javascript - Promise.then 在前一个 .then 之前被解决，foreach 完成

Question

我正在运行一个半复杂的承诺链，中间有一个 foreach 循环。我遇到的问题是 final.then()在 foreach 循环完成之前被击中，导致一个完全空的dailyTotals数组。

fs.readdir(csvPath)
    .then(files => {
        // Define storage array
        var csvFiles = [];

        // Loop through and remove non-csv
        files.forEach(file => {
            if (file !== "README.md" && file !== ".gitignore") {
                csvFiles.push(file);
            }
        });

        return csvFiles;
    })
    .then(files => {
        var dailyTotals = [];

        files.forEach(filename => {
            const loadedFile = fs.createReadStream(csvPath + filename);

            var totalCases = 0;
            var totalDeaths = 0;
            var totalRecovered = 0;

            papa.parse(loadedFile, {
                header: true,
                worker: true,
                step: r => {
                    totalCases += parseIntegerValue(r.data.Confirmed);
                    totalDeaths += parseIntegerValue(r.data.Deaths);
                    totalRecovered += parseIntegerValue(r.data.Recovered);
                },
                complete: () => {
                    var dailyTotal = {
                        date: filename.replace(".csv", ""),
                        parsed: {
                            confirmed: totalCases,
                            deaths: totalDeaths,
                            recovered: totalRecovered
                        }
                    };

                    dailyTotals.push(dailyTotal);
                }
            });
        });

        return dailyTotals;
    })
    .then(dailyTotals => {
        console.log(dailyTotals);
    });

有没有办法在解决下一个之前等待该 foreach 循环完成.then()？问题直接在foreach和final上console.log(dailyTotals);

标签： javascriptasynchronousforeachpromise

Answer 1

首先，我不得不提一下，还有一些替代方法Array#forEach：

• Array#map它旨在通过映射原始数组（与您所做的相同.forEach）来创建一个新数组，并且
• Array#filter（它的名字不言自明）。

问题是，你有一个承诺链和一个基于回调的模块。要混合它们，请使用Promise构造函数并使用Promise.all, 将它们组合成一个 Promise：

fs.readdir(csvPath)
    .then(files => {

        // Filter out non-csv
        return files.filter(file => (file !== "README.md" && file !== ".gitignore"))
    })
    .then(files => {
        //Returning a Promise from then handler will be awaited
        return Promise.all(files.map(filename => {
            const loadedFile = fs.createReadStream(csvPath + filename);
            return new Promise(resolve => {
                var totalCases = 0;
                var totalDeaths = 0;
                var totalRecovered = 0;

                papa.parse(loadedFile, {
                    header: true,
                    worker: true,
                    step: r => {
                        totalCases += parseIntegerValue(r.data.Confirmed);
                        totalDeaths += parseIntegerValue(r.data.Deaths);
                        totalRecovered += parseIntegerValue(r.data.Recovered);
                    },
                    complete: () => {
                        resolve( {
                          date: filename.replace(".csv", ""),
                          parsed: {
                            confirmed: totalCases,
                            deaths: totalDeaths,
                            recovered: totalRecovered
                          }
                        });
                    }
                });
            });
        }));
    })
    .then(dailyTotals => {
        console.log(dailyTotals);
    });

您甚至可以省略第一个 .then处理程序，就像.filter同步操作一样，并在单个 中执行所有操作.then：

fs.readdir(csvPath)
    .then(files => {
        //Returning a Promise from then handler will be awaited
        return Promise.all(
            files
                //Filter here
                .filter(file => (file !== "README.md" && file !== ".gitignore"))
                //And map without a second then
                .map(filename => {
                    const loadedFile = fs.createReadStream(csvPath + filename);
                    return new Promise(resolve => {
                        var totalCases = 0;
                        var totalDeaths = 0;
                        var totalRecovered = 0;

                        papa.parse(loadedFile, {
                            header: true,
                            worker: true,
                            step: r => {
                                totalCases += parseIntegerValue(r.data.Confirmed);
                                totalDeaths += parseIntegerValue(r.data.Deaths);
                                totalRecovered += parseIntegerValue(r.data.Recovered);
                            },
                            complete: () => {
                                resolve( {
                                  date: filename.replace(".csv", ""),
                                  parsed: {
                                    confirmed: totalCases,
                                    deaths: totalDeaths,
                                    recovered: totalRecovered
                                  }
                                });
                            }
                        });
                    });
                })
        );
    })
    .then(dailyTotals => {
        console.log(dailyTotals);
    });