Javascript設置變量值

Question

我想從if if else block中設置stat的值，但是當我設置它並提醒它時，它對我說「undefined」。我如何設置統計值。這是我的代碼。

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); 
}; 

由於提前

Answer 1

你必須把警報（或其他）到異步回調：

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); 
    }); 
} 

當你調用API，它會立即返回。因此，如果您有外部警報，則立即調用它。然後，稍後，調用您的回調函數（您作爲第三個參數傳遞的函數）。

編輯：你不能從deleteComment返回stat。相反，這樣做：

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

你可以稱之爲一樣：

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

Answer 2

你的函數調用asynchronuous。這意味着，您的代碼中的alert()在HTTP請求尚未返回時運行。

不要在回調函數的警報，因爲只有這樣，它有一個值：

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，這意味着你傳遞給FP.api來電會後的回調函數，API時通話已結束，但在撥打FB.api後，您的提醒將立即運行，這當然意味着回叫功能尚未運行，因此stat仍爲Don't Know。

要使其工作，你必須把alert回調中：

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 
}