How to create a write stream in Node.js?

Published September 15, 2020

How to create a write stream in Node.js?

Write stream is primarily used to write some data to something as the data comes in.

In the last blog, we looked at How to create a read stream in Node.js?.

We need the knowledge on reading stream to work with write stream because after reading a file then we will be writing those chunks of data into another file using the write streams.

So let's look at how it's done,

To create a write stream in Node.js, you need to first import or require the native filesystem module. In Node.js filesystem module is named as fs.

// import filesystem module
const fs = require("fs");

Now first let's create a read stream to read some data from a file called hello.txt.

// import filesystem module
const fs = require("fs");

// create a read stream to read data from hello.txt
const rs = fs.createReadStream("./hello.txt");

After creating a read stream, now we need to make a write stream to write the read data into that file.

For that Now we need to make use of a method called createWriteStream() on the fs object.

  • The method requires a valid path to a file to write to as a string type.
  • The method returns a WritableStream Object.
// import filesystem module
const fs = require("fs");

// create a read stream to read data from hello.txt
const rs = fs.createReadStream("./hello.txt");

// create a write stream
// to write the read data into a file called toWrite.txt
const ws = fs.createWriteStream("./toWrite.txt");

Now to write the content into that file, we need to join the two streams together i.e, to connect the read stream into the write stream.

Tip: Visualise streams just like the streams in nature 🦄. What we are doing is diverting a stream of read data into a write stream.

For that, we need to first listen for the data event on rs (or ReadableStream) and write the data into the Write stream as it comes in.

// import filesystem module
const fs = require("fs");

// create a read stream to read data from hello.txt
const rs = fs.createReadStream("./hello.txt");

// create a write stream
// to write the read data into a file called toWrite.txt
const ws = fs.createWriteStream("./toWrite.txt");

// listen for data event on ReadbelStream
// to write the data into WritableStream
// as the data comes in
rs.on("data", (data) => {
  console.log("New chunq Recieved");
});

Now we need to use the write() method in ws (WritableStream) object to write the data to the WritableStream.

// import filesystem module
const fs = require("fs");

// create a read stream to read data from hello.txt
const rs = fs.createReadStream("./hello.txt");

// create a write stream
// to write the read data into a file called toWrite.txt
const ws = fs.createWriteStream("./toWrite.txt");

// listen for data event on ReadbelStream
// to write the data into WritableStream
// as the data comes in
rs.on("data", (data) => {
  console.log("New chunq Recieved");

  // write the data
  // into the Write stream
  ws.write(data);
});

Now if you look into the contents of the toWrite.txt file, you can observe the contents are the same as in the hello.txt file.

See this snippet live in repl.it.

Feel free to share if you found this useful 😃.


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