javascript ajax在浏览器控制台中返回值“被盗”

Question

I've got following javascript code that I execute in the browser:

function run(request) {
    var res;
    $.ajax({
      url:'http://custom-host:8080/',
      type: "POST",
      async: false,
      data: request
    }).done(function(data, textStatus, jqXHR){
      console.log(textStatus);
      res = data;
    });
    return res;
}

It just asks my custom server for a response that it gets. The Chrome Console log looks like this:

> var a = run({command:'version'}); // executing custom function
success dev.pycached-admin/:14 // this is the console log
undefined // this is the console returned value
> a // ask for value a
"1.1" // value a (returned from ajax)

The question is: how come undefined is returned in the console, when actual value of '1.1' is returned afterwards (the value is correctly assigned)?

If I add debugger statement inside the done function:

}).done(function(data, textStatus, jqXHR){
  console.log(textStatus);
  debugger;
  res = data;
});

then I can see Apple's code which probably maintains the chrome console ( VM files in the script tab). Anyway, the AJAX call is synchronous, so why is the value not returned for the first time?

Answer 1

...so why is the value not returned for the first time?

It is, but var is a statement, not an expression, and so it cannot have a result for the console to display. You can see this if you just do this:

> var a = "foo";
undefined
> a
"foo"

So just do this:

> var a
undefined
> a = run({command:'version'});

...which should give you:

success dev.pycached-admin/:14 // this is the console log
"1.1"