The definition of the callback function is to pass the b function as a parameter to a for execution. At this time, b is the callback function. I suddenly have a question, what is the difference between it and directly calling b in the a function?
I wrote a demo myself

function a(){
    b();
    console.log('hello world');
}
function b(){
    var n = 0;
    for(var i = 0;i < 1000000000;i++){
        n++;
    }
    return n;
}
a(); //hello world

It takes about 3 seconds to see the output result, and then rewrite it in the form of a callback function

function a(callback){
    callback.call(this);
    console.log('hello world');
}
function b(){
    var n = 0;
    for(var i = 0;i < 1000000000;i++){
    }
    console.log(1);
}
a(b); //hello world

The output result can only be seen after 3 seconds.
I have found many articles, all of which explain that when b is executed as the callback function of function a, it does not hinder the normal execution of function a. According to this logic, the second The method should be to output hello world immediately. Did I use the wrong callback function? Still have trouble understanding?