JSFiddle - React, Tailwind, and code Playground

HTML

<span class="res"></span>

JavaScript

let arr = ['пн', 'вт', 'ср', 'чт', 'пт', 'сб', 'вс'];
var res = document.querySelector('.res');
for (let i = 0; i < arr.length; i++) {
	res.append(arr[i] + ' ');
  
  res.style.fontWeight = 800;
}