Stream writable.writableLength Property in Node.js

The writable.writableLength property in Node.js helps you monitor how much data is currently queued in a writable stream before being flushed. This is especially helpful when you're dealing with high-throughput write operations and want to track backpressure or buffer size.

What Is writable.writableLength?

This property returns the number of bytes currently buffered for writing in a writable stream. It includes all data that has been passed to write() but not yet handled by the underlying system.

Syntax

writable.writableLength

No parameters are needed. It's a simple property you can access at any time on a writable stream instance.

Why Use writableLength?

• Monitor Buffer Load: Helps you understand how full the internal buffer is.
• Manage Backpressure: Helps in implementing throttling mechanisms.
• Optimize Performance: Useful in high-performance scenarios to avoid overloading the stream.

Example: Tracking writableLength

Below is a simple example that writes data to a stream and logs the buffered length after each write.

const fs = require('fs');

const stream = fs.createWriteStream('output.txt');

stream.write('First Line\n');
console.log('Buffered bytes:', stream.writableLength);

stream.write('Second Line\n');
console.log('Buffered bytes:', stream.writableLength);

stream.end();
    

Each time you write, the writableLength property reflects the size of data yet to be written to the file.

How It Helps

If you're writing data faster than the destination can handle, the buffer will grow. Monitoring writable.writableLength lets you slow down or pause writes when needed, avoiding memory overflow or lag.

Note

• This property is read-only.
• It’s available only on writable streams.
• The value is measured in bytes.

Conclusion

The writable.writableLength property in Node.js gives you insights into the current buffer load of a writable stream. It’s a small but powerful tool for developers working with stream-heavy or performance-critical applications.