javascript - Node.js - foreach 内的 mysql 查询

Question

我有这样的代码结构:

connection.query(query1,function(err,rows) {
    var response = [];
    //doing something with rows
    rows.forEach(function(item) {
        connection.query(queryItem,function(err,rows) {
            //doing something
            result = rows[0].field;
            //and want to push it to an array
            response.push(result);
        });
    });
    console.log(response); //empty
});

我知道 forEach 是阻塞的，但查询是非阻塞的。我尝试使用 promise :

connection.query(query1,function(err,rows) {
    var response = [];
    //doing something with rows
    rows.forEach(function(item) {
        var promise = new Promise(function(resolve,reject) {
            connection.query(queryItem,function(err,rows) {
                //doing something
                result = rows[0].field;
                //and want to push it to an array
                resolve(result);
            });
        });
        promise.then(function(result) {
            console.log(result); //ok
            response.push(result) //not ok, result is empty
        });
    });
    console.log(response); //empty
});

但这并没有帮助。如何从非阻塞函数将值插入数组并在之后使用它？

Answer 1

创建 Promise 是朝着正确方向迈出的一步，但您需要更进一步，将 Promise 聚合到一个数组中并传递给 Promise.all，然后等待它们全部完成，然后再尝试访问响应。

connection.query(query1,function(err,rows) {
    var response = [];
    //doing something with rows
    Promise.all(rows.map(function(item) {
        var promise = new Promise(function(resolve,reject) {
            connection.query(queryItem,function(err,rows) {
                //doing something
                result = rows[0].field;
                //and want to push it to an array
                resolve(result);
            });
        });
        return promise.then(function(result) {
            console.log(result); //ok
            response.push(result) //ok
        });
    }).then(function () {
        console.log(response); //not empty
    });
});