变量在node.js的有效性与sqlite3

Validity of variable in node.js with sqlite3

本文关键字：有效性 sqlite3 js node 变量更新时间：2023-09-26

我在node.js中对sqlite3执行select语句。我想有变量"数据"的结果，这是在sqlite代码块之外定义的，但它保持空。在sqlite代码块中，数据变量具有正确的值。有人知道我哪里做错了吗?

谢谢。

    /* Client connects via socket.io */
    io.on('connection', function(client) {
        console.log('Client connected');
        /* Client needs data... */
        client.on('needData', function(fields) {
            var data = [];
            var sql_stmt = "SELECT ...";
            if(fs.existsSync(db_file)) {
                try {
                    var db = new sqlite3.Database(db_file);
                    db.all(sql_stmt, function(err, all) {
                        data = all;
                        console.log(data); //--> data has valid values
                    }); 
                    db.close();
                    console.log(data); //--> data is empty
                }
                catch(e) {
                    console.log("Error with database. Error: ", e); 
                }   
            }
            else {
                console.log("Database file not found.");
            }
            client.emit('data', data);
        });
    });

这是因为Node.js的asynchronous性质你可以用promises来处理

我建议使用瀑布方法异步模块

 var async=require('async');
  /* Client connects via socket.io */
  io.on('connection', function(client) {
    console.log('Client connected');
    /* Client needs data... */
    client.on('needData', function(fields) {
      async.waterfall([function(next) {
        var sql_stmt = "SELECT ...";
        if (fs.existsSync(db_file)) {
          try {
            var db = new sqlite3.Database(db_file);
            db.all(sql_stmt, function(err, all) {
            next(null,all)
            });
            db.close();
          } catch (e) {
            console.log("Error with database. Error: ", e);
            next();
          }
        } else {
          console.log("Database file not found.");
          next();
        }
      }], function(err, data) {
        if (!err) {
      client.emit('data', data);
        }
      })
    });
  });