javascript - 返回函数 JavaScript

Question

我在返回查询时遇到了一些问题。

在这里，我想做的事情:

//If the email hasn't a good format
if(email_not_good_format())
   //I do something
else if(email_already_exists_in_mysql(email))
  //I do something


function email_already_exists_in_mysql(email){
connection.query('SELECT COUNT(*) AS nb FROM user WHERE emailUser = ' + connection.escape(email), function(err, rows, fields) {
        if (err) throw err;
        if(rows[0].nb == 0)
            return false;
        else
            return true;
    });
}

我在不同的帖子上看到回调函数，但它不适用于我想做的事情。

Answer 1

I saw on different posts callback function but it doesn't work for what I want to do.

是的，您只需要改变您思考代码的方式即可。您应该编写一个名为 if_email_already_exists_in_mysql 的函数，而不是编写 email_already_exists_in_mysql:

/* Executes callback if email
 * already exists in mysql:
 */
function if_email_already_exists_in_mysql (email,callback) {
    connection.query(
        'SELECT COUNT(*) AS nb FROM user WHERE emailUser = ' +
        connection.escape(email),

        function(err, rows, fields) {
            if(rows[0].nb != 0) {
                callback();
            }
        }
    )
}

然后不要写这个:

//If the email hasn't a good format
if(email_not_good_format()) {
    //I do something
}
else if(email_already_exists_in_mysql(email)) {
    //I do something
}

你可以这样写:

//If the email hasn't a good format
if(email_not_good_format()) {
    //I do something
}
else {if_email_already_exists_in_mysql(email),function(){
    //I do something
})}

现在，您可能会问自己，如果之后还有其他情况怎么办？那么，您需要修改 if_email_already_exists_in_mysql 函数，使其行为类似于 and if...else 而不是仅仅 and if:

function if_email_already_exists_in_mysql (email,callback,else_callback) {
    connection.query(
        'SELECT COUNT(*) AS nb FROM user WHERE emailUser = ' +
        connection.escape(email),

        function(err, rows, fields) {
            if(rows[0].nb != 0) {
                callback();
            }
            else if(else_callback) {
                else_callback();
            }
        }
    )
}

这样你就可以这样调用它:

//If the email hasn't a good format
if(email_not_good_format()) {
    //I do something
}
else {
    if_email_already_exists_in_mysql(email),function(){
        //I do something
    },
    // else
    function(){
        //I do something else
    }
)}

您可以编写异步代码来执行几乎任何常规代码只能执行的操作，而不是返回在回调中传递的值。请记住:

同步代码中的返回 == 在异步代码中传入回调。

因此，代码结构必须不同，但正如我上面演示的，您想要实现的逻辑可以完全相同。