WhatCentury.js

// Write a function that takes a year as input and returns the century. The return value should be a string that begins with the century number, and ends with 'st', 'nd', 'rd', or 'th' as appropriate for that number.
// New centuries begin in years that end with 01. So, the years 1901 - 2000 comprise the 20th century.

const century = (year) => {

    let centuryCalc = Math.floor(year / 100) + 1;

    if (year % 100 === 0) {
        centuryCalc -= 1;
    }

    return String(centuryCalc) + ending(centuryCalc) ;       ;

}

const ending = (centuryCalc) => {
    let lastNum = centuryCalc % 10;

    if (centuryCalc % 100 >= 11 && centuryCalc % 100 <= 13) {
        return 'th';
    }

    switch (lastNum) {
        case 1: return 'st';
        case 2: return 'nd';
        case 3: return 'rd';
        default: return 'th';
    }
}

console.log(century(1965));        // "20th"
console.log(century(2001));        // "21st"
console.log(century(256));         // "3rd"
console.log(century(2000));        // "20th"
console.log(century(5));           // "1st"
console.log(century(10103));       // "102nd"
console.log(century(1052));        // "11th"
console.log(century(1127));        // "12th"
console.log(century(11201));       // "113th"