Promise.all() 在解决所有承诺时永远不会触发

Question

我有以下代码，它异步循环遍历表列表，按顺序执行每个表的每个查询。我确实知道，由于console.log的输出延迟，所有dbTableQueryPromises承诺都会解决，但promise.all永远不会触发。我现在只有一张桌子users.代码和输出如下。

const dropAllDatabaseTables = () => {
// tableRequiredList is an object that contains a list of SQL queries, among other things not important to the issue.
const errorList = [
'test'
];
return new Promise((allQueriesResolve, allQueriesReject) => {
const dbTableQueryPromises = tableRequiredList.map((databaseTable) => {
return new Promise((tableResolve, tableReject) => {
const dbQueries = databaseTable.table.dropTableQuery;
dbQueries.reduce((previousPromise, dbTableQuery) => {
return previousPromise.then(() => {
return new Promise((resolve, reject) => {
Logger.info(`Query (${databaseTable.name}): ${dbTableQuery}`);
dbConn.query(dbTableQuery, function(err, results, fields) {
if (err) {
Logger.error(`Exec error '${databaseTable.name}'. Error reported: ${err}`);
errorList.push({ err, dbTableQuery });
reject(err);
return null;
}
resolve({ results, fields });
});
});
}).catch((err) => {
Logger.error(`Exec error '${databaseTable.name}'. Error reported: ${err}`);
});
}, Promise.resolve()).then(() => {
Logger.info(`Table Drop Completed: ${databaseTable.name}`);
tableResolve(errorList);
}).catch((err) => {
Logger.error(`Exec loop error '${databaseTable.name}'. Error reported: ${err}`);
tableResolve(errorList);
});
});
});
console.log(1111, dbTableQueryPromises) // debug code
setTimeout(() => { // debug code
console.log(2222, dbTableQueryPromises) // debug code
}, 1000) // debug code
Promise.all(dbTableQueryPromises, (results) => {
console.log(3333, dbTableQueryPromises) // debug code
Logger.info(`All tables dropped.`);
allQueriesResolve(results);
});
});
};

输出：

1111 [ Promise { <pending> } ]
info: Query (users.js): DROP TABLE IF EXISTS `users`;
info: Table Drop Completed: users.js
2222 [ Promise { [ 'test' ] } ]

Answer 1

Promise.all是基于 Promise 的，而不是基于回调的。它只接受一个参数：要等待的承诺数组。然后，它返回一个承诺，当该数组中的所有承诺都已解析时解析(或者，一旦其中一个承诺拒绝，它就会拒绝(。

在Promise.all上调用.then，而不是尝试向其传递回调：

Promise.all(dbTableQueryPromises).then((results) => {
console.log(3333, dbTableQueryPromises) // debug code
Logger.info(`All tables dropped.`);
allQueriesResolve(results);
});