Stack in js

// Stack class
class Stack {

// Array is used to implement stack
constructor()
{
    this.items = [];
}

// Functions to be implemented
// push(item)
// pop()
// peek()
// isEmpty()
// printStack()
// push function
push(element)
{
    // push element into the items
    this.items.push(element);
}
pop()
{
    // return top most element in the stack
    // and removes it from the stack
    // Underflow if stack is empty
    if (this.items.length == 0)
        return "Underflow";
    return this.items.pop();
}
peek()
{
    // return the top most element from the stack
    // but does'nt delete it.
    return this.items[this.items.length - 1];
}
isEmpty()
{
    // return true if stack is empty
    return this.items.length == 0;
}
printStack()
{
    var str = "";
    for (var i = 0; i < this.items.length; i++)
        str += this.items[i] + " ";
    return str;
}

}
function postFixEvaluation(exp)
{
var stack = new Stack();
for (var i = 0; i < exp.length; i++) {
var c = exp[i];
if (!isNaN(c)){
stack.push(c - '0');
}
else {
var val1 = stack.pop();
var val2 = stack.pop();
if (val1 == "Underflow" || val2 == "Underflow")
return "Can't perform postfix evaluation";
switch (c) {
case '+':
stack.push(val2 + val1);
break;

        case '-':
            stack.push(val2 - val1);
            break;

        case '/':
            stack.push(val2 / val1);
            break;

        case '*':
            stack.push(val2 * val1);
            break;
        }
    }
      
}
  console.log(stack.printStack())

return stack.pop();

}

console.log(postFixEvaluation("235*+8-"));