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I calculated to get the first day of the month but go the last

I start by getting the date of the beginning of month:
var date = new Date();
var firstDay = new Date(date.getFullYear(), date.getMonth(), 1);
Then I convert it to ISO:
firstDay = firstDay.toISOString();
Why did I get 2019-05-31 as the first day instead of 2019-06-01?

You could use a simple regex to format the string using replace:
/(\d{4})-(\d{2})-(\d{2}).+/
// Set the inital date to a UTC date
var date = new Date(new Date().toLocaleString("en-US", {timeZone: "UTC"}))
// Update the day without affecting the month/day when using toISOString()
date.setDate(1)
// Format the date
let formatted = date.toISOString().replace(/(\d{4})-(\d{2})-(\d{2}).+/, '$3-$2-$1')
console.log(formatted)

The default javascript date uses your local timezone, by converting it to something else you can end up with a different date.

You can do it
var firstDay = new Date().toISOString().slice(0, 8) + '01';
console.log(firstDay);

The date object in javascript can be somewhat tricky. When you create a date, it is created in your local timezone, but toISOString() gets the date according to UTC. The following should convert the date to ISO but keep it in your own time zone.
var date = new Date();
var firstDay = new Date(date.getFullYear(), date.getMonth(), 1);
var day = 0;
if (firstDay.getDate() < 10) {
day = '0' + firstDay.getDate();
}
var month = 0;
if ((firstDay.getMonth() + 1) < 10) {
//months are zero indexed, so we have to add 1
month = '0' + (firstDay.getMonth() + 1);
}
firstDay = firstDay.getFullYear() + '-' + month + '-' + day;
console.log(firstDay);

How do I get the date to print only the day of the week?

How do i print only the date of the week with JS. Or maybe the day and month?
I was playing around with some functions but I only get the whole date.
let today = new Date()
today.getDate()
let tomorrow = new Date();
tomorrow.getDay(today.getDate()+1)
Then I will have to use the variable as props in a react component and it should be a string.
So should I use toUTCstring?

You can use the weekday option of toLocaleString:
mydateobject.toLocaleString("en", { weekday: "long" })
The month name can be got in a similar way:
mydateobject.toLocaleString("en", { month: "long" })

getDay takes no arguments. You're looking for setDate to set the date to tomorrow, and then use getDay without arguments:
let tomorrow = new Date();
tomorrow.setDate(tomorrow.getDate() + 1);
let days = ['sun', 'mon', 'tue', 'wed', 'thu', 'fri', 'sat']
console.log('tomorrow is: ' + days[tomorrow.getDay()])

You can use the getDay() method. This will return a number. You can use this number to get the name of the day. Something like this
var days = ['Sunday','Monday','Tuesday','Wednesday','Thursday','Friday','Saturday'];
let date = new Date();
var day_of_week = date.getDay();
var day_name = days[day_of_week];

the javascript Date object has many methods to get what you want: you can use getDay() to get the day of the week and getMonth() to get the month.
To pass this as a string parameter I believe the best would be to use toISOString() and you can use this string as a parameter to the Date constructor on the other side.

For handling dates I recommend using moment js library
you can install it with npm install moment and then
moment().format('E');
or
moment().utc.().format('E');
or just moment().day()

Looks like toDateString will always return the 3 letters of the day if the week, so you can:
// Today
new Date().toDateString().substring(0,3)
// Tomorrow
new Date(Date.now()+24*3600*1e3).toDateString().substring(0,3)
Or, to get the number (zero based):
// Today
new Date().getDay()
// Tomorrow
new Date(Date.now()+24*3600*1e3).getDay()

If you use momentjs library, you can use let day = moment(tomorrow).format('dddd');
let tomorrow = new Date();
tomorrow.setDate(tomorrow.getDate() + 1);
console.log(tomorrow)
let day = moment(tomorrow).format('dddd');
console.log(day);
let tomorrow = new Date();
tomorrow.setDate(tomorrow.getDate() + 1);
console.log(tomorrow)
let day = moment(tomorrow).format('dddd');
console.log(day);
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.24.0/moment.js"></script>

Using Date.toDateString() you can format into dd mmm yy (or any way you want):
let date = new Date
str = date.toDateString() // 'Mon May 06 2019
let arr = str.split(" ") // ['Mon', 'May', '06', '2019']
let formatted = parseInt(arr[2]).toString() + " " + arr[1] + " " + arr[3] //6 May 2019
console.log(formatted) //6 May 2019
Using Date.toLocaleString() gives weekday:
let date = new Date
let weekdayLong = date.toLocaleString('en-gb', {weekday: 'long'}) //Monday
let weekdayShort = date.toLocaleString('en-gb', {weekday: 'short'}) //Mon

Javascript get start of the month

How to get the start of the month in date format e.g. 01-05-2017?
I have already seen
Get first and last date of current month with javascript or jquery

If I understand correctly toLocaleString() might do what you want.
For example:
lastDay.toLocaleString('en-GB', {day: 'numeric', month: '2-digit', year: 'numeric'});
For more info (MDN)
Replace locale with any that suits your desired formatting.

