在node.js中一次读取一行文件？

小哥宝儿2020-03-12

我正在尝试一次读取一行大文件。我在Quora上发现了一个与该主题有关的问题，但我缺少一些联系以使整个组件组合在一起。

 var Lazy=require("lazy");
 new Lazy(process.stdin)
     .lines
     .forEach(
          function(line) { 
              console.log(line.toString()); 
          }
 );
 process.stdin.resume();

我想弄清楚的一点是，如何一次从文件而不是如本示例中的STDIN读取一行。

我试过了：

 fs.open('./VeryBigFile.csv', 'r', '0666', Process);

 function Process(err, fd) {
    if (err) throw err;
    // DO lazy read 
 }

但它不起作用。我知道，在紧急情况下，我可能会转而使用PHP之类的东西，但是我想弄清楚这一点。

我不认为其他答案会起作用，因为该文件比我在其上运行的具有内存的服务器大得多。

Davaid阳光小卤蛋2020/03/12 15:36:46

I was frustrated by the lack of a comprehensive solution for this, so I put together my own attempt (git / npm). Copy-pasted list of features:

Interactive line processing (callback-based, no loading the entire file into RAM)
Optionally, return all lines in an array (detailed or raw mode)
Interactively interrupt streaming, or perform map/filter like processing
Detect any newline convention (PC/Mac/Linux)
Correct eof / last line treatment
Correct handling of multi-byte UTF-8 characters
Retrieve byte offset and byte length information on per-line basis
Random access, using line-based or byte-based offsets
Automatically map line-offset information, to speed up random access
Zero dependencies
Tests

NIH? You decide :-)

飞云2020/03/12 15:36:46

Edit:

Use a transform stream.

With a BufferedReader you can read lines.

new BufferedReader ("lorem ipsum", { encoding: "utf8" })
    .on ("error", function (error){
        console.log ("error: " + error);
    })
    .on ("line", function (line){
        console.log ("line: " + line);
    })
    .on ("end", function (){
        console.log ("EOF");
    })
    .read ();

老丝小胖蛋蛋2020/03/12 15:36:46

function createLineReader(fileName){
    var EM = require("events").EventEmitter
    var ev = new EM()
    var stream = require("fs").createReadStream(fileName)
    var remainder = null;
    stream.on("data",function(data){
        if(remainder != null){//append newly received data chunk
            var tmp = new Buffer(remainder.length+data.length)
            remainder.copy(tmp)
            data.copy(tmp,remainder.length)
            data = tmp;
        }
        var start = 0;
        for(var i=0; i<data.length; i++){
            if(data[i] == 10){ //\n new line
                var line = data.slice(start,i)
                ev.emit("line", line)
                start = i+1;
            }
        }
        if(start<data.length){
            remainder = data.slice(start);
        }else{
            remainder = null;
        }
    })

    stream.on("end",function(){
        if(null!=remainder) ev.emit("line",remainder)
    })

    return ev
}


//---------main---------------
fileName = process.argv[2]

lineReader = createLineReader(fileName)
lineReader.on("line",function(line){
    console.log(line.toString())
    //console.log("++++++++++++++++++++")
})

LEYStafan2020/03/12 15:36:46

您不必访问open文件，而是必须创建一个ReadStream。

fs.createReadStream

然后将该流传递给 Lazy

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