how to change local variable in javascript by ajax?

Question

function check_login(){
    var authenticated  = false;
    $.ajax({type:"POST",url:"/account/islogin/",data:"",async:"false",
        success:function(data){
            if(data=="null"){
                signup();
                authenticated =  false;
            }
            authenticated =  true;   
        }
    });
    return authenticated;
};

why the function always return false,even authenticated=true is executed.

the success function is called asychronously. check_login() will always return false, because the success function is called later — x4rf41
– x4rf41, Commented Jun 12, 2013 at 13:28
You really should use callbacks or promises instead of making a synchronous call. — Felix Kling
– Felix Kling, Commented Jun 12, 2013 at 13:31
But if you really want to make a synchronous request (although you shouldn't), you have to set the async option properly. Use a boolean instead of a string: async: false. — Felix Kling
– Felix Kling, Commented Jun 12, 2013 at 13:39

David Ziemann · Accepted Answer · 2013-06-12 13:40:06Z

3

The issue is that you run you if statement, and then set authenticated to true every time. It sounds like you are looking for an if else statement.

function check_login(){
    var authenticated  = false;
    $.ajax({type:"POST",url:"/account/islogin/",data:"",async:false,
        success:function(data){
            if(data=="null"){
                signup();
                authenticated =  false;
            } else {
                authenticated =  true; 
            }
        }
    });
    return authenticated;
};

or even better, you can set up the authenticated to default to true and then only set it to false when the data is null.

function check_login(){
    var authenticated  = false;
    $.ajax({type:"POST",url:"/account/islogin/",data:"",async:false,
        success:function(data){
            authenticated =  true;
            if(data=="null"){
                signup();
                authenticated =  false;
            }
        }
    });
    return authenticated;
};

edited Jun 12, 2013 at 13:40

answered Jun 12, 2013 at 13:27

David Ziemann

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5 Comments

Felix Kling Over a year ago

I think another issue is that the request is asynchronous. "false" is probably regarded as truthy value.

David Ziemann Over a year ago

I think that has more to do with how the ajax call is fired. stackoverflow.com/questions/1478295/…

Felix Kling Over a year ago

I don't understand what you are trying to tell me, so I will elaborate: The OP's code will only work if the request is synchronous. That's why they set the async option, but they passed a (non-empty) string as value instead of a boolean. I think we can assume that internally jQuery tests the option simply with if (option.async). Therefore async: "false" will be equivalent to async: true i.e. the request will be asynchronous and the function will always return false.

David Ziemann Over a year ago

Ahh, you are correct! I just assumed it was in relation to the async and not the actual value. i will update my answer.

Ian Over a year ago

@FelixKling You're right! In the source, the only time they check for the async property, it's if ( !s.async ) { callback(); }, which is right after calling .send() on the XMLHttpRequest. So since it's their way of making sure the success is executed immediately for async requests (and obviously wouldn't run if s.async is truthy

punkrockbuddyholly · Accepted Answer · 2013-06-12 13:29:01Z

-2

Because the authenticated variable inside your Ajax function and the one inside check_login() are in different scopes. They can't affect one another.

answered Jun 12, 2013 at 13:29

punkrockbuddyholly

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4 Comments

Felix Kling Over a year ago

The callback passed to $.ajax is a closure. It has access to authenticated defined in check_login.

punkrockbuddyholly Over a year ago

I could have sworn I had this exact issue the other day where even though to me it looked like a closure and should therefore work it wouldn't and the two variables couldn't see each other. Of course judging by the two downvotes I must be mistaken.

Igor Over a year ago

@MrMisterMan - this would happen if you redeclared the variable with var

punkrockbuddyholly Over a year ago

@Igor ha! I think that must have been what I was doing #facepalm#. I'll leave this incorrect answer here in case these comments help anyone else out.

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