JSFiddle - React, Tailwind, and code Playground

HTML

<div id="cos">A word  - nopthing will be appended</div>
<div id="4">4</div>
<div id="4">6</div>
<div id="4">12</div>

JavaScript

var daysArray = ["1", "2", "3", "4", "5", "12"];
var courseHwork = ["4", "8", "15", "16", "23", "42", "12"];

for (var i = 0; i < courseHwork.length; i++) {
    for (var j = 0; j < daysArray.length; j++) {
        if (courseHwork[i] == daysArray[j]) {
          $('div:contains("'+daysArray[j]+'")').append("<div class='assignment'>"+courseHwork[i]+" - appended</div>");
        }
    }
}