javascript - JS setInterval not starting right away but otherwise working well

Question

Can someone tell me why using this setInterval takes 15 seconds to start running and how to make it run without delay? It works well in repeating every 15 seconds like it should but I want to remove the start

function pollServer() {
    intervalId = window.setInterval(function ()  {
        callE();
        callA();
        callC();
        callB();
        callD();
        
     }, 15000);  
    
 }
 pollServer();

Answer 1

To fix this, call the function before setting the interval like this:

function foo() {
    callE();
    callA();
    callC();
    callB();
    callD();
}
foo();
setInterval(foo, 15000);