fast-csv

What is fast-csv?

The fast-csv npm package is a comprehensive library for working with CSV data in Node.js. It provides functionalities for parsing CSV files and strings, formatting data to CSV, and transforming data during the parse and format process.

What are fast-csv's main functionalities?

Parsing CSV

This feature allows you to parse CSV files or strings. The code sample demonstrates how to read a CSV file using a stream, parse it with headers, and log each row to the console.

const fs = require('fs');
const fastcsv = require('fast-csv');

fs.createReadStream('data.csv')
  .pipe(fastcsv.parse({ headers: true }))
  .on('data', row => console.log(row))
  .on('end', () => console.log('CSV file successfully processed'));

Formatting Data to CSV

This feature allows you to format JavaScript data arrays or streams to CSV. The code sample shows how to take an array of objects and write it to a CSV file with headers.

const fs = require('fs');
const fastcsv = require('fast-csv');

const data = [{ id: 1, name: 'John Doe' }, { id: 2, name: 'Jane Doe' }];

fastcsv
  .write(data, { headers: true })
  .pipe(fs.createWriteStream('out.csv'))
  .on('finish', () => console.log('Write to out.csv successfully!'));

Transforming Data

This feature allows you to transform data during the parse or format process. The code sample demonstrates how to read a CSV file, transform each row by combining first and last names into a full name, and then write the transformed data to a new CSV file.

const fs = require('fs');
const fastcsv = require('fast-csv');

fs.createReadStream('data.csv')
  .pipe(fastcsv.parse({ headers: true }))
  .transform(row => ({ fullName: row.firstName + ' ' + row.lastName }))
  .pipe(fastcsv.format({ headers: true }))
  .pipe(fs.createWriteStream('out.csv'))
  .on('finish', () => console.log('Transformed file successfully written.'));

Other packages similar to fast-csv

papaparse

PapaParse is a robust and powerful CSV parser for JavaScript with a similar feature set to fast-csv. It supports browser and server-side parsing, auto-detection of delimiters, and stream parsing. Compared to fast-csv, PapaParse has a stronger emphasis on browser-side parsing and provides a more user-friendly configuration for handling large files in the browser.

csv-parser

csv-parser is a Node.js module for parsing CSV data. It can handle large datasets and supports stream-based processing. While fast-csv offers both parsing and formatting capabilities, csv-parser focuses primarily on parsing CSV data and may be preferred for its simplicity when only parsing functionality is needed.

csv-writer

csv-writer is a CSV writing library for Node.js that provides functionality to serialize arrays and objects into CSV files. Unlike fast-csv, which offers both parsing and formatting, csv-writer is specialized for CSV output, making it a good choice if the primary requirement is to generate CSV files from data.

README

Fast-csv

This is a library that provides CSV parsing and formatting.

NOTE As of v0.2.0 fast-csv supports multi-line values.

Installation

npm install fast-csv

Usage

Parsing

All methods accept the following options

• objectMode=true: Ensure that data events have an object emitted rather than the stringified version set to false to have a stringified buffer.
• headers=false: Set to true if you expect the first line of your CSV to contain headers, alternatly you can specify an array of headers to use. You can also specify a sparse array to omit some of the columns.
• ignoreEmpty=false: If you wish to ignore empty rows.
• discardUnmappedColumns=false: If you want to discard columns that do not map to a header.
• strictColumnHandling=false: If you want to consider empty lines/lines with too few fields as errors - Only to be used with headers=true
• delimiter=',': If your data uses an alternate delimiter such as ; or \t.
  • NOTE When specifying an alternate delimiter you may only pass in a single character delimiter
• quote='"': The character to use to escape values that contain a delimiter. If you set to null then all quoting will be ignored
• escape='"': The character to use when escaping a value that is quoted and contains a quote character.
  • i.e: 'First,"Name"' => '"First,""name"""'
• The following are options for parsing only.
  • trim=false: If you want to trim all values parsed set to true.
  • rtrim=false: If you want to right trim all values parsed set to true.
  • ltrim=false: If you want to left trim all values parsed set to true.
  • comment=null: If your CSV contains comments you can use this option to ignore lines that begin with the specified character (e.g. #).

events

• data: Emitted when a record is parsed.
• data-invalid: Emitted if there was invalid row encounted, only emitted if the validate function is used or strictColumnHandling=true.
• data: Emitted with the object or stringified version if the objectMode is set to false.

