How to get the day of the week from the day number in JavaScript?

This will give you a day based on the index you pass:

var weekday=new Array(7);
weekday[0]="Monday";
weekday[1]="Tuesday";
weekday[2]="Wednesday";
weekday[3]="Thursday";
weekday[4]="Friday";
weekday[5]="Saturday";
weekday[6]="Sunday";
console.log("Today is " + weekday[3]);

Outputs "Today is Thursday"

You can alse get the current days index from JavaScript with getDay() (however in this method, Sunday is 0, Monday is 1, etc.):

var d=new Date();
console.log(d.getDay());

Outputs 1 when it's Monday.

A much more elegant way which allows you to also show the weekday by locale if you choose to is available starting the latest version of ECMA scripts and is running in all latest browsers and node.js:

console.log(new Date().toLocaleString('en-us', {  weekday: 'long' }));

/**
* I convert a day string to an number.
*
* @method dayOfWeekAsInteger
* @param {String} day
* @return {Number} Returns day as number
*/
function dayOfWeekAsInteger(day) {
  return ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"].indexOf(day);
}

This code is a modified version of what is given above. It returns the string representing the day instead

/**
* Converts a day number to a string.
*
* @param {Number} dayIndex
* @return {String} Returns day as string
*/
function dayOfWeekAsString(dayIndex) {
  return ["Sunday", "Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"][dayIndex] || '';
}

For example

dayOfWeekAsString(0) returns "Sunday"