javascript - 循环写入文件 - JS

Question

尝试在循环中写入文件时似乎遇到问题，即使第一个文件尚未创建，循环仍在迭代(我认为我要么不理解 promise ，要么不理解脚本的异步性质)

因此，我将在命令行上运行node write_file_script.js prime_league

// teams.js
module.exports = {
premier_league: [
  { team_name: "Team 1", friendly_name: "name 1"},
  { team_name: "Team 2", friendly_name: "name 2"}
 ]
}

我的脚本

const args = process.argv.slice(2);
const TEAM = require('./teams');

const Excel = require('exceljs');
const workbook = new Excel.Workbook();

for (var team = 0; team < TEAM[args].length; team++) {
  console.log("Starting Excel File generation for " + TEAM[args][team]['team_name']);
  var fhcw = require('../data_files/home/fhcw/' + TEAM[args][team]['friendly_name'] + '_home' + '.json');
  fhcw = fhcw.map(Number);

  workbook.xlsx.readFile('./excel_files/blank.xlsx')
  .then(function() {
    var worksheet = workbook.getWorksheet(1);
    // Write FHCW
    for (i=0; i < fhcw.length; i++) {
      col = i+6;
      worksheet.getCell('E'+ col).value = fhcw[i];
    }

    console.log(TEAM[args][team])
    workbook.xlsx.writeFile('./excel_files/' + TEAM[args] + '/' + TEAM[args][team]['friendly_name'] + '.xlsx');

  });

}

运行时得到的输出是

Starting Excel File generation for Team 1
Starting Excel File generation for Team 2
undefined
(node:75393) UnhandledPromiseRejectionWarning: Unhandled promise rejection 
(rejection id: 1): TypeError: Cannot read property 'friendly_name' of undefined

所以看起来文件没有被写入但循环仍在继续，我如何确保在进入下一个循环之前文件已被写入？

谢谢

Answer 1

如果函数返回一个Promise，并且您想要串行执行它们(一次一个)而不是并行执行(同时开始所有操作)，则需要等待在开始下一个之前使用 then()。

另外，请注意，您的 TEAM 只是一个数组的导出(至少如所呈现的那样)，因此您不能给它 args，这是其他错误来自。

当您有一系列要做的事情时，最好的方法是建立一个队列，运行该队列直到文件用完。在这种情况下，您的 TEAM 数组看起来像是您的队列，但由于这是一个导出，我建议不一定要更改它，而是将其复制到您可以更改的另一个数组:

const args = process.argv.slice(2);
const TEAM = require('./teams');

const Excel = require('exceljs');
const workbook = new Excel.Workbook();

const writeNextFile = (queue) => {
  // If we have nothing left in the queue, we're done.
  if (!queue.length) {
    return Promise.resolve();
  }

  const team = queue.shift(); // get the first element, take it out of the array

  console.log("Starting Excel File generation for " + team.team_name);
  var fhcw = require('../data_files/home/fhcw/' + team.friendly_name + '_home' + '.json');
  fhcw = fhcw.map(Number);

  // return this promise chain
  return workbook.xlsx.readFile('./excel_files/blank.xlsx')
  .then(function() {
    var worksheet = workbook.getWorksheet(1);
    // Write FHCW
    for (i=0; i < fhcw.length; i++) {
      col = i+6;
      worksheet.getCell('E'+ col).value = fhcw[i];
    }

    console.log(team);
    // not sure what you thought this would TEAM[args] would have translated to, but it wouldn't have been a string, so I put ??? for now
    // also, making the assumption that writeFile() returns a Promise.
    return workbook.xlsx.writeFile('./excel_files/' + team.??? + '/' + team.friendly_name + '.xlsx');
  }).then(() => writeNextFile(queue));
}

writeNextFile(TEAM.slice(0)) // make a copy of TEAM so we can alter it
  .then(() => console.log('Done'))
  .catch(err => console.error('There was an error', err)); 

基本上，我们的函数采用一个数组并写入第一个团队，然后递归调用自身来调用下一个团队。最终一切都会解决，你最终会得到一个最终解决的 Promise。

当涉及到 Promise 时，您基本上总是需要将它们链接在一起。您将无法对它们使用 for 循环，或任何其他标准循环。

如果您想一次编写所有它们，它会更干净一些，因为您可以为每个文件做一个非递归映射，并使用 Promise.all 来知道它们何时完成。

const writeNextFile = (team) => {
  console.log("Starting Excel File generation for " + team.team_name);
  var fhcw = require('../data_files/home/fhcw/' + team.friendly_name + '_home' + '.json');
  fhcw = fhcw.map(Number);

  // return this promise chain
  return workbook.xlsx.readFile('./excel_files/blank.xlsx')
  .then(function() {
    var worksheet = workbook.getWorksheet(1);
    // Write FHCW
    for (i=0; i < fhcw.length; i++) {
      col = i+6;
      worksheet.getCell('E'+ col).value = fhcw[i];
    }

    console.log(team);
    // not sure what you thought this would TEAM[args] would have translated to, but it wouldn't have been a string, so I put ??? for now
    // also, making the assumption that writeFile() returns a Promise.
    return workbook.xlsx.writeFile('./excel_files/' + team.??? + '/' + team.friendly_name + '.xlsx');
  });
}

Promise.all(TEAM.map(writeTeamFile))
  .then(() => console.log('Done')
  .catch(err => console.error('Error'));

Promise 用于异步，通常当您有一组事情时，您希望并行执行它们，因为这样更快。这就是为什么并行版本要干净得多；我们不必递归地调用事物。