Day Name from Date in JS

Ahum, three years later...

Why nobody uses the methods provided by the standard javascript Date class (except Callum Linington)?

See https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Date/toLocaleDateString

Getting the day name from a date:

function getDayName(dateStr, locale)
{
    var date = new Date(dateStr);
    return date.toLocaleDateString(locale, { weekday: 'long' });        
}

var dateStr = '05/23/2014';
var day = getDayName(dateStr, "nl-NL"); // Gives back 'Vrijdag' which is Dutch for Friday.

Getting all weekdays in an array:

function getWeekDays(locale)
{
    var baseDate = new Date(Date.UTC(2017, 0, 2)); // just a Monday
    var weekDays = [];
    for(i = 0; i < 7; i++)
    {       
        weekDays.push(baseDate.toLocaleDateString(locale, { weekday: 'long' }));
        baseDate.setDate(baseDate.getDate() + 1);       
    }
    return weekDays;
}

var weekDays = getWeekDays('nl-NL'); // Gives back { 'maandag', 'dinsdag', 'woensdag', 'donderdag', 'vrijdag', 'zaterdag', 'zondag'} which are the days of the week in Dutch.

For American dates use 'en-US' as locale.

You could use the Date.getDay() method, which returns 0 for sunday, up to 6 for saturday. So, you could simply create an array with the name for the day names:

var days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
var d = new Date(dateString);
var dayName = days[d.getDay()];

Here dateString is the string you received from the third party API.

Alternatively, if you want the first 3 letters of the day name, you could use the Date object's built-in toString method:

var d = new Date(dateString);
var dayName = d.toString().split(' ')[0];

That will take the first word in the d.toString() output, which will be the 3-letter day name.

let weekday = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'][new Date().getDay()]

use the Date.toLocaleString() method :

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