Zlib

The node:zlib module provides compression functionality implemented using Gzip, Deflate/Inflate, Brotli, and Zstd.

To access it:

import zlib from 'node:zlib';

const zlib = require('node:zlib');

Compression and decompression are built around the Node.js Streams API.

Compressing or decompressing a stream (such as a file) can be accomplished by piping the source stream through a zlib Transform stream into a destination stream:

import {
  createReadStream,
  createWriteStream,
} from 'node:fs';
import process from 'node:process';
import { createGzip } from 'node:zlib';
import { pipeline } from 'node:stream';

const gzip = createGzip();
const source = createReadStream('input.txt');
const destination = createWriteStream('input.txt.gz');

pipeline(source, gzip, destination, (err) => {
  if (err) {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  }
});

const {
  createReadStream,
  createWriteStream,
} = require('node:fs');
const { createGzip } = require('node:zlib');
const { pipeline } = require('node:stream');

const gzip = createGzip();
const source = createReadStream('input.txt');
const destination = createWriteStream('input.txt.gz');

pipeline(source, gzip, destination, (err) => {
  if (err) {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  }
});

Or, using the promise pipeline API:

import {
  createReadStream,
  createWriteStream,
} from 'node:fs';
import { createGzip } from 'node:zlib';
import { pipeline } from 'node:stream/promises';

async function do_gzip(input, output) {
  const gzip = createGzip();
  const source = createReadStream(input);
  const destination = createWriteStream(output);
  await pipeline(source, gzip, destination);
}

await do_gzip('input.txt', 'input.txt.gz');

const {
  createReadStream,
  createWriteStream,
} = require('node:fs');
const { createGzip } = require('node:zlib');
const { pipeline } = require('node:stream/promises');

async function do_gzip(input, output) {
  const gzip = createGzip();
  const source = createReadStream(input);
  const destination = createWriteStream(output);
  await pipeline(source, gzip, destination);
}

do_gzip('input.txt', 'input.txt.gz')
  .catch((err) => {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  });

It is also possible to compress or decompress data in a single step:

import process from 'node:process';
import { Buffer } from 'node:buffer';
import { deflate, unzip } from 'node:zlib';

const input = '.................................';
deflate(input, (err, buffer) => {
  if (err) {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  }
  console.log(buffer.toString('base64'));
});

const buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');
unzip(buffer, (err, buffer) => {
  if (err) {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  }
  console.log(buffer.toString());
});

// Or, Promisified

import { promisify } from 'node:util';
const do_unzip = promisify(unzip);

const unzippedBuffer = await do_unzip(buffer);
console.log(unzippedBuffer.toString());

const { deflate, unzip } = require('node:zlib');

const input = '.................................';
deflate(input, (err, buffer) => {
  if (err) {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  }
  console.log(buffer.toString('base64'));
});

const buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');
unzip(buffer, (err, buffer) => {
  if (err) {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  }
  console.log(buffer.toString());
});

// Or, Promisified

const { promisify } = require('node:util');
const do_unzip = promisify(unzip);

do_unzip(buffer)
  .then((buf) => console.log(buf.toString()))
  .catch((err) => {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  });

All zlib APIs, except those that are explicitly synchronous, use the Node.js internal threadpool. This can lead to surprising effects and performance limitations in some applications.

Creating and using a large number of zlib objects simultaneously can cause significant memory fragmentation.

import zlib from 'node:zlib';
import { Buffer } from 'node:buffer';

const payload = Buffer.from('This is some data');

// WARNING: DO NOT DO THIS!
for (let i = 0; i < 30000; ++i) {
  zlib.deflate(payload, (err, buffer) => {});
}

const zlib = require('node:zlib');

const payload = Buffer.from('This is some data');

// WARNING: DO NOT DO THIS!
for (let i = 0; i < 30000; ++i) {
  zlib.deflate(payload, (err, buffer) => {});
}

In the preceding example, 30,000 deflate instances are created concurrently. Because of how some operating systems handle memory allocation and deallocation, this may lead to significant memory fragmentation.

It is strongly recommended that the results of compression operations be cached to avoid duplication of effort.

The node:zlib module can be used to implement support for the gzip, deflate, br, and zstd content-encoding mechanisms defined by HTTP.