After getting the date we need to convert it to desired string format.
01 is the day and may be a constant since every beginning of the month starts from 1.
getMonth returns number of the month starting from 0 so we need a little function to format it.
var date = new Date();
var startDate = new Date(date.getFullYear(), date.getMonth(), 1);
var startDateString = '01-' + fotmatMonth(startDate.getMonth()) + '-' + startDate.getFullYear();
console.log(startDateString);
function fotmatMonth(month) {
month++;
return month < 10 ? '0' + month : month;
}

var date = new Date();
console.log(getStartOfMonthDateAsString(date));
function getStartOfMonthDateAsString() {
function zerosPad(number, numOfZeros) {
var zero = numOfZeros - number.toString().length + 1;
return Array(+(zero > 0 && zero)).join("0") + number;
}
var day = zerosPad(1, 2);
var month = zerosPad((date.getMonth() + 1), 2);
var year = date.getFullYear();
return day + '-' + month + '-' + year;
}

How to convert timestamp to readable date/time?

I have an API result giving out timestamp like this 1447804800000. How do I convert this to a readable format using Javascript/jQuery?

You can convert this to a readable date using new Date() method
if you have a specific date stamp, you can get the corresponding date time format by the following method
var date = new Date(timeStamp);
in your case
var date = new Date(1447804800000);
this will return
Wed Nov 18 2015 05:30:00 GMT+0530 (India Standard Time)

Call This function and pass your date :
JS :
function getDateFormat(date) {
var d = new Date(date),
month = '' + (d.getMonth() + 1),
day = '' + d.getDate(),
year = d.getFullYear();
if (month.length < 2)
month = '0' + month;
if (day.length < 2)
day = '0' + day;
var date = new Date();
date.toLocaleDateString();
return [day, month, year].join('-');
}
;

In my case, the REST API returned timestamp with decimal. Below code snippet example worked for me.
var ts= 1551246342.000100; // say this is the format for decimal timestamp.
var dt = new Date(ts * 1000);
alert(dt.toLocaleString()); // 2/27/2019, 12:45:42 AM this for displayed

A twofold javascript function to convert a long date and return today's date in mm-dd-yyyy format

I need your help,
I can't seem to find any other help on this on the internet, because it seems its either one way ot the other. What I would like to be able to do is to create a combined, two-fold javascript function that would convert a long date string into the mm-dd-yyyy format, and when the same function is called again with no string specified to convert, to just return todays date in mm-dd-yyyy format.
Example:
getDate(Fri May 22 2015 13:32:25 GMT-0400)
would return: 05-22-2015
getDate()
would return today's date of 05-23-2015

Hi this should do the trick
FORMAT: mm-dd-yyyy
function addZeroIfRequired(dayOrMonth) {
return dayOrMonth > 9 ? dayOrMonth : "0"+dayOrMonth;
}
function getDate(dateString) {
var date = dateString ? new Date(dateString) : new Date();
return addZeroIfRequired((date.getUTCMonth()+1)) + "-" +
addZeroIfRequired(date.getDate())+ "-" +
date.getUTCFullYear();
}
console.log(getDate()); // 05-23-2015
console.log(getDate("Fri May 22 2015 13:32:25 GMT-0400")); 05-22-2015
NOTE: the +1 after the getUTCMonth().
JSFiddle. Open the console to see the result. https://jsfiddle.net/wt79yLo0/2/
ISO FORMAT: yyyy-mm-dd
Just in case someone is interested in the opposite format, the code would be much nicer and neater:
function getDate(dateString) {
var date = dateString ? new Date(dateString) : new Date();
return date.toISOString().substring(0, 10);
}
console.log(getDate());
console.log(getDate("Fri May 22 2015 13:32:25 GMT-0400"));
https://jsfiddle.net/wt79yLo0/

First I would recommend a very powerful library for JS called Moment.js which solves all this kind of problems.
But if you only want a snippet, here is my proposal:
function twoDigits(num) {
return ("0" + num).substr(-2);
}
function getFormattedDateDMY(date, separator) {
var day = twoDigits(date.getDate());
var month = twoDigits(date.getMonth());
var year = date.getFullYear();
return [day,month,year].join(separator);
}
function getFormattedDateMDY(date, separator) {
var day = twoDigits(date.getDate());
var month = twoDigits(date.getMonth());
var year = date.getFullYear();
return [month,day,year].join(separator);
}
console.log(getFormattedDateDMY(new Date(), "-")); //23-04-2015
console.log(getFormattedDateMDY(new Date(), "-")); //04-23-2015

With getDate(), getMonth() and getFullYear(). You have to set a "0" before the months and days which are < 10. GetMonth() starts with 0, therefore (getMonth() + 1).
function getFormattedDate() {
var date = new Date();
var day = date.getDate() > 9 ? date.getDate() : "0" + date.getDate();
var month = (date.getMonth() + 1) > 9 ? (date.getMonth() + 1) : "0" + (date.getMonth() + 1);
var year = date.getFullYear();
var formattedDate = day + "-" + month + "-" + year;
return formattedDate;
}
console.log(getFormattedDate());
Demo