Javascript for 循环中的嵌套回调

Question

我有嵌套函数，它们不应阻塞 I/O。

 for(let i = 1; i < category_amount;i++){
        pool.query('SELECT COUNT(1) FROM threads_mphp WHERE category = ?',i,function(error, results1, fields) {
            console.log(i);
            pool.query('UPDATE category SET posts=? where category=?',[results1[0]['COUNT(1)'],i],function() {
                console.log(i);
            });
        });
    }

两个查询都使用相同的索引 i。我不能将 let 用于此类目的。 console.logs 没有按预期连续输出两个相同的数字。我该如何解决这个问题？

Answer 1

我认为这里没有问题。只有在 pool.query 的结果完成时，才会调用回调函数 (function(error, results1, fields)...) 和后续的 UPDATE 语句。因此，日志可能会出现乱序，因为您正在以允许您进入循环的速度快速生成 pool.query 调用。

在 pool.query 之前添加一条日志记录语句以准确查看发生了什么。我相信日志记录看起来像

1 query! - called from loop
2 query! - called from loop
3 query! - called from loop
1 SELECT finished! - 1st callback is invoked and UPDATE dispatched.
4 query! - loop is still running!
2 SELECT finished!
1 UPDATE finished!
3 SELECT finished!
2 UPDATE finished!
...

for(let i = 1; i < category_amount;i++){
        console.log(`${i} query!`);
        pool.query('SELECT COUNT(1) FROM threads_mphp WHERE category = ?',i,function(error, results1, fields) {
            console.log(`${i} SELECT finished`);
            pool.query('UPDATE category SET posts=? where category=?',[results1[0]['COUNT(1)'],i],function() {
                console.log(`${i} UPDATE finished`);
            });
        });
    }

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