The HTTP Accept-Encoding header is used within an HTTP request to identify the compression encodings accepted by the client. The Content-Encoding header is used to identify the compression encodings actually applied to a message.

The examples given below are drastically simplified to show the basic concept. Using zlib encoding can be expensive, and the results ought to be cached. See Memory usage tuning for more information on the speed/memory/compression tradeoffs involved in zlib usage.

// Client request example
import fs from 'node:fs';
import zlib from 'node:zlib';
import http from 'node:http';
import process from 'node:process';
import { pipeline } from 'node:stream';

const request = http.get({ host: 'example.com',
                           path: '/',
                           port: 80,
                           headers: { 'Accept-Encoding': 'br,gzip,deflate,zstd' } });
request.on('response', (response) => {
  const output = fs.createWriteStream('example.com_index.html');

  const onError = (err) => {
    if (err) {
      console.error('An error occurred:', err);
      process.exitCode = 1;
    }
  };

  switch (response.headers['content-encoding']) {
    case 'br':
      pipeline(response, zlib.createBrotliDecompress(), output, onError);
      break;
    // Or, just use zlib.createUnzip() to handle both of the following cases:
    case 'gzip':
      pipeline(response, zlib.createGunzip(), output, onError);
      break;
    case 'deflate':
      pipeline(response, zlib.createInflate(), output, onError);
      break;
    case 'zstd':
      pipeline(response, zlib.createZstdDecompress(), output, onError);
      break;
    default:
      pipeline(response, output, onError);
      break;
  }
});

// Client request example
const zlib = require('node:zlib');
const http = require('node:http');
const fs = require('node:fs');
const { pipeline } = require('node:stream');

const request = http.get({ host: 'example.com',
                           path: '/',
                           port: 80,
                           headers: { 'Accept-Encoding': 'br,gzip,deflate,zstd' } });
request.on('response', (response) => {
  const output = fs.createWriteStream('example.com_index.html');

  const onError = (err) => {
    if (err) {
      console.error('An error occurred:', err);
      process.exitCode = 1;
    }
  };

  switch (response.headers['content-encoding']) {
    case 'br':
      pipeline(response, zlib.createBrotliDecompress(), output, onError);
      break;
    // Or, just use zlib.createUnzip() to handle both of the following cases:
    case 'gzip':
      pipeline(response, zlib.createGunzip(), output, onError);
      break;
    case 'deflate':
      pipeline(response, zlib.createInflate(), output, onError);
      break;
    case 'zstd':
      pipeline(response, zlib.createZstdDecompress(), output, onError);
      break;
    default:
      pipeline(response, output, onError);
      break;
  }
});

// server example
// Running a gzip operation on every request is quite expensive.
// It would be much more efficient to cache the compressed buffer.
import zlib from 'node:zlib';
import http from 'node:http';
import fs from 'node:fs';
import { pipeline } from 'node:stream';

http.createServer((request, response) => {
  const raw = fs.createReadStream('index.html');
  // Store both a compressed and an uncompressed version of the resource.
  response.setHeader('Vary', 'Accept-Encoding');
  const acceptEncoding = request.headers['accept-encoding'] || '';

  const onError = (err) => {
    if (err) {
      // If an error occurs, there's not much we can do because
      // the server has already sent the 200 response code and
      // some amount of data has already been sent to the client.
      // The best we can do is terminate the response immediately
      // and log the error.
      response.end();
      console.error('An error occurred:', err);
    }
  };

  // Note: This is not a conformant accept-encoding parser.
  // See https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
  if (/\bdeflate\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'deflate' });
    pipeline(raw, zlib.createDeflate(), response, onError);
  } else if (/\bgzip\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'gzip' });
    pipeline(raw, zlib.createGzip(), response, onError);
  } else if (/\bbr\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'br' });
    pipeline(raw, zlib.createBrotliCompress(), response, onError);
  } else if (/\bzstd\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'zstd' });
    pipeline(raw, zlib.createZstdCompress(), response, onError);
  } else {
    response.writeHead(200, {});
    pipeline(raw, response, onError);
  }
}).listen(1337);

