返回一个对象给出未定义，使用 db.all() sqlite3 javascript

Question

我不明白为什么它的控制台记录一个未定义的值而不是对象


function getusername( username ){ 
  let currentUsernameObj;
  sqlDB.all(sql, [username], (err, rows) => {
  if (err) {
    throw err;
  }
  if (!(rows.length === 0)&& username === rows[0].username) {
    console.log(username +" sqn " +rows[0].username );
    console.log(rows[0]);
    currentUsernameObj=rows[0];

  } else {
    console.log("user not found")
  }
  console.log(typeof(currentUsernameObj))
  return currentUsernameObj;
})};
var x=getusername("user2@s.com");
console.log(x+" yyyyyy");

这是我的输出

我期待显示一个对象，但不知道该怎么做

Answer 1

您的回调（见下文）将在函数返回后执行，因为您没有将 currentUsernameObj 初始化为任何东西 undefined。

// this callback is called after SQL returns the data
// as DB access is slow this will only be executed after your function has already returned
(err, rows) => {
  if (err) {
    throw err;
  }
  if (!(rows.length === 0)&& username === rows[0].username) {
    console.log(username +" sqn " +rows[0].username );
    console.log(rows[0]);
    currentUsernameObj=rows[0];

  } else {
    console.log("user not found")
  }
  console.log(typeof(currentUsernameObj))
  return currentUsernameObj;
}

您将不得不等待这些结果。不幸的是，SQLite 没有基于 API 的 Promise 来简化这一切，因此您必须自己创建 Promise。

async function getusername(username) {
  // wrap the function in a Promise
  // resolve the promise on success
  // reject the promise on error
  return new Promise((resolve, reject) => {
    sqlDB.all(sql, [username], (err, rows) => {
      if (err) {
        // promise needs to be rejected as an error occurred
        reject(err)
      }
      if (!(rows.length === 0) && username === rows[0].username) {
        console.log(username + " sqn " + rows[0].username);
        console.log(rows[0]);
        currentUsernameObj = rows[0];
      } else {
        console.log("user not found");
      }
      console.log(typeof currentUsernameObj);
      resolve(currentUsernameObj);
    });
  });
}

// in order to be able to call an async method with await the call has to be in an async method itself
(async () => {
  // we can now use await to wait for the result
  var x = await getusername("user2@s.com");
  console.log(x + " yyyyyy");
})();

有关 async/ await 以及如何正确处理错误的更多信息，请参阅 JavaScript docs, this tutorial 或详细介绍的无数 YouTube 视频之一。只需搜索 async/ await 或 Promises.

返回一个对象给出未定义，使用 db.all() sqlite3 javascript

returning a object gives undefined, using db.all() sqlite3 javascript

javascript

sqlite

node.js