JSFiddle - React, Tailwind, and code Playground

HTML

<input name="semana" type="text" class="tb836" id="semana" size="5" readonly="readonly" onclick="this.value=getWeekNr();" />

JavaScript

document.getElementById('semana').value=getWeekNr();
function getWeekNr(){
	var now=new Date(),i=0,f,sem=(new Date(now.getFullYear(), 0,1).getDay()>0)?1:0;
	while( (f=new Date(now.getFullYear(), 0, ++i)) < now ){
		if(!f.getDay()){
			sem++;
		}
	}
	return sem;
}