How do global variable work in javascript (callback)?

问题

Usually, defining a variable outside functions is enough to let it be "global". In my case, however, situation seems to be different.

var username = null;

function myFunction_1() {
    username="pippo";
    myobject.myfunction(function (data){ username="mickey" })
    console.log("1: ", username);
}
myFunction_1();

I expected this code to log "1: mickey". However, if i set the variable inside a callback function logs "1: pippo", that is setting inside the callback gets ignored. What i'm missing? Setting variable in this way is not enough?

回答1:

Your code is being executed from the top of the page as following :
username gets declared and set to = null -> myFunction_1() get's defined -> myFunction_1() gets called -> username gets set to 'pippo' -> console.logs "1: pippo" -> console.logs "2: pippo" -> myFunction_2() get's defined -> myFunction_2() gets called -> console.logs "3: pippo" this happens in this sequence assuming that this code runs, which it does not in your case.

Assuming that salvaUsername() looks like function salvaUsername(){ return username; } username is null as it have never reached the point of assignment that happens in myFunction_1(). (It's actually surprising that output is not undefined but null).

UPDATE In this case myFunction_1() never runs so username doesn't get set to 'pippo' hence the outcome.

回答2:

Assuming that code is not inside a function you haven't shown, username is a global variable. If you look at the value of username before calling myFunction_1, it will be null. If you call myFunction_1, then look at username, it will be "pippo" (and it will stay "pippo" until you do something not shown to change it). So if you're seeing null when you expect "pippo", it tells you that either A) myFunction_1 has not (yet?) been called, or B) You've done something since calling myFunction_1 that set username back to null.

来源：https://stackoverflow.com/questions/24912542/how-do-global-variable-work-in-javascript-callback