Read and write to files

  • 4 minutes

Reading data from files and writing data to files are core concepts in Node.js.

Tailwind Traders needs to write the total of all the individual store sales files to a new file. This file will then be loaded into the company's sales system.

Here, you'll learn how to use the fs module to read and write to files.

Read data from files

Files are read through the readFile method on the fs module.

await fs.readFile("stores/201/sales.json");

The return object from the readFile method is a Buffer object. It contains the contents of the file that was read, but in binary format. For instance, assume that you have a file called sales.json with the following contents.

{
  "total": 22385.32
}

Logging out the return value of the readFile method would give you the Buffer value.

console.log(await fs.readFile("stores/201/sales.json"));
// <Buffer 7b 0a 20 20 22 74 6f 74 61 6c 22 3a 20 32 32 33 38 35 2e 33 32 0a 7d>

That result isn't helpful. You might have read the file, but you certainly can't "read" this data. That's OK, though. JavaScript can convert a Buffer value to a string value that you can read. To do that, invoke the String object and pass in the buffer.

const bufferData = await fs.readFile("stores/201/sales.json");
console.log(String(bufferData));
// {
//   "total": 22385.32
// }

Parse data in files

This data in its string format doesn't do you much good. It's still just characters, but now in a format that you can read. You want the ability to parse this data into a format that you can use programmatically.

JavaScript includes a built-in parser for JSON files. You don't need to include anything to use it. Just use the JSON object. As a bonus, you don't need to convert a Buffer value to a string before you parse it. The JSON.parse method can do that.

const data = JSON.parse(await fs.readFile("stores/201/sales.json"));
console.log(data.total);
// 22385.32

Tip

Files come a variety of formats. JSON files are the most desirable to work with because of the built-in support in the language. However, you might encounter files that are .csv, fixed width, or some other format. In that case, it's best to search npmjs.org for a parser for that file type.

Write data to files

You learned how to write files in the previous exercise. It's just that you wrote an empty one. To write data to a file, use the same writeFile method, but pass in the data that you want to write as the third parameter.

const data = JSON.parse(await fs.readFile("stores/201/sales.json"));

// write the total to the "totals.json" file
await fs.writeFile("salesTotals/totals.txt"), data.total);

// totals.txt
// 22385.32

Append data to files

In the preceding example, the file is overwritten every time you write to it. Sometimes you don't want that. Sometimes you want to append data to the file, not replace it entirely. You can do this by passing a flag to the writeFile method. By default, the flag is set to w, which means "replace the file." To append to the file instead, pass the a flag, which means "append."

const data = JSON.parse(await fs.readFile("stores/201/sales.json"));

// write the total to the "totals.json" file
await fs.writeFile(path.join("salesTotals/totals.txt"), `${data.total}\r\n`, {
  flag: "a"
});

// totals.json
// 22385.32
// 22385.32

Tip

In the preceding code example, \r\n tells JavaScript to put the value on its own line. If you didn't pass this value (known as a carriage return line feed), you would get all the numbers squished together on the same line.

In the next exercise, you'll finish the sales-total project for Tailwind Traders by reading all the sales files and writing the grand total to a .txt file. The company's commerce system can then process the file.