// server example
// Running a gzip operation on every request is quite expensive.
// It would be much more efficient to cache the compressed buffer.
const zlib = require('node:zlib');
const http = require('node:http');
const fs = require('node:fs');
const { pipeline } = require('node:stream');

http.createServer((request, response) => {
  const raw = fs.createReadStream('index.html');
  // Store both a compressed and an uncompressed version of the resource.
  response.setHeader('Vary', 'Accept-Encoding');
  const acceptEncoding = request.headers['accept-encoding'] || '';

  const onError = (err) => {
    if (err) {
      // If an error occurs, there's not much we can do because
      // the server has already sent the 200 response code and
      // some amount of data has already been sent to the client.
      // The best we can do is terminate the response immediately
      // and log the error.
      response.end();
      console.error('An error occurred:', err);
    }
  };

  // Note: This is not a conformant accept-encoding parser.
  // See https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
  if (/\bdeflate\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'deflate' });
    pipeline(raw, zlib.createDeflate(), response, onError);
  } else if (/\bgzip\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'gzip' });
    pipeline(raw, zlib.createGzip(), response, onError);
  } else if (/\bbr\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'br' });
    pipeline(raw, zlib.createBrotliCompress(), response, onError);
  } else if (/\bzstd\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'zstd' });
    pipeline(raw, zlib.createZstdCompress(), response, onError);
  } else {
    response.writeHead(200, {});
    pipeline(raw, response, onError);
  }
}).listen(1337);

By default, the zlib methods will throw an error when decompressing truncated data. However, if it is known that the data is incomplete, or the desire is to inspect only the beginning of a compressed file, it is possible to suppress the default error handling by changing the flushing method that is used to decompress the last chunk of input data:

// This is a truncated version of the buffer from the above examples
const buffer = Buffer.from('eJzT0yMA', 'base64');

zlib.unzip(
  buffer,
  // For Brotli, the equivalent is zlib.constants.BROTLI_OPERATION_FLUSH.
  // For Zstd, the equivalent is zlib.constants.ZSTD_e_flush.
  { finishFlush: zlib.constants.Z_SYNC_FLUSH },
  (err, buffer) => {
    if (err) {
      console.error('An error occurred:', err);
      process.exitCode = 1;
    }
    console.log(buffer.toString());
  });

This will not change the behavior in other error-throwing situations, e.g. when the input data has an invalid format. Using this method, it will not be possible to determine whether the input ended prematurely or lacks the integrity checks, making it necessary to manually check that the decompressed result is valid.

From zlib/zconf.h, modified for Node.js usage:

The memory requirements for deflate are (in bytes):

(1 << (windowBits + 2)) + (1 << (memLevel + 9));

That is: 128K for windowBits = 15 + 128K for memLevel = 8 (default values) plus a few kilobytes for small objects.

For example, to reduce the default memory requirements from 256K to 128K, the options should be set to:

const options = { windowBits: 14, memLevel: 7 };

This will, however, generally degrade compression.

The memory requirements for inflate are (in bytes) 1 << windowBits. That is, 32K for windowBits = 15 (default value) plus a few kilobytes for small objects.

This is in addition to a single internal output slab buffer of size chunkSize, which defaults to 16K.

The speed of zlib compression is affected most dramatically by the level setting. A higher level will result in better compression, but will take longer to complete. A lower level will result in less compression, but will be much faster.

In general, greater memory usage options will mean that Node.js has to make fewer calls to zlib because it will be able to process more data on each write operation. So, this is another factor that affects the speed, at the cost of memory usage.

There are equivalents to the zlib options for Brotli-based streams, although these options have different ranges than the zlib ones:

• zlib's level option matches Brotli's BROTLI_PARAM_QUALITY option.
• zlib's windowBits option matches Brotli's BROTLI_PARAM_LGWIN option.

See below for more details on Brotli-specific options.

There are equivalents to the zlib options for Zstd-based streams, although these options have different ranges than the zlib ones:

• zlib's level option matches Zstd's ZSTD_c_compressionLevel option.
• zlib's windowBits option matches Zstd's ZSTD_c_windowLog option.

See below for more details on Zstd-specific options.

Calling .flush() on a compression stream will make zlib return as much output as currently possible. This may come at the cost of degraded compression quality, but can be useful when data needs to be available as soon as possible.

