我有 2 个问题:我正在尝试在代码中插入 Promise.all，我需要 Promise.all 来进行异步数据库调用。
第一个问题是，通过使用我将在下面发布的代码(代码A)，当我记录“idArticolo”变量时，在控制台中结果是“getIdArticolo()”函数的所有代码，(我的意思是它只是将我记录在要执行的书面代码中，而不是我想要的变量的值)，也许我不必使用 getIdArticolo 函数，而是使用变量？
我尝试使用变量var getIdArticolo = new Promise(代码B)，它给了我另一个错误:
为什么？"TypeError: Cannot read property 'id' of undefined", referring to the code "resolve (result [0] .id ); "," result "is undefined!
实际上，它是完全相同的代码，只不过一个是在一个函数中，另一个是变量的赋值。显然我要么使用A代码，要么使用B代码，对我不想执行的代码进行注释。
所有代码:
var mysql = require('mysql')
var http = require('http')
var url = require('url');
var express = require("express");
var cors = require('cors')
var app = express();
var sql = "";
var connection = mysql.createConnection({
host: '127.0.0.1',
user: 'andrea',
password: 'password',
database: 'spesa'
});
connection.connect();
function LinkMysql() {
var data = '';
return new Promise(function(resolve,reject) {
connection.query(sql, function(err, result) {
if(err)
console.log("error: " + err);
resolve(result);
});
});
}
var idArticolo;
var idCategoria;
var nuovoId;
var articolo, costo, quantita, negozio, data;
app.use(cors());
app.get('/inserisciDati',function(request, response){
console.log("/inserisciDati");
/*
console.log("/inserisciDati");
console.log("articolo="+request.query.articolo);
console.log("costo="+request.query.costo);
console.log("quantita="+request.query.quantita);
console.log("negozio="+request.query.negozio);
console.log("data="+request.query.data);
*/
articolo = request.query.articolo;
costo = request.query.costo;
quantita = request.query.quantita;
negozio = request.query.negozio;
data = request.query.data;
//INSERIRE RISPOSTA ALLA PAGINA HTML CON ERRORE SE C'È
//recuperare gli id dell' articolo e del negozio, l' id categoria dell'articolo e il nuovo id del registro
getValues();
//INSERIRE I DATI IN TABLLA MYSQL
});
function getValues(){
Promise.all([getIdArticolo, getIdCategoria, getLastId]).then(function(values){
idArticolo = values[0];
idCategoria = values[1];
nuovoId = values[2];
console.log("idArticolo="+idArticolo);
sql="INSERT INTO registro VALUES ('" + nuovoId + "','" + articolo + "','" + idCategoria +
"','" + idArticolo + "','" + costo + "','" + quantita +
"','" + negozio + "','" + data + "')";
//console.log("sql="+sql);
});
}
var getIdArticolo = new Promise(function(resolve, reject){
sql = "SELECT id FROM articoli WHERE articolo = '" + articolo + "'";
connection.query(sql, function(err, result) {
if(err)
console.log("error: " + err);
resolve(result[0].id);
});
});
function getIdArticolo(){
return new Promise(function(resolve,reject) {
sql = "SELECT id FROM articoli WHERE articolo = '" + articolo + "'";
connection.query(sql, function(err, result) {
if(err)
console.log("error: " + err);
resolve(result[0].id);
});
});
}
function getIdCategoria(){
return new Promise(function(resolve, reject){
sql = "SELECT id_categoria FROM articoli WHERE articolo = '" + articolo + "'";
connection.query(sql, function(err, result) {
if(err)
console.log("error: " + err);
resolve(result[0].id_categoria);
});
});
}
function getLastId(){
return new Promise(function(resolve, reject){
sql = "SELECT id FROM registro ORDER BY id DESC LIMIT 1";
connection.query(sql, function(err, result) {
if(err)
console.log("error: " + err);
resolve(result[0].id);
});
});
}
app.get('/getArticoli', function(request, response){
console.log("/getArticoli");
response.setHeader("Access-Control-Allow-Origin", "*");
response.setHeader("Access-Control-Allow-Headers", "X-Requested-With");
response.setHeader("Access-Control-Allow-Methods","PUT,POST,GET,DELETE,OPTIONS");
response.setHeader("X-Powered-By",' 3.2.1');
response.setHeader("Content-Type", "application/json");
sql = 'SELECT articolo FROM articoli ORDER BY articolo ASC';
LinkMysql().then(function(val) {
response.writeHead(200, {'content-Type': 'text/plain; charset=utf-8'});
response.end(JSON.stringify(val));
});
});
app.get('/getNegozi', function(request, response){
console.log("/getNegozi");
response.setHeader("Access-Control-Allow-Origin", "*");
response.setHeader("Access-Control-Allow-Headers", "X-Requested-With");
response.setHeader("Access-Control-Allow-Methods","PUT,POST,GET,DELETE,OPTIONS");
response.setHeader("X-Powered-By",' 3.2.1');
response.setHeader("Content-Type", "application/json");
sql = 'SELECT negozio FROM negozi ORDER BY negozio ASC';
LinkMysql().then(function(val) {
response.writeHead(200, {'content-Type': 'text/plain; charset=utf-8'});
response.end(JSON.stringify(val));
});
});
app.listen(8080);
代码 A 摘录:
function getIdArticolo(){
return new Promise(function(resolve,reject) {
sql = "SELECT id FROM articoli WHERE articolo = '" + articolo + "'";
connection.query(sql, function(err, result) {
if(err)
console.log("error: " + err);
resolve(result[0].id);
});
});
}
代码 B 摘录:
var getIdArticolo = new Promise(function(resolve, reject){
sql = "SELECT id FROM articoli WHERE articolo = '" + articolo + "'";
connection.query(sql, function(err, result) {
if(err)
console.log("error: " + err);
resolve(result[0].id);
});
});
谢谢!
最佳答案
这两种方法似乎都存在一些问题:
看起来您忘记了调用返回 promise 的函数。
例如:代码 A 是 getIdArticolo，必须调用 getIdArticolo() 以返回一个 Promise，您可以在 Promise.all< 内部等待该 Promise/ 声明。
Promise.all([getIdArticolo(), getIdCategoria()]).then(result => {
});
另外，请记住，当将某些内容声明为变量时，如 var getIdArticolo = new Promise() 中，变量声明为 hoisted 。
关于javascript - 类型错误 : Cannot read property 'id' of undefined & bad variable log，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/57396475/