Node.js callback function question.

var i = 0; function a(callback){ i +=1; callback(i); }; setInterval(a((num)=>{ console.log(num); }),1000);

The above is the code I made. I expected that it would come out as 1, 2, 3... every second, but with the error TypeError: "callback" argument must be a function, only 1 is printed and finished.

Why is this happening?