JSFiddle - React, Tailwind, and code Playground

JavaScript

function formatStr(str) {
		str = str.replace(/(\.(.*))/g, '');
		var arr = str.split('');
		var str_temp = '';
		if (str.length > 3) {
			for (var i = arr.length - 1, j = 1; i >= 0; i--, j++) {
				str_temp = arr[i] + str_temp;
				if (j % 3 == 0) {
					str_temp = ' ' + str_temp;
				}
			}
			return str_temp;
		} else {
			return str;
		}
	}

alert(formatStr('10545'));