hello i am very new in java script, Trying to make first calculator function. please explain why it is not working on browser

Question

var a = prompt ("enter first num");
var b = prompt ("enter second num");
var operator = prompt ("enter operator");

//this is calculator function

function calculator (a,b,operator) { var c = operator(a,b); return c;}
function add (a,b){return a+b;}

this is pass function

 var res = calculator (num1,num2,operator){

 function add (num1,num2){ return num1+num2};
 function substract (num1,num2){ return num1-num2};
 function multiply (num1,num2){return num1*num2};
 function devide (num1,num2){ return num1/num2};
 return calculator(num1,num2,operator);
 }

 document.write(calculator(a,b,operator));