Question

我无法绕过异步/等待语法

我似乎无法弄清楚为什么在这段代码中，＆＃34;连接＆＃34;即使连接到mysql时出错，也会记录到控制台。

mongoose.Promise = Promise;
function connection() {
    //return a promise to be consumed in an async function
    return new Promise((resolve, reject) => {
        mongoose.connect('mongodb://localhost/testdatabase');
        let db = mongoose.connection;
        let sw = mysql.createConnection({
            host: 'localhost',
            user: 'reporting',
            Promise: Promise,
            database: 'testdatabase'
        });

        //Only resolve if both database connections are made successfully
        sw.then(() => {
            db.on('error', (err) => {
                reject(err);
            });
            db.once('open', () => {
                resolve(db);
            });
        }).catch((e) => {

            reject(e);
        })
    });
}

//Await connection and return true or false for more synchronous code style
async function connect() {
    let connected = false;
    try {
        //should pause execution until resolved right?
        await connection();
        connected = true;

    } catch (e) {
        //Makes it here successfully
        console.error(e);
        connected = false
    } finally {
        return connected
    } 
}

if (connect()) {
    //This code also gets fired?
    console.log('connected');
}

Answer 1

请记住，async function仍然是异步的，它们不会阻止任何事情或在它们返回之前神奇地等待。调用connect()总是会返回一个承诺，它始终是真实的，因此在每种情况下都会立即记录“已连接”。

如果您想等待连接，则必须await结果并将if语句包含在async function中：

async function connect() {
    try {
        await connection();
        return true;
    } catch (e) {
        console.error(e);
        return false
    } 
}

(async function() {
    if (await connect()) {
//      ^^^^^
        console.log('connected');
    }
}());

Answer 2

异步函数返回promises，所以这个

if (connect()) { // <--- connect() will return a promise
  // and a promise always evaluates to a truthy value, 
  // that's why this code always runs
  console.log('connected');
}

可能是：

connect().then(isConnected => {
  if (isConnected) {
    console.log('connected');
  }
});

或者你可以等待connect()从SI返回的承诺，如Bergi demonstrated in his answer。

异步/等待预期行为

2 个答案: