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node.js: read a text file into an array. (Each line an item in the array.)

Posted by: admin November 28, 2017 Leave a comment

Questions:

I would like to read a very very large file into a JavaScript array in node.js.

So if the file is like this:

first line
two 
three
...
...

I would have the array:

['first line','two','three', ... , ... ] 

The function would look like this:

var array = load(filename); 

Therefore the idea of loading it all as a string and then split it is not acceptable.

Answers:

If you can fit the final data into an array then wouldn’t you also be able to fit it in a string and split it, as has been suggested?
In any case if you would like to process the file one line at a time you can also try something like this:

var fs = require('fs');

function readLines(input, func) {
  var remaining = '';

  input.on('data', function(data) {
    remaining += data;
    var index = remaining.indexOf('\n');
    while (index > -1) {
      var line = remaining.substring(0, index);
      remaining = remaining.substring(index + 1);
      func(line);
      index = remaining.indexOf('\n');
    }
  });

  input.on('end', function() {
    if (remaining.length > 0) {
      func(remaining);
    }
  });
}

function func(data) {
  console.log('Line: ' + data);
}

var input = fs.createReadStream('lines.txt');
readLines(input, func);

EDIT: (in response to comment by phopkins) I think (at least in newer versions) substring does not copy data but creates a special SlicedString object (from a quick glance at the v8 source code). In any case here is a modification that avoids the mentioned substring (tested on a file several megabytes worth of “All work and no play makes Jack a dull boy”):

function readLines(input, func) {
  var remaining = '';

  input.on('data', function(data) {
    remaining += data;
    var index = remaining.indexOf('\n');
    var last  = 0;
    while (index > -1) {
      var line = remaining.substring(last, index);
      last = index + 1;
      func(line);
      index = remaining.indexOf('\n', last);
    }

    remaining = remaining.substring(last);
  });

  input.on('end', function() {
    if (remaining.length > 0) {
      func(remaining);
    }
  });
}

Questions:
Answers:

Synchronous:

var fs = require('fs');
var array = fs.readFileSync('file.txt').toString().split("\n");
for(i in array) {
    console.log(array[i]);
}

Asynchronous:

var fs = require('fs');
fs.readFile('file.txt', function(err, data) {
    if(err) throw err;
    var array = data.toString().split("\n");
    for(i in array) {
        console.log(array[i]);
    }
});

Questions:
Answers:

Using the Node.js readline module.

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

var filename = process.argv[2];
readline.createInterface({
    input: fs.createReadStream(filename),
    terminal: false
}).on('line', function(line) {
   console.log('Line: ' + line);
});

Questions:
Answers:

With a BufferedReader, but the function should be asynchronous:

var load = function (file, cb){
    var lines = [];
    new BufferedReader (file, { encoding: "utf8" })
        .on ("error", function (error){
            cb (error, null);
        })
        .on ("line", function (line){
            lines.push (line);
        })
        .on ("end", function (){
            cb (null, lines);
        })
        .read ();
};

load ("file", function (error, lines){
    if (error) return console.log (error);
    console.log (lines);
});

Questions:
Answers:

use readline (documentation). here’s an example reading a css file, parsing for icons and writing them to json

var results = [];
  var rl = require('readline').createInterface({
    input: require('fs').createReadStream('./assets/stylesheets/_icons.scss')
  });


  // for every new line, if it matches the regex, add it to an array
  // this is ugly regex :)
  rl.on('line', function (line) {
    var re = /\.icon-icon.*:/;
    var match;
    if ((match = re.exec(line)) !== null) {
      results.push(match[0].replace(".",'').replace(":",''));
    }
  });


  // readline emits a close event when the file is read.
  rl.on('close', function(){
    var outputFilename = './icons.json';
    fs.writeFile(outputFilename, JSON.stringify(results, null, 2), function(err) {
        if(err) {
          console.log(err);
        } else {
          console.log("JSON saved to " + outputFilename);
        }
    });
  });

Questions:
Answers:

This is a variation on the answer above by @mtomis.

It creates a stream of lines. It emits ‘data’ and ‘end’ events, allowing you to handle the end of the stream.

var events = require('events');

var LineStream = function (input) {
    var remaining = '';

    input.on('data', function (data) {
        remaining += data;
        var index = remaining.indexOf('\n');
        var last = 0;
        while (index > -1) {
            var line = remaining.substring(last, index);
            last = index + 1;
            this.emit('data', line);
            index = remaining.indexOf('\n', last);
        }
        remaining = remaining.substring(last);
    }.bind(this));

    input.on('end', function() {
        if (remaining.length > 0) {
            this.emit('data', remaining);
        }
        this.emit('end');
    }.bind(this));
}

LineStream.prototype = new events.EventEmitter;

Use it as a wrapper:

var lineInput = new LineStream(input);

lineInput.on('data', function (line) {
    // handle line
});

lineInput.on('end', function() {
    // wrap it up
});

Questions:
Answers:

file.lines with JFile package

Pseudo

var JFile=require('jfile');

var myF=new JFile("./data.txt");
myF.lines // ["first line","second line"] ....

Don’t forget before :

npm install jfile --save

Questions:
Answers:

i just want to add @finbarr great answer, a little fix in the asynchronous example:

Asynchronous:

var fs = require('fs');
fs.readFile('file.txt', function(err, data) {
    if(err) throw err;
    var array = data.toString().split("\n");
    for(i in array) {
        console.log(array[i]);
    }
    done();
});

@MadPhysicist, done() is what releases the async. call.

Questions:
Answers:

I had the same problem, and I have solved it with the module line-by-line

https://www.npmjs.com/package/line-by-line

At least for me works like a charm, both in synchronous and asynchronous mode.

Also, the problem with lines terminating not terminating \n can be solved with the option:

{ encoding: 'utf8', skipEmptyLines: false }

Synchronous processing of lines:

var LineByLineReader = require('line-by-line'),
    lr = new LineByLineReader('big_file.txt');

lr.on('error', function (err) {
    // 'err' contains error object
});

lr.on('line', function (line) {
    // 'line' contains the current line without the trailing newline character.
});

lr.on('end', function () {
    // All lines are read, file is closed now.
}); 

Questions:
Answers:

To read a big file into array you can read line by line or chunk by chunk.

line by line refer to my answer here

var fs = require('fs'),
    es = require('event-stream'),

var lines = [];

var s = fs.createReadStream('filepath')
    .pipe(es.split())
    .pipe(es.mapSync(function(line) {
        //pause the readstream
        s.pause();
        lines.push(line);
        s.resume();
    })
    .on('error', function(err) {
        console.log('Error:', err);
    })
    .on('end', function() {
        console.log('Finish reading.');
        console.log(lines);
    })
);

chunk by chunk refer to this article

var offset = 0;
var chunkSize = 2048;
var chunkBuffer = new Buffer(chunkSize);
var fp = fs.openSync('filepath', 'r');
var bytesRead = 0;
while(bytesRead = fs.readSync(fp, chunkBuffer, 0, chunkSize, offset)) {
    offset += bytesRead;
    var str = chunkBuffer.slice(0, bytesRead).toString();
    var arr = str.split('\n');

    if(bytesRead = chunkSize) {
        // the last item of the arr may be not a full line, leave it to the next chunk
        offset -= arr.pop().length;
    }
    lines.push(arr);
}
console.log(lines);

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