NodeJS Add New Rows To CSV (Append Prepend Insert)

Welcome to a tutorial on how to add new rows to an existing CSV file in NodeJS. So you need to update a CSV file? Read on for the examples!

 

 

TABLE OF CONTENTS

 

 

EXAMPLE CODE DOWNLOAD

Source code on GitHub Gist

Just click on “download zip” or do a git clone. I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

 

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1) APPEND NEW ROWS

// (A) LOAD MODULES
// npm install csv-stringify
// https://www.npmjs.com/package/csv-stringify
const fs = require("fs"),
      csvs = require("csv-stringify");

// (B) DATA TO APPEND
var data = [["A", "B"]];

// (C) CREATE CSV FILE
csvs.stringify(data, (err, output) => {
  fs.appendFileSync("x-dummy.csv", output);
  console.log("OK");
});

Appending new rows (to the end of the CSV file) should not be too much trouble:

1. Load the required modules. We will use the file system fs and csv-stringify modules in this example.
2. Define the rows that you want to append in a nested array.
3. For beginners who are lost –
   • csvs.stringify(DATA, CALLBACK) will simply turn DATA into a “CSV encoded string”.
   • CALLBACK is a function that will be called when stringify() is done “converting” the DATA.
   • Very simply, we append the converted “CSV string” to the end of the CSV file.

 

 

2) PREPEND NEW ROWS

// (A) LOAD MODULES
// npm install csv-stringify csv-parser
// https://www.npmjs.com/package/csv-stringify
// https://www.npmjs.com/package/csv-parser
const fs = require("fs"),
      csvp = require("csv-parser"),
      csvs = require("csv-stringify");

// (B) READ CSV FILE
var rows = [];
fs.createReadStream("x-dummy.csv")
.pipe(csvp({ headers : false }))
.on("data", data => rows.push(data))
.on("end", () => {
  // (C) REARRANGE TO FLAT ARRAY
  rows.forEach((r,i) => rows[i] = Object.values(r));

  // (D) PREPEND
  rows.splice(0, 0, ["C", "D"]);

  // (E) WRITE
  csvs.stringify(rows, (err, output) => {
    fs.writeFileSync("x-dummy.csv", output);
    console.log("OK");
  });
});

Appending new rows is easy, but sadly, prepend is a different story… There are no native NodeJS functions to prepend data to a file, thus a roundabout solution:

1. Load the required modules – File system, CSV parser (CSV to array), and CSV stringify (array to CSV).
2. Read the entire CSV file into a rows array.
3. Do some “data yoga” so that rows is in a “nice nested array” of [["COL", "COL"], ["COL", "COL"], ...]
4. To prepend, simply add a new row to the top of rows.
5. Lastly, write the updated rows back into the CSV file.

 

 

3) INSERT NEW ROWS

// (A) LOAD MODULES
// npm install csv-stringify csv-parser
// https://www.npmjs.com/package/csv-stringify
// https://www.npmjs.com/package/csv-parser
const fs = require("fs"),
      csvp = require("csv-parser"),
      csvs = require("csv-stringify");
 
// (B) READ CSV FILE
var rows = [];
fs.createReadStream("x-dummy.csv")
.pipe(csvp({ headers : false }))
.on("data", data => rows.push(data))
.on("end", () => {
  // (C) REARRANGE TO FLAT ARRAY
  rows.forEach((r,i) => rows[i] = Object.values(r));

  // (D) INSERT AT EXACT ROW
  rows.splice(3, 0, ["E", "F"]);
 
  // (E) SEARCH & INSERT AT ROW
  let at = null;
  for (let [i,r] of Object.entries(rows)) {
    if (r.includes("Jou Doe")) { at = i; break; }
  }
  if (at !== null) { rows.splice(at, 0, ["G", "H"]); }

  // (F) WRITE
  csvs.stringify(rows, (err, output) => {
    fs.writeFileSync("x-dummy.csv", output);
    console.log("OK");
  });
});

Finally, if you need to insert into “somewhere in the middle” – It’s pretty much the same as prepend.

• (A To C) Load the modules, and read the CSV file into an array.
• (D) Use the same old ARRAY.splice(AT, REMOVE, [ROW]) to insert a new row.
• (E) If you do not know where to insert, the only way is to search through all the rows to find the row number…
• (F) Save the updated CSV.

 

 

 

PERFORMANCE NOTES

Take note that the above “prepend” and “insert” examples read the entire CSV file into the memory. While it works, it will take a lot of system resources to process massive CSV files. So do your own changes when it comes to dealing with such cases. There are quite a few options:

• Find ways to limit the number of rows in the CSV file.
• Change the process. Instead of feeding the entire file stream into CSV parse, read line by line instead.
• Similarly, instead of “stringifying” the entire file – Do it line by line, or by batch.

 

LINKS & REFERENCES

• Read CSV Files – Code Boxx
• Write CSV Files – Code Boxx
• CSV Stringify – NPM
• CSV Parse – NPM