setInterval javascript - how to call function

Question

Here is the simple example of two similar use of setInterval function:

http://codepen.io/anon/pen/MwYOOJ

In the first example setInterval function does not work, in the second version where below syntax was used it's working fine.

/* This part does not work */
var i = 0;
function displayNumber() {
  i++;
  $('#result').html(i);
}

setInterval(displayNumber(), 500);

/* This part does work */
var j = 0;
function displayNumberOk() {
  j++;
  $('#result-ok').html(j);
}

setInterval(function() { displayNumberOk() }, 500);

In specification I can find:

setInterval(function,milliseconds,param1,param2,...)

Why can't I use function name directly?

Answer 1

In the first example, where you have

setInterval(displayNumber(), 500);

you are calling displayNumber directly, instead of passing it as parameter.

In fact, using

setInterval(displayNumber, 500);

should be sufficient (notice the lack of (), so that it is not called but rather passed as parameter).