承诺无法解决

Question

承诺的第二部分(内部then)永远不会运行.当我在不使用Promise的情况下运行数据库查询时(在我运行的节点脚本中,node myscript.js它返回数据,但控制台永远不会返回提示 - 控制台只是挂起,我必须手动发送一个中断.因此,当我把它放在Promise中,我认为Promise不知道数据库查询是否完整,即使它似乎已经返回了所有数据,因此Promise的第二部分没有运行(我想).如果这是问题,如何编写数据库查询以使其不挂起并且Promise可以运行完成？

const sqlite = require('/usr/local/lib/node_modules/sqlite3');
const express = require('/usr/local/lib/node_modules/express')
const promise = require('/usr/local/lib/node_modules/promise')

app.get('/', (request, res) => {

  var res = [];

  function getData() {
    return new Promise(function(resolve, reject) {
      db.each('SELECT column_a, column_b FROM trips group by column_a', (e, rows) => {

        var d = {
          a: rows['column_a'],
          b: rows['column_b']
        }


        res.push(d)
      });
    });

  }
  getData().then(function(data) {
    console.log("never run....", res, data) //never run
  });

})

Answer 1

您需要通过调用它的构造函数调用它在回调中提供的函数来解析promise.

const promise = new Promise((resolve, reject) => {
  // you must call resolve() or reject() here
  // otherwise the promise never resolves
});

否则它将始终处于Pending状态,并且永远不会调用您传入的回调then.

promise.then(() => {
  // this never gets called if we don't resolve() or reject()
});

此外,promises允许您使用值来解析,因此通常不需要维护全局变量,您只需传递结果即可.

最后,db.each将为每一行调用一次回调函数,因此您需要通过在获得所有行后解析promise来处理它

以下是编写代码的方法:

function getData() {
  const data = [];
  return new Promise((resolve, reject) => {
    db.each('SELECT column_a, column_b FROM trips group by column_a', (e, row) => {
      if (e) {
        // error reading a row, reject the Promise immediately
        // optionally you could accumulate errors here in a similar manner to rows
        reject(e); 
        return;
      } 

      // success reading a row, store the row result
      data.push({
        a: row['column_a'],
        b: row['column_b']
      });

    }, (e, rowCount) => { // the complete handler called when the operation is done, see docs: https://github.com/mapbox/node-sqlite3/wiki/API#databaseeachsql-param--callback-complete

      if (e) { 
        // operation finished, there was an error
        reject(e);
        return;
      }

      // operation succeeded, resolve with rows
      resolve(data);
    });
  });
}

app.get('/', (request, res) => {  

  getData().then((data) => {
    // here `data` is an array of row objects
  }, (e) => {
    console.error(`Database error: ${e}`);
  });
});

边注

不知道为什么你重新声明参数res为[],但有没有必要做var res = [].既然你已经有了res,你可以说res = []要指向res一个新的数组.当然,这将覆盖响应对象,因此我假设您只是为了本示例的目的而这样做.如果没有,您应该创建一个新变量.