nodejs逐行读取文件并在末尾获取其内容

Question

我想使用 nodejs 逐行读取文件，然后在完成后将 returned 结果作为 json 字符串获取。我试图这样做，但最后 console.log 打印未定义而不是列表。我得到了列表和承诺的结尾，但是如何 return 它到 main.js 中的调用函数？

我有我的 main.js 文件：

var fct = require('./romlist-parser');

console.log(fct.myfunction());

并且romlist-parser.js有以下内容：

var fs = require('fs');
var readline = require('readline');

exports.myfunction = function() {

    var promise = new Promise(function(resolve,reject) {
        var rd = readline.createInterface({
            input: fs.createReadStream('Atari 2600.txt'),
            console: false
        });

        var games = [];

        rd.on('line', function(line) {
            var arr = line.split(";");
            games.push({name:arr[0], title:arr[1]});
        });

        rd.on('close', function() {
            var json = JSON.stringify(games);
            resolve(games);
        });

    });

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

Answer 1

试试这个：

exports.myfunction = function() {

    var promise = new Promise(function(resolve,reject) {

        var games = [];
        var rl = readline('./Atari 2600.txt'); // provide correct file path

        rl.on('line', function (line, lineCount, byteCount) {
            // console.log(lineCount, line, byteCount);
            var arr = line.split(";");
            games.push({name:arr[0], title:arr[1]});
        })
        .on('close', function() {
            var json = JSON.stringify(games);
            resolve(games); // resolve(json); may be?? 
        })
        .on('error', function (e) {
            console.log("error", e);
            // something went wrong
        });
    });

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

P.S。此代码可以进一步改进，但为了简单起见，您的理解答案仅限于 post 中的 style/code posted。否则它可以根据样式改变样式。

Answer 2

我会将设置和累积结果的变量移动到封闭范围中，然后，最重要的是，return 来自创建它的函数的承诺。所以...

exports.myfunction = function(filename) {
    let games = [];
    let rd = readline.createInterface({
        input: fs.createReadStream(filename),
        console: false
    });

    return new Promise(function(resolve,reject) {
        rd.on('line', function(line) {
            var arr = line.split(";");
            games.push({name:arr[0], title:arr[1]});
        });

        rd.on('close', function() {
            var json = JSON.stringify(games);
            resolve(games);
        });
        // on 'error' call reject(error)
    });
};

// then elsewhere

const fct = require('./romlist-parser');

function someFunction() {
    let promise = fct.myfunction('Atari 2600.txt');
    return promise.then(result => {
        console.log(result);
        return resolveResult
    });
}

Answer 3

我还需要处理大文件中行的好方法。

所以我做了这个。您可以轻松地逐行迭代文件、流、字符串、缓冲区

还支持反向模式！

请尝试一下，如果喜欢，请给我一个star！

https://github.com/sharpart555/nexline

const nl = nexline({
  input: fs.openSync(path_to_file, 'r'), // input can be file, stream, string and buffer
});

console.log(await nl.next()); // 'foo'
console.log(await nl.next()); // 'bar'
console.log(await nl.next()); // 'baz'
console.log(await nl.next()); // null, If all data is read, then return null

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