([options]) or .parse(options)

If you use fast-csv as a function it returns a transform stream that can be piped into.

var stream = fs.createReadStream("my.csv");

var csvStream = csv()
    .on("data", function(data){
         console.log(data);
    })
    .on("end", function(){
         console.log("done");
    });

stream.pipe(csvStream);

//or

var csvStream = csv
    .parse()
    .on("data", function(data){
         console.log(data);
    })
    .on("end", function(){
         console.log("done");
    });

stream.pipe(csvStream);

fs.createReadStream("my.csv")
    .pipe(csv())
    .on("data", function(data){
        console.log(data);
    })
    .on("end", function(){
        console.log("done");
    });

var fileStream = fs.createReadStream("my.csv"),
    parser = fastCsv();

fileStream
    .on("readable", function () {
        var data;
        while ((data = fileStream.read()) !== null) {
            parser.write(data);
        }
    })
    .on("end", function () {
        parser.end();
    });

parser
    .on("readable", function () {
        var data;
        while ((data = parser.read()) !== null) {
            console.log(data);
        }
    })
    .on("end", function () {
        console.log("done");
    });

.fromPath(path[, options])

This method parses a file from the specified path.

var csv = require("fast-csv");

csv
 .fromPath("my.csv")
 .on("data", function(data){
     console.log(data);
 })
 .on("end", function(){
     console.log("done");
 });

.fromString(string[, options])

This method parses a string

var csv = require("fast-csv");

var CSV_STRING = 'a,b\n' +
                 'a1,b1\n' +
                 'a2,b2\n';

csv
 .fromString(CSV_STRING, {headers: true})
 .on("data", function(data){
     console.log(data);
 })
 .on("end", function(){
     console.log("done");
 });

.fromStream(stream[, options])

This accepted a readable stream to parse data from.

var stream = fs.createReadStream("my.csv");

csv
 .fromStream(stream)
 .on("data", function(data){
     console.log(data);
 })
 .on("end", function(){
     console.log("done");
 });

If you expect the first line your CSV to be headers you may pass in a headers option. Setting the headers option will cause change each row to an object rather than an array.

var stream = fs.createReadStream("my.csv");

csv
 .fromStream(stream, {headers : true})
 .on("data", function(data){
     console.log(data);
 })
 .on("end", function(){
     console.log("done");
 });

You may alternatively pass an array of header names which must match the order of each column in the CSV, otherwise the data columns will not match.

var stream = fs.createReadStream("my.csv");

csv
 .fromStream(stream, {headers : ["firstName", "lastName", "address"]})
 .on("data", function(data){
     console.log(data);
 })
 .on("end", function(){
     console.log("done");
 });

To omit some of the data columns you may not need, pass a sparse array as headers.

var stream = fs.createReadStream("my.csv");

csv
 .fromStream(stream, {headers : ["firstName" , , "address"]})
 .on("data", function(data){
     console.log(data);
 })
 .on("end", function(){
     console.log("done");
 });

If your data may include empty rows, the sort Excel might include at the end of the file for instance, you can ignore these by including the ignoreEmpty option.

Any rows consisting of nothing but empty strings and/or commas will be skipped, without emitting a 'data' or 'error' event.

var stream = fs.createReadStream("my.csv");

csv
 .fromStream(stream, {ignoreEmpty: true})
 .on("data", function(data){
     console.log(data);
 })
 .on("end", function(){
     console.log("done");
 });

Validating

You can validate each row in the CSV by providing a validate handler. If a row is invalid then a data-invalid event will be emitted with the row and the index.

var stream = fs.createReadStream("my.csv");

csv
 .fromStream(stream, {headers : true})
 .validate(function(data){
     return data.age < 50; //all persons must be under the age of 50
 })
 .on("data-invalid", function(data){
     //do something with invalid row
 })
 .on("data", function(data){
     console.log(data);
 })
 .on("end", function(){
     console.log("done");
 });

If your validation is async then your validation function

var stream = fs.createReadStream("my.csv");

csv
 .fromStream(stream)
 .validate(function(data, next){
     MyModel.findById(data.id, function(err, model){
        if(err){
            next(err);
        }else{
            next(null, !model); //valid if the model does not exist
        }
     });
 })
 .on("data", function(data){
     console.log(data);
 })
 .on("end", function(){
     console.log("done");
 });

Transforming

You can transform data by providing a transform function. What is returned from the transform function will be provided to validate and emitted as a row.

var stream = fs.createReadStream("my.csv");

csv
 .fromStream(stream)
 .transform(function(data){
     return data.reverse(); //reverse each row.
 })
 .on("data", function(data){
     console.log(data);
 })
 .on("end", function(){
     console.log("done");
 });

If your transform function expects two arguments then a callback function will be provided and should be called once the validation is complete. This is useful when doing async validation

var stream = fs.createReadStream("my.csv");

csv
 .fromStream(stream)
 .transform(function(data, next){
     MyModel.findById(data.id, next);
 })
 .on("data", function(data){
     console.log(data);
 })
 .on("end", function(){
     console.log("done");
 });

Formatting

fast-csv also allows you to create a CSV from data.

