javascript - 如何在继续之前等待所有文件被读取并执行所有回调

Question

我正在使用 node.js 并尝试同时读取 3 个 xlsx 文件，并且仅当所有 3 个文件均已读取并执行回调时，才以数组形式返回所有文件。

我尝试使用 Promise.all 但我在回调完成之前获取了数组。

这是代码

const path = require('path');
const extractor = require("xlsx-to-json");

(async () =>{
  let x = [];

  const dir = 'my/path';

  const files = [
      { name:'File1.xlsx'},
      { name:'File2.xlsx' },
      { name:'File3.xlsx'}
  ];

  await Promise.all(
          files.map(async (file)=>{   
          console.log('init: %s',file.name);
          let start = new Date();

          let filePath = path.resolve(dir,file.name);

          extractor({
              input: filePath,
              output: null
          }, function(err, result) {
              if(err) {
                  console.error(err);
              } else {
                  x.push(result);
                  let end = new Date() - start;
                  console.info('Extraction time of [%s]: %dms', file.name, end);
              }
          });
      })
  );  
  console.log(x);    
})();

我需要的输出是:

init: File1.xlsx
init: File2.xlsx
init: File3.xlsx
Extraction time of [File1.xlsx]: 17233ms
Extraction time of [File2.xlsx]: 16615ms
Extraction time of [File3.xlsx]: 15266ms
[ some json objects ]

但我得到以下信息:

init: File1.xlsx
init: File2.xlsx
init: File3.xlsx
[] //empty array
Extraction time of [File1.xlsx]: 17233ms
Extraction time of [File2.xlsx]: 16615ms
Extraction time of [File3.xlsx]: 15266ms

Answer 1

使用Promise.all是正确的，但你没有正确使用它。

请参阅下面的内嵌评论

const path = require('path');
const extractor = require("xlsx-to-json");

(async () => {
    //let x = [];

    const dir = 'my/path';

    const files = [
        { name: 'File1.xlsx' },
        { name: 'File2.xlsx' },
        { name: 'File3.xlsx' }
    ];

    // all 3 extracted results will be returned from `await Promise.all()`
    let x = await Promise.all(

        // `async` not needed here
        files.map(/*async*/ (file) => {
            console.log('init: %s', file.name);
            let start = new Date();

            let filePath = path.resolve(dir, file.name);

            // Promise wrapper
            return new Promise((resolve, reject) => {
                extractor({
                    input: filePath,
                    output: null
                }, function (err, result) {
                    if (err) {
                        console.error(err);

                        // return error by rejecting the Promise
                        reject(err);      

                    } else {
                        // x.push(result);

                        // return result by resolving the Promise
                        resolve(result);  

                        let end = new Date() - start;
                        console.info('Extraction time of [%s]: %dms', file.name, end);
                    }
                });
            });
        })
    );

    console.log(x);
})();