How to convert a string to int in Javascript?

in principle the solution would be to convert the intervening variables to whole by parseInt() but first you should see if you are actually bringing a number you may be bringing anything would validate the values by the function isNaN ()

A better approach would be

let cantidad = $tds.eq(2).find("#cant").val();
if (!isNaN(cantidad)) {
    total_bultos = !isNaN(total_bultos) ? parseInt(total_bultos, 10) : 0; //si es una cadena vacia o cualquier cosa que no sea numero total = 0
    total_bultos = parseInt(cantidad, 10) + parseInt(total_bultos, 10);  
    document.getElementById("total_bultos").value = total_bultos;
} else {
    console.error('Error, cantidad no valida');
}

I assign 0 to total_bultos by default if it is not a numeric value, because it may happen that the parameter is not initialized, but you can remove this check ':

total_bultos = !isNaN(total_bultos) ? parseInt(total_bultos, 10) : 0;

Have you checked the value of total_bulks per console before the sum?

Try the following:

 var cantidad = $tds.eq(2).find("#cant").val();
 total_bultos = parseInt(cantidad) + parseInt(total_bultos);  
 document.getElementById("total_bultos").value = total_bultos;

If you notice, we do parseInt of both values. I usually solve it in that way to make sure everything is numeric.

What I recommend is that you always check if the values are numeric with the function isNaN

And also always occupy parseInt with all your values so you avoid the sum of characters and you get the sum of numbers.

if (!isNaN(cantidad) && !isNaN(total_bultos)) {
    total_bultos = parseInt(cantidad) + parseInt(total_bultos);
} else {
    console.log("Los valores no son numéricos");
}