Formatting accepts the same options as parsing with an additional transform option.

• transform(row[, cb]): A function that accepts a row and returns a transformed one to be written, or your function can accept an optional callback to do async transformations.
• rowDelimiter='\n': Specify an alternate row delimiter (i.e \r\n)
• includeEndRowDelimiter=false: Set to true to include a row delimiter at the end of the csv.
• quoteHeaders=false
  • If true then all headers will be quoted.
  • If it is an object then each key that has a true value will be quoted (see example below)
  • If it is an array then each item in the array that is true will have the header at the corresponding index quoted (see example below)
• quoteColumns=false
  • If true then columns and headers will be quoted (unless quoteHeaders is specified).
  • If it is an object then each key that has a true value will be quoted ((unless quoteHeaders is specified)
  • If it is an array then each item in the array that is true will have the column at the corresponding index quoted (unless quoteHeaders is specified)

Data Types

When creating a CSV fast-csv supports a few data formats.

Objects

You can pass in object to any formatter function if your CSV requires headers the keys of the first object will be used as the header names.

[
    {
        a: "a1",
        b: "b1",
        c: "c1"
    }
]

//Generated CSV
//a,b,c
//a1,b1,c1

Arrays

You can also pass in your rows as arrays. If your CSV requires headers the first row passed in will be the headers used.

[
    ["a", "b", "c"],
    ["a1", "b1", "c1"]
]
//Generated CSV
//a,b,c
//a1,b1,c1

Arrays of Array

This is the least commonly used format but can be useful if you have requirements to generate a CSV with headers with the same column name (Crazy we know but we have seen it).

[
    [
        ["a", "a1"],
        ["a", "a2"],
        ["b", "b1"],
        ["b", "b2"],
        ["c", "c1"],
        ["c", "c2"]
    ]
]

//Generated CSV
//a,a,b,b,c,c
//a1,a2,b1,b2,c1,c2

Formatting Functions

createWriteStream(options) or .format(options)

This is the lowest level of the write methods, it creates a stream that can be used to create a CSV of unknown size and pipe to an output CSV.

var csvStream = csv.createWriteStream({headers: true}),
    writableStream = fs.createWriteStream("my.csv");

writableStream.on("finish", function(){
  console.log("DONE!");
});

csvStream.pipe(writableStream);
csvStream.write({a: "a0", b: "b0"});
csvStream.write({a: "a1", b: "b1"});
csvStream.write({a: "a2", b: "b2"});
csvStream.write({a: "a3", b: "b4"});
csvStream.write({a: "a3", b: "b4"});
csvStream.end();

//or

var csvStream = csv.format({headers: true}),
    writableStream = fs.createWriteStream("my.csv");

writableStream.on("finish", function(){
  console.log("DONE!");
});

csvStream.pipe(writableStream);
csvStream.write({a: "a0", b: "b0"});
csvStream.write({a: "a1", b: "b1"});
csvStream.write({a: "a2", b: "b2"});
csvStream.write({a: "a3", b: "b4"});
csvStream.write({a: "a3", b: "b4"});
csvStream.end();

If you wish to transform rows as writing then you can use the .transform method.

var csvStream = csv
    .createWriteStream({headers: true})
    .transform(function(row){
        return {
           A: row.a,
           B: row.b
        };
    }),
    writableStream = fs.createWriteStream("my.csv");

writableStream.on("finish", function(){
  console.log("DONE!");
});

csvStream.pipe(writableStream);
csvStream.write({a: "a0", b: "b0"});
csvStream.write({a: "a1", b: "b1"});
csvStream.write({a: "a2", b: "b2"});
csvStream.write({a: "a3", b: "b4"});
csvStream.write({a: "a3", b: "b4"});
csvStream.end();

