Javascript设置变量值

Question

我想从if if else block中设置stat的值，但是当我设置它并提醒它时，它对我说“undefined”。我如何设置统计值。这是我的代码。

deleteComment = function(postId){ 
    var stat = "Don't Know"; 
    FB.api(postId, 'delete', function(response) { 
    if (!response || response.error) { 
     stat = "Error2"; 
    } else { 
     stat = "Deleted" 
    } 
    }); 

    alert(stat); 
}; 

由于提前

Answer 1

你必须把警报（或其他）到异步回调：

deleteComment = function(postId){ 
    var stat = "Don't Know"; 
    FB.api(postId, 'delete', function(response) { 
    if (!response || response.error) { 
     stat = "Error2"; 
    } else { 
     stat = "Deleted" 
    } 
    alert(stat); 
    }); 
} 

当你调用API，它会立即返回。因此，如果您有外部警报，则立即调用它。然后，稍后，调用您的回调函数（您作为第三个参数传递的函数）。

编辑：你不能从deleteComment返回stat。相反，这样做：

deleteComment = function(postId, callback){ 
    FB.api(postId, 'delete', function(response) { 
    if (!response || response.error) { 
     stat = "Error2"; 
    } else { 
     stat = "Deleted" 
    } 
    callback(stat); 
    }); 
} 

你可以称之为一样：

deleteComment(postid, function(stat) 
{ 
    // use stat 
}); 

Answer 2

你的函数调用asynchronuous。这意味着，您的代码中的alert()在HTTP请求尚未返回时运行。

不要在回调函数的警报，因为只有这样，它有一个值：

deleteComment = function(postId){ 
    FB.api(postId, 'delete', function(response) { 
    var stat = "Don't Know"; 
    if (!response || response.error) { 
     stat = "Error2"; 
    } else { 
     stat = "Deleted"; 
    } 
    alert(stat); 
    }); 
} 

Answer 3

Facebook的API是asynchronous，这意味着你传递给FP.api来电会后的回调函数，API时通话已结束，但在拨打FB.api后，您的提醒将立即运行，这当然意味着回叫功能尚未运行，因此stat仍为Don't Know。

要使其工作，你必须把alert回调中：

deleteComment = function(postId){ 


    var stat = "Don't Know"; 

    // call is made... 
    FB.api(postId, 'delete', function(response) { 

     // if we land here, the callback has been called 
     if (!response || response.error) { 
      stat = "Error2"; 

     } else { 
      stat = "Deleted" 
     } 
     alert(stat); // now - inside the callback - stat is actually set to the new value 
    }); 

    // but execution continues 
}