In the following example, flush() is used to write a compressed partial HTTP response to the client:

import zlib from 'node:zlib';
import http from 'node:http';
import { pipeline } from 'node:stream';

http.createServer((request, response) => {
  // For the sake of simplicity, the Accept-Encoding checks are omitted.
  response.writeHead(200, { 'content-encoding': 'gzip' });
  const output = zlib.createGzip();
  let i;

  pipeline(output, response, (err) => {
    if (err) {
      // If an error occurs, there's not much we can do because
      // the server has already sent the 200 response code and
      // some amount of data has already been sent to the client.
      // The best we can do is terminate the response immediately
      // and log the error.
      clearInterval(i);
      response.end();
      console.error('An error occurred:', err);
    }
  });

  i = setInterval(() => {
    output.write(`The current time is ${Date()}\n`, () => {
      // The data has been passed to zlib, but the compression algorithm may
      // have decided to buffer the data for more efficient compression.
      // Calling .flush() will make the data available as soon as the client
      // is ready to receive it.
      output.flush();
    });
  }, 1000);
}).listen(1337);

const zlib = require('node:zlib');
const http = require('node:http');
const { pipeline } = require('node:stream');

http.createServer((request, response) => {
  // For the sake of simplicity, the Accept-Encoding checks are omitted.
  response.writeHead(200, { 'content-encoding': 'gzip' });
  const output = zlib.createGzip();
  let i;

  pipeline(output, response, (err) => {
    if (err) {
      // If an error occurs, there's not much we can do because
      // the server has already sent the 200 response code and
      // some amount of data has already been sent to the client.
      // The best we can do is terminate the response immediately
      // and log the error.
      clearInterval(i);
      response.end();
      console.error('An error occurred:', err);
    }
  });

  i = setInterval(() => {
    output.write(`The current time is ${Date()}\n`, () => {
      // The data has been passed to zlib, but the compression algorithm may
      // have decided to buffer the data for more efficient compression.
      // Calling .flush() will make the data available as soon as the client
      // is ready to receive it.
      output.flush();
    });
  }, 1000);
}).listen(1337);

All of the constants defined in zlib.h are also defined on require('node:zlib').constants. In the normal course of operations, it will not be necessary to use these constants. They are documented so that their presence is not surprising. This section is taken almost directly from the zlib documentation.

Previously, the constants were available directly from require('node:zlib'), for instance zlib.Z_NO_FLUSH. Accessing the constants directly from the module is currently still possible but is deprecated.

Allowed flush values.

• zlib.constants.Z_NO_FLUSH
• zlib.constants.Z_PARTIAL_FLUSH
• zlib.constants.Z_SYNC_FLUSH
• zlib.constants.Z_FULL_FLUSH
• zlib.constants.Z_FINISH
• zlib.constants.Z_BLOCK

Return codes for the compression/decompression functions. Negative values are errors, positive values are used for special but normal events.

• zlib.constants.Z_OK
• zlib.constants.Z_STREAM_END
• zlib.constants.Z_NEED_DICT
• zlib.constants.Z_ERRNO
• zlib.constants.Z_STREAM_ERROR
• zlib.constants.Z_DATA_ERROR
• zlib.constants.Z_MEM_ERROR
• zlib.constants.Z_BUF_ERROR
• zlib.constants.Z_VERSION_ERROR

Compression levels.

• zlib.constants.Z_NO_COMPRESSION
• zlib.constants.Z_BEST_SPEED
• zlib.constants.Z_BEST_COMPRESSION
• zlib.constants.Z_DEFAULT_COMPRESSION

Compression strategy.

• zlib.constants.Z_FILTERED
• zlib.constants.Z_HUFFMAN_ONLY
• zlib.constants.Z_RLE
• zlib.constants.Z_FIXED
• zlib.constants.Z_DEFAULT_STRATEGY

There are several options and other constants available for Brotli-based streams:

The following values are valid flush operations for Brotli-based streams:

• zlib.constants.BROTLI_OPERATION_PROCESS (default for all operations)
• zlib.constants.BROTLI_OPERATION_FLUSH (default when calling .flush())
• zlib.constants.BROTLI_OPERATION_FINISH (default for the last chunk)
• zlib.constants.BROTLI_OPERATION_EMIT_METADATA
  • This particular operation may be hard to use in a Node.js context, as the streaming layer makes it hard to know which data will end up in this frame. Also, there is currently no way to consume this data through the Node.js API.

