JSFiddle - React, Tailwind, and code Playground

JavaScript

function solution(str) {
	return [].join.call(
		$( str = str.split("") ).map(function (index) {
			return !index || $(str).get(index - 1) === " " ? this.toUpperCase() : this;
		})
	, "");
}

alert( solution("hellou, world") );