How Javascript exactly flows

Hi I am trying to learn JS.

let currentResult=defaultResult;
currentResult=currentResult + 10**3;

let calculateDescription= `(+ ${defaultResult} + +10**3 )`

outputResult(currentResult, calculateDescription);

function add(num1 ,num2)
{
    const result= num1 +num2 ;
    return result;    
}

currentResult= add(3,12);

Now what I gather is the answer should be 15 add(3,12) but it is coming 1000(10**3). Any idea why?