
console.log(d.format("DD-MM-YYYY"))
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.15.1/moment-with-locales.min.js"></script>

Use Date.parse() to parse the date and get the Date object.
var dateObj = new Date('October 02, 2016')
Now, you can perform all the Date operations on dateObj

Given that everything is static here, I thing the best case for you might be to keep a map of your month's name against there number i.e say Oktober : 8. This way you will easily get around of any locale issue in any library.
Once above map is done, you can use .substring to separate your string for blank space and commas to get date and year easily.
Once you have all this you can use new Date constructor with months date and year field.
Hope this all is easily understood, so I am skipping any code here.

Using new Date() can be converted from string to date.
And using setDate() you can add days in the date and can convert the date back to string,
Please check below snippet for more understanding.
var someDate = new Date('October 02, 2016');
console.log(someDate);
console.log(new Date(someDate.setDate(someDate.getDate() + 5)).toString());