if statement

The command-line still evaluates everything in order, as you type it... The if statement won't evaluate again unless you tell it to.

You could do something like:

setInterval(function() {
 if (a != b) {
    var b = a;
    var c = 1;
    var d = 1;
    var e = 1;
    var f = b+c+d+e;
 }
},100);

And now, that code will keep being called every 100ms so if you write a=20, everything will update as you expect.