Javascript设置变量值

Question

我想从 if else 块设置 stat 的值，但是当我设置它并提醒它时，它对我说“未定义”。如何设置 stat 的值？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);
};

提前致谢

Answer 1

您的函数调用是异步的。这意味着，代码中的 alert() 在 HTTP 请求尚未返回时运行。

在回调函数中执行警报，因为只有这样它才有值：

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 2

Facebook API 是异步，这意味着您传递给 FP 的回调函数。 api 调用稍后会在 API 调用完成后进行，但您的警报将在您调用 FB.api 后立即运行，这当然意味着回调函数尚未运行因此 stat 仍然是不知道。

要使其正常工作，您必须将 alert 放入回调中：

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
}

Answer 3

您必须将警报（或其他内容）带入异步回调中：

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);
  });
}

当您调用 API 时，它会立即返回。因此，如果您在外面收到警报，系统会立即呼叫。然后，稍后，您的回调（作为第三个参数传递的函数）被调用。

编辑：您无法从deleteComment返回stat。相反，做：

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

你可以这样称呼它：

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