Unable to assign a variable as a arithmetic operator in javascript while making a calculator

Hello I was making a calculator
But i am unable to play arithmetic operators in it please solve it. because it was not checking it as a arithmetic operator it was checking it as string. And if i assisgn + like this var a = + ; it gives me an error that unexpected token }

Html code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <ulnk rel="stylesheet" href="css/index.css">
  <script src="js/index.js"></script>
  <title>CALCULATOR </title>
</head>
<body>
<!------------------------------------------->

<input readonly="true" type="text" id="value"></input>
<button id="result" ></button>

<ul><button onclick="one()" >1</button></ul>    
<ul><button onclick="four()" type="submit">4</button></ul>
<ul><button onclick="seven()" type="submit">7</button></ul>
<ul><button onclick="zero()" type="submit">0</button></ul>
<ul><button onclick="Add()" type="submit">+</button></ul>
<ul><button onclick="RESULT()" type="submit">=</button></ul>
<!------------------------------------------->
</body>
</html>

Js code

function one() {
var value = document.getElementById("value")
var input = value.value +="1"

}
function RESULT() {
var value = document.getElementById("value")
var input = value.value
var result = value.value
}
function Add() {
var value = document.getElementById("value")
var input = value.value +='+'
}
function four() {
var value = document.getElementById("value")
var input = value.value +=4
}
function seven() {
var value = document.getElementById("value")
var input = value.value +=7
}
function zero() {
var value = document.getElementById("value")
var input = value.value +=0
}
function RESULT() {
var result = document.getElementById("result")
var value = document.getElementById("value").value
var results_value =parseInt(value);
var results_value2 = result.innerHTML=results_value
}