There are several options that can be set on Brotli encoders, affecting compression efficiency and speed. Both the keys and the values can be accessed as properties of the zlib.constants object.

The most important options are:

• BROTLI_PARAM_MODE
  • BROTLI_MODE_GENERIC (default)
  • BROTLI_MODE_TEXT, adjusted for UTF-8 text
  • BROTLI_MODE_FONT, adjusted for WOFF 2.0 fonts
• BROTLI_PARAM_QUALITY
  • Ranges from BROTLI_MIN_QUALITY to BROTLI_MAX_QUALITY, with a default of BROTLI_DEFAULT_QUALITY.
• BROTLI_PARAM_SIZE_HINT
  • Integer value representing the expected input size; defaults to 0 for an unknown input size.

The following flags can be set for advanced control over the compression algorithm and memory usage tuning:

• BROTLI_PARAM_LGWIN
  • Ranges from BROTLI_MIN_WINDOW_BITS to BROTLI_MAX_WINDOW_BITS, with a default of BROTLI_DEFAULT_WINDOW, or up to BROTLI_LARGE_MAX_WINDOW_BITS if the BROTLI_PARAM_LARGE_WINDOW flag is set.
• BROTLI_PARAM_LGBLOCK
  • Ranges from BROTLI_MIN_INPUT_BLOCK_BITS to BROTLI_MAX_INPUT_BLOCK_BITS.
• BROTLI_PARAM_DISABLE_LITERAL_CONTEXT_MODELING
  • Boolean flag that decreases compression ratio in favour of decompression speed.
• BROTLI_PARAM_LARGE_WINDOW
  • Boolean flag enabling “Large Window Brotli” mode (not compatible with the Brotli format as standardized in RFC 7932).
• BROTLI_PARAM_NPOSTFIX
  • Ranges from 0 to BROTLI_MAX_NPOSTFIX.
• BROTLI_PARAM_NDIRECT
  • Ranges from 0 to 15 << NPOSTFIX in steps of 1 << NPOSTFIX.

These advanced options are available for controlling decompression:

• BROTLI_DECODER_PARAM_DISABLE_RING_BUFFER_REALLOCATION
  • Boolean flag that affects internal memory allocation patterns.
• BROTLI_DECODER_PARAM_LARGE_WINDOW
  • Boolean flag enabling “Large Window Brotli” mode (not compatible with the Brotli format as standardized in RFC 7932).

There are several options and other constants available for Zstd-based streams:

The following values are valid flush operations for Zstd-based streams:

• zlib.constants.ZSTD_e_continue (default for all operations)
• zlib.constants.ZSTD_e_flush (default when calling .flush())
• zlib.constants.ZSTD_e_end (default for the last chunk)

There are several options that can be set on Zstd encoders, affecting compression efficiency and speed. Both the keys and the values can be accessed as properties of the zlib.constants object.

The most important options are:

• ZSTD_c_compressionLevel
  • Set compression parameters according to pre-defined cLevel table. Default level is ZSTD_CLEVEL_DEFAULT==3.
• ZSTD_c_strategy
  • Select the compression strategy.
  • Possible values are listed in the strategy options section below.

The following constants can be used as values for the ZSTD_c_strategy parameter:

• zlib.constants.ZSTD_fast
• zlib.constants.ZSTD_dfast
• zlib.constants.ZSTD_greedy
• zlib.constants.ZSTD_lazy
• zlib.constants.ZSTD_lazy2
• zlib.constants.ZSTD_btlazy2
• zlib.constants.ZSTD_btopt
• zlib.constants.ZSTD_btultra
• zlib.constants.ZSTD_btultra2

Example:

const stream = zlib.createZstdCompress({
  params: {
    [zlib.constants.ZSTD_c_strategy]: zlib.constants.ZSTD_btultra,
  },
});

It's possible to specify the expected total size of the uncompressed input via opts.pledgedSrcSize, which must be a non-negative safe integer. If the size doesn't match at the end of the input, compression will fail with the code ZSTD_error_srcSize_wrong.

