Why can I not enter operand into the input box so that the last two numbers in the stack get added, subtracted, divided, or multiplied by their respective operands $(+,-,/,*)$ Also combine the push to stack function and evaluate function into one function. I will thumbs up if the code works!

RPN Calculator

class Node $\{$

constructor$($data$)\{$

this.data $=$ data;

this.next $=$ null;

$\}$

$\}$

class Stack $\{$

constructor$()\{$

this.top $=$ null;

$\}$

push$($data$)\{$

const newNode $=$ new Node$($data$)$;

newNode.next $=$ this.top;

this.top $=$ newNode;

this.display$()$;

$\}$

pop$()\{$

if $(!$this.top$)\{$

this.displayOutput$("$Stack is empty"$)$;

return null;

$\}$

const poppedData $=$ this.top.data;

this.top $=$ this.top.next;

this.display$()$;

return poppedData;

$\}$

display$()\{$

let current $=$ this.top;

const stackContents $=[]$;

while $($current$)\{$

stackContents.push$($current$.$data$)$;

current $=$ current.next;

$\}$

this.displayOutput$("$Contents of Stack: $"+$ stackContents.join$(""))$;

$\}$

displayOutput$($message$)\{$

document.getElementById$("$stackOutput$").$innerText $=$ message;

$\}$

$\}$

const stack $=$ new Stack$()$;

function pushToStack$()\{$

const input $=$ document.getElementById$("$stackInput$").$value;

if $(!$isNaN$($input$))\{$

stack.push$($parseFloat$($input$))$;

$\}$ else $\{$

stack.displayOutput$("$Invalid input: Please enter a number"$)$;

$\}$

document.getElementById$("$stackInput$").$value $=\text{'}\text{'}$; $//$ Clear the input box after push

$\}$

function evaluate$()\{$

const input $=$ document.getElementById$("$stackInput$").$value;

let result;

if $(["+","-","*","/"].$includes$($input$))\{$

const operand$2=$ stack.pop$()$;

const operand$1=$ stack.pop$()$;

if $($operand$1!==$ null && operand$2!==$ null$)\{$

switch$($input$)\{$

case $\text{'}+\text{'}$:

result $=$ operand$1+$ operand$2$;

break;

case $\text{'}-\text{'}$:

result $=$ operand$1-$ operand$2$;

break;

case $\text{'}*\text{'}$:

result $=$ operand$1*$ operand$2$;

break;

case $\text{'}/\text{'}$:

if $($operand$2===0)\{$

stack.displayOutput$("$Error: Division by zero"$)$;

return;

$\}$

result $=$ operand$1/$ operand$2$;

break;

$\}$

stack.push$($result$)$;

$\}$ else $\{$

stack.displayOutput$("$Error: Not enough operands"$)$;

$\}$

$\}$ else $\{$

stack.displayOutput$("$Invalid operator: Please enter $+,-,*,$ or $/")$;

$\}$

document.getElementById$("$stackInput$").$value $=\text{'}\text{'}$; $//$ Clear the input box after evaluation

$\}$