DynamicDayName.js

function dayName(day) {

    // Load the day names from a file
    const dayNames = require('./weekday.json');

    // Were there seven names in the array?
    if (dayNames.length != 7)
        throw 'Error in weekday.json';
    
    // Is day value between 1-7?
    if (day < 1 || day > 7)
       throw 'Invalid day number';

    // Array start with 0, Mon -> 1
    return dayNames[day-1]; 
}