These advanced options are available for controlling decompression:

• ZSTD_d_windowLogMax
  • Select a size limit (in power of 2) beyond which the streaming API will refuse to allocate memory buffer in order to protect the host from unreasonable memory requirements.

Each zlib-based class takes an options object. No options are required.

Some options are only relevant when compressing and are ignored by the decompression classes.

See the deflateInit2 and inflateInit2 documentation for more information.

Each Brotli-based class takes an options object. All options are optional.

For example:

const stream = zlib.createBrotliCompress({
  chunkSize: 32 * 1024,
  params: {
    [zlib.constants.BROTLI_PARAM_MODE]: zlib.constants.BROTLI_MODE_TEXT,
    [zlib.constants.BROTLI_PARAM_QUALITY]: 4,
    [zlib.constants.BROTLI_PARAM_SIZE_HINT]: fs.statSync(inputFile).size,
  },
});

class zlib.BrotliCompress extends undefined

Compress data using the Brotli algorithm.

class zlib.BrotliDecompress extends undefined

Decompress data using the Brotli algorithm.

class zlib.Deflate extends undefined

Compress data using deflate.

class zlib.DeflateRaw extends undefined

Compress data using deflate, and do not append a zlib header.

class zlib.Gunzip extends undefined

Decompress a gzip stream.

class zlib.Gzip extends undefined

Compress data using gzip.

class zlib.Inflate extends undefined

Decompress a deflate stream.

class zlib.InflateRaw extends undefined

Decompress a raw deflate stream.

class zlib.Unzip extends undefined

Decompress either a Gzip- or Deflate-compressed stream by auto-detecting the header.

The ZIP archive API is experimental. Using any part of it (this class among them) emits an experimental warning the first time; merely importing node:zlib does not.

An in-memory, zero-copy view over the entries of a ZIP archive already held in a Buffer, TypedArray, DataView, or ArrayBuffer. Its set of entries can be edited - entries added or removed - but, unlike ZipFile, those edits are not written into the source buffer: a newly added entry is held as a separate in-memory ZipEntry (the passed buffer is a fixed-size view with no room to append to), and removal just drops the entry from ZipBuffer's index. The original bytes are never modified. zipBuffer.toBuffer() serializes the current set of entries into a fresh archive.

ZipBuffer does not copy the archive you hand it. It keeps a view onto that memory and reads each entry's content lazily and directly from it, which is what makes construction cheap regardless of archive size. The trade-off is that you must not modify or reuse that memory - including the ArrayBuffer backing a TypedArray/DataView - while the ZipBuffer, or any ZipEntry obtained from it, is still in use: a later read would observe the change and may fail or return corrupt data. Pass a copy (for example Buffer.from(source)) if the source might be mutated or reused.

add() and toBuffer() each have a *Sync counterpart (addSync(), toBufferSync()) that performs the same compression work synchronously. As with the synchronous node:fs APIs, these block the Node.js event loop and further JavaScript execution until the operation completes; use them only where synchronous execution is appropriate (for example, short-lived scripts or startup code), not in code that must stay responsive.

import { ZipBuffer } from 'node:zlib';
import { readFileSync, writeFileSync } from 'node:fs';
import { Buffer } from 'node:buffer';

const zip = new ZipBuffer(readFileSync('archive.zip'));
for (const [name, entry] of zip) {
  console.log(name, entry.size);
}
await zip.add('hello.txt', Buffer.from('Hello, world!'));
zip.delete('unwanted.txt');
writeFileSync('archive.zip', await zip.toBuffer());

const { ZipBuffer } = require('node:zlib');
const { readFileSync, writeFileSync } = require('node:fs');

async function main() {
  const zip = new ZipBuffer(readFileSync('archive.zip'));
  for (const [name, entry] of zip) {
    console.log(name, entry.size);
  }
  await zip.add('hello.txt', Buffer.from('Hello, world!'));
  zip.delete('unwanted.txt');
  writeFileSync('archive.zip', await zip.toBuffer());
}
main();

new zlib.ZipBuffer(buffer): zlib.ZipBuffer

Parses the archive's central directory. Throws an ERR_ZIP_INVALID_ARCHIVE or ERR_ZIP_UNSUPPORTED_FEATURE error if buffer is not a well-formed, supported archive.

