zlib.constants Property in Node.js

Introduction to zlib.constants

The zlib.constants property in Node.js is part of the zlib module, which provides compression and decompression functionalities using Gzip and Deflate algorithms. This property exposes a set of predefined constants that control how compression and decompression behave. These constants are mostly derived from the zlib C library and are crucial for advanced configuration.

Why zlib.constants Matter

While the zlib module works well with default settings, sometimes you may need to tweak its behavior. For instance, you might want to change compression levels or strategies. This is where zlib.constants comes in — it gives you full control over the fine-tuning of your compression logic.

How to Access zlib.constants

You can access the constants like this:

const zlib = require('zlib');

console.log(zlib.constants.Z_BEST_COMPRESSION); // Output: 9

The constants are available as properties under zlib.constants. They include compression levels, flush modes, return codes, and more.

Commonly Used Constants

• Z_NO_FLUSH (0): No flush operation.
• Z_SYNC_FLUSH (2): Flush output to maintain stream synchronization.
• Z_FULL_FLUSH (3): Flush all output and reset compression state.
• Z_FINISH (4): Finish the compression operation.
• Z_OK (0): Operation completed successfully.
• Z_STREAM_END (1): End of stream reached.
• Z_DEFAULT_COMPRESSION (-1): Use the default compression level.
• Z_BEST_COMPRESSION (9): Maximize compression ratio.
• Z_BEST_SPEED (1): Optimize for speed over compression ratio.

Practical Example Using Constants

Here's how you can use one of the constants to control compression level:

const zlib = require('zlib');
const fs = require('fs');

const input = fs.createReadStream('input.txt');
const output = fs.createWriteStream('input.txt.gz');

const gzip = zlib.createGzip({
    level: zlib.constants.Z_BEST_COMPRESSION
});

input.pipe(gzip).pipe(output);

In this example, we’re compressing a file with the best compression ratio by passing the corresponding constant to the createGzip() method.

When to Use zlib.constants

• When you need precise control over compression behavior.
• For performance tuning — balancing speed and compression quality.
• To respond correctly to different return statuses from zlib methods.

Conclusion

The zlib.constants property in Node.js gives developers access to low-level compression configuration and status codes. While not required for basic tasks, these constants become essential when you need more control over how data is compressed or decompressed, especially in performance-sensitive or high-volume environments.