how to return/pass value from javascript function?

Try

var numberOfElements= numberOfDivs();   
console.log(numberOfElements);

As the function is to return a value, while we invoke the function we assigned a variable to capture the result

var numberOfElements;    
function numberOfDivs(){
    numberOfElements = $("#div").children().length; //this count how many divs exist
            }

    $("#element").click(function(){
             console.log(numberOfElements);// here I need to return the number but getting error :(
    });

One way is : define numberOfElements in global scope like this :

var numberOfElements;
function numberOfDivs(){
 numberOfElements = $("#div").children().length; //this count how many divs exist
 return numberOfElements;
}

$("#element").click(function(){
  numberOfDivs();   
  console.log(numberOfElements);// here I need to return the number but getting error :(
});

Or another way is : assign the result in one variable and use that

$("#element").click(function(){
      var output = numberOfDivs();   
      console.log(output);// here I need to return the number but getting error :(
    });

Try using Callback functions, consider a scenario where incase your numberofDivs function takes time to return the value, it will not give the appropriate results as Jquery is an asynchronous. See this demo for the use of callback to return the data CALLBACK DEMO

function AppendJson(callback) {
   var numberOfElements = "AppendJson";
   callback(numberOfElements)
}


AppendJson(function (ReturnValue) {
   alert(ReturnValue);
});