//or
var csvStream = csv
    .format({headers: true})
    .transform(function(row){
        return {
           A: row.a,
           B: row.b
        };
    }),
    writableStream = fs.createWriteStream("my.csv");

writableStream.on("finish", function(){
  console.log("DONE!");
});

csvStream.pipe(writableStream);
csvStream.write({a: "a0", b: "b0"});
csvStream.write({a: "a1", b: "b1"});
csvStream.write({a: "a2", b: "b2"});
csvStream.write({a: "a3", b: "b4"});
csvStream.write({a: "a3", b: "b4"});
csvStream.end();

Transform can also be async by accepting a callback.

var csvStream = csv
    .createWriteStream({headers: true})
    .transform(function(row, next){
        setImmediate(function(){
            next(null, {A: row.a, B: row.b});
        });;
    }),
    writableStream = fs.createWriteStream("my.csv");

writableStream.on("finish", function(){
  console.log("DONE!");
});

csvStream.pipe(writableStream);
csvStream.write({a: "a0", b: "b0"});
csvStream.write({a: "a1", b: "b1"});
csvStream.write({a: "a2", b: "b2"});
csvStream.write({a: "a3", b: "b4"});
csvStream.write({a: "a3", b: "b4"});
csvStream.end();

//or

var csvStream = csv
    .format({headers: true})
    .transform(function(row, next){
        setImmediate(function(){
            next(null, {A: row.a, B: row.b});
        });;
    }),
    writableStream = fs.createWriteStream("my.csv");

writableStream.on("finish", function(){
  console.log("DONE!");
});

csvStream.pipe(writableStream);
csvStream.write({a: "a0", b: "b0"});
csvStream.write({a: "a1", b: "b1"});
csvStream.write({a: "a2", b: "b2"});
csvStream.write({a: "a3", b: "b4"});
csvStream.write({a: "a3", b: "b4"});
csvStream.end();

Writing Data

Each of the following methods accept an array of values to be written, however each value must be an array of arrays or objects.

write(arr[, options])

Create a readable stream to read data from.

var ws = fs.createWriteStream("my.csv");
csv
   .write([
       ["a", "b"],
       ["a1", "b1"],
       ["a2", "b2"]
   ], {headers: true})
   .pipe(ws);

var ws = fs.createWriteStream("my.csv");
csv
   .write([
       {a: "a1", b: "b1"},
       {a: "a2", b: "b2"}
   ], {headers: true})
   .pipe(ws);

var ws = fs.createWriteStream("my.csv");
csv
   .write([
       {a: "a1", b: "b1"},
       {a: "a2", b: "b2"}
   ], {
        headers: true
        transform: function(row){
            return {
                A: row.a,
                B: row.b
            };
        }
   })
   .pipe(ws);

writeToStream(stream, arr[, options])

Write an array of values to a WritableStream

csv
   .writeToStream(fs.createWriteStream("my.csv"), [
       ["a", "b"],
       ["a1", "b1"],
       ["a2", "b2"]
   ], {headers: true});

csv
   .writeToStream(fs.createWriteStream("my.csv"), [
       {a: "a1", b: "b1"},
       {a: "a2", b: "b2"}
   ], {headers: true})
   .pipe(ws);

csv
   .writeToStream(fs.createWriteStream("my.csv"), [
       {a: "a1", b: "b1"},
       {a: "a2", b: "b2"}
   ], {
        headers: true,
        transform: function(row){
            return {
                A: row.a,
                B: row.b
            };
        }
   })
   .pipe(ws);

writeToPath(path, arr[, options])

Write an array of values to the specified path

csv
   .writeToPath("my.csv", [
       ["a", "b"],
       ["a1", "b1"],
       ["a2", "b2"]
   ], {headers: true})
   .on("finish", function(){
       console.log("done!");
   });

csv
   .writeToPath("my.csv", [
       {a: "a1", b: "b1"},
       {a: "a2", b: "b2"}
   ], {headers: true})
   .on("finish", function(){
      console.log("done!");
   });

csv
   .writeToPath("my.csv", [
       {a: "a1", b: "b1"},
       {a: "a2", b: "b2"}
   ], {
        headers: true,
        transform: function(row){
            return {
                A: row.a,
                B: row.b
            };
        }
   })
   .on("finish", function(){
      console.log("done!");
   });

writeToString(arr[, options], cb)

csv.writeToString(
    [
        ["a", "b"],
        ["a1", "b1"],
        ["a2", "b2"]
    ],
    {headers: true},
    function(err, data){
        console.log(data); //"a,b\na1,b1\na2,b2\n"
    }
);

csv.writeToString(
    [
        {a: "a1", b: "b1"},
        {a: "a2", b: "b2"}
    ],
    {headers: true},
    function(err, data){
        console.log(data); //"a,b\na1,b1\na2,b2\n"
    }
);

csv.writeToString(
    [
        {a: "a1", b: "b1"},
        {a: "a2", b: "b2"}
    ],
    {
        headers: true,
        transform: function (row) {
            return {
                A: row.a,
                B: row.b
            };
        }
    },
    function (err, data) {
        console.log(data); //"A,B\na1,b1\na2,b2\n"
    }
);

Piping from Parser to Writer

You can use fast-csv to pipe the output from a parsed CSV to a transformed CSV by setting the parser to objectMode and using createWriteStream.

csv
   .fromPath("in.csv", {headers: true})
   .pipe(csv.createWriteStream({headers: true}))
   .pipe(fs.createWriteStream("out.csv", {encoding: "utf8"}));

When piping from a parser to a formatter the transforms are maintained also.

csv
   .fromPath("in.csv", {headers: true})
   .transform(function(obj){
        return {
            name: obj.Name,
            address: obj.Address,
            emailAddress: obj.Email_Address,
            verified: obj.Verified
        };
   })
   .pipe(csv.createWriteStream({headers: true}))
   .pipe(fs.createWriteStream("out.csv", {encoding: "utf8"}));

The output will contain formatted result from the transform function.

If you want to transform on the formatting side

var formatStream = csv
        .createWriteStream({headers: true})
        .transform(function(obj){
            return {
                name: obj.Name,
                address: obj.Address,
                emailAddress: obj.Email_Address,
                verified: obj.Verified
            };
        });
csv
   .fromPath("in.csv", {headers: true})
   .pipe(formatStream)
   .pipe(fs.createWriteStream("out.csv", {encoding: "utf8"}));

Quoting Columns

Sometimes you may need to quote columns is certain ways in order meet certain requirements. fast-csv can quote columns and headers almost anyway you may need.

Note in the following example we use writeToString but the options option are valid for any of the formatting methods.

quoteColumns

//quote all columns including headers
var objectData = [{a: "a1", b: "b1"}, {a: "a2", b: "b2"}],
    arrayData = [["a", "b"], ["a1", "b1"], ["a2", "b2"]];
csv.writeToString(objectData, {headers: true, quoteColumns: true}, function(err, data){
    console.log(data); //"a","b"
                       //"a1","b1"
                       //"a2","b2"
});

//only quote the "a" column
csv.writeToString(objectData, {headers: true, quoteColumns: {a: true}}, function(err, data){
    console.log(data); //"a",b
                       //"a1",b1
                       //"a2",b2
});

//only quote the second column
csv.writeToString(arrayData, {headers: true, quoteColumns: [false, true]}, function(err, data){
    console.log(data); //a,"b"
                       //a1,"b1"
                       //a2,"b2"
});

quoteHeaders

//quote all columns including headers
var objectData = [{a: "a1", b: "b1"}, {a: "a2", b: "b2"}],
    arrayData = [["a", "b"], ["a1", "b1"], ["a2", "b2"]];
csv.writeToString(objectData, {headers: true, quoteHeaders: true}, function(err, data){
    console.log(data); //"a","b"
                       //a1,b1
                       //a2,b2
});

//only quote the "a" column
csv.writeToString(objectData, {headers: true, quoteHeaders: {a: true}}, function(err, data){
    console.log(data); //"a",b
                       //a1,b1
                       //a2,b2
});

//only quote the second column
csv.writeToString(arrayData, {headers: true, quoteHeaders: [false, true]}, function(err, data){
    console.log(data); //a,"b"
                       //a1,b1
                       //a2,b2
});

//quoting columns and not headers

//only quote the second column
csv.writeToString(arrayData, {headers: true, quoteHeaders: false, quoteColumns: true}, function(err, data){
    console.log(data); //a,b
                       //"a1","b1"
                       //"a2","b2"
});

License

MIT https://github.com/C2FO/fast-csv/raw/master/LICENSE

##Meta

• Code: git clone git://github.com/C2FO/fast-csv.git
• Website: http://c2fo.com
• Twitter: http://twitter.com/c2fo - 877.465.4045

Changelog

v2.4.0

• Allow renaming headers. #175