jQuery: running setInterval only when a given ID is present -

in jquery, execute code:

setinterval((function() {   return alert('hello'); }), 3000); 

but if dom contains element, such as

<div id="say_hello">say hello</div> 

this must basic, cannot seem right.

if( $( '#say_hello' ).length > 0 ) {   setinterval((function() {     return alert('hello');   }), 3000); }