whats the problem in my code? I want to implement a JavaScript stack function and it resulted to NaN

//2
function Stack(){
    this.dataStore=[];
    this.top = 0;
    this.push = push;
    this.pop = pop;
    this.peek = peek;
    this.length = length;
    this.clear = clear;
}
function push(element){
    this.dataStore[this.top++] = element;
}
function pop(){
    this.dataStore[--this.top];
}
function peek(){
    return this.dataStore[this.top-1];
}
function length(){
    return this.top;
}
function clear(){
    this.top = 0;
}

var input = "100/5";
var operands = new Stack();
var operators = new Stack();

var operandsFlip = new Stack();
var postfixStr = "";

function convertInfixToPostfix(input) {
  var numStr = "";

  for (var i = 0; i < input.length; i++) {
    var curr = input[i];
    if (curr === "+" || curr === "-" || curr === "*" || curr === "/") {
      operators.push(curr);
      operands.push(numStr);
      numStr = "";
    } else {
      numStr += curr; 
    }
  }
  operands.push(numStr); 

  
  while (operands.length() > 0) {
    operandsFlip.push(operands.pop());
  }

  var operand1 = operandsFlip.pop();
  var operand2 = operandsFlip.pop();
  var operator = operators.pop();

  console.log("Postfix expression: " + operand1 + " " + operand2 + " " + operator);

  var result = eval(operand1 + operator + operand2);
  console.log("Posfix evaluated: " + result);
}

convertInfixToPostfix(input);

A postfix expression evaluator works on arithmetic expressions taking the following
form:
op1 op2 operator
Using two stacks—one for the operands and one for the operators—design and
implement a JavaScript function that converts infix expressions to postfix expres‐
sions, and then use the stacks to evaluate the expression.

and this is the output:

Postfix expression: undefined undefined undefined Posfix evaluated: NaN

>Solution :

Your pop() function misses the return keyword and therefore always returns undefined.

function pop(){
    return this.dataStore[--this.top];
}