buffer is not copied: the ZipBuffer retains a zero-copy view of it (for a TypedArray, DataView, or ArrayBuffer, of the underlying ArrayBuffer) and reads entry content directly from it on demand. Do not mutate or reuse that memory while the ZipBuffer or any entry read from it is still live; pass a copy if it might change.

zipBuffer.add(filename, data, options?): Promise

Equivalent to zipBuffer.addEntry(await zlib.ZipEntry.create(filename, data, options)).

zipBuffer.addSync(filename, data, options?): ZipEntry

The synchronous version of zipBuffer.add(). Equivalent to zipBuffer.addEntry(zlib.ZipEntry.createSync(filename, data, options)).

zipBuffer.addEntry(entry): ZipEntry

Adds an already-built entry, keyed by its own zipEntry.name. Replaces any existing entry of that name.

zipBuffer.clear(): void

Removes every entry.

The archive-level comment, preserved byte-for-byte across zipBuffer.toBuffer() calls unless overridden. The bytes are decoded as UTF-8 when they are valid UTF-8 and as CP437 otherwise (the field carries no encoding flag of its own).

zipBuffer.delete(name): boolean

zipBuffer.entries(): Iterator

zipBuffer.forEach(callback, thisArg?): void

Calls callback once for each entry, in the order the archive lists them.

zipBuffer.get(name): ZipEntry

Throws ERR_ZIP_ENTRY_NOT_FOUND if the archive has no entry named name.

zipBuffer.has(name): boolean

zipBuffer.keys(): Iterator

The number of entries in the archive.

zipBuffer.toBuffer(options?): Promise

Serializes the current set of entries - in the order they were added or read - into a fresh archive, switching to Zip64 structures automatically as needed (see zlib.createZipArchive()).

zipBuffer.toBufferSync(options?): Buffer

The synchronous version of zipBuffer.toBuffer() (see zlib.createZipArchiveSync()).

zipBuffer.values(): Iterator

Always true.

The ZIP archive API is experimental. Using any part of it (this class among them) emits an experimental warning the first time; merely importing node:zlib does not.

A single file or directory inside a ZIP archive. Instances are produced by ZipBuffer and ZipFile, or created directly for writing with ZipEntry.create()/ZipEntry.createStream().

create() and content() each have a *Sync counterpart (the streaming contentIterator() does not). As with the synchronous node:fs APIs, these block the Node.js event loop and further JavaScript execution until the operation (including any deflate/inflate pass) completes; use them only where synchronous execution is appropriate (for example, short-lived scripts or startup code), not in code that must stay responsive.

zlib.ZipEntry.create(filename, data, options?): Promise

Compresses data (unless method is 'store', or compression would not reduce its size) and computes its CRC-32.

When the entry ends up stored uncompressed (because method is 'store', or because compression would not reduce the size), the entry retains a zero-copy view of data rather than a copy, and its CRC-32 has already been recorded. Do not mutate data after creating the entry; pass a copy if it might change.

The MS-DOS date/time fields ZIP uses for modified have 2-second resolution and no time zone. When modified does not fall on a whole 2-second boundary, an Info-ZIP extended-timestamp extra field is written as well, recording the whole (UTC) second so the time round-trips more precisely (see zipEntry.modified). This applies to every entry-creation path.

zlib.ZipEntry.createStream(filename, source, options?): ZipEntry

Creates an entry whose content is compressed on the fly as it is serialized by zlib.createZipArchive(), without buffering source in memory. Its size, compressedSize, and crc32 only become available once serialization has finished. There is no synchronous counterpart: streaming entries only make sense with an asynchronous, incrementally-produced source.

source is drained exactly once, during serialization. Until that happens the entry has no readable content, so zipEntry.content(), zipEntry.contentSync(), and zipEntry.contentIterator() throw ERR_INVALID_STATE. If the entry is serialized by adding it to a writable ZipFile with zipFile.addEntry() (or addEntrySync()), it is then promoted in place to a file-backed entry pointing at the copy just written, so it becomes readable (and can be serialized again) for as long as that ZