JS回调函数callback用法

<script>
function a(callback) 
{ 
    div1.innerHTML += "a"; 
      callback(); 
} 
function b(){ 
    div1.innerHTML+="b"; 
} 
 
var div1;
window.onload = function () { 
    div1 = document.getElementById("div1"); 
    setTimeout("a(b);", 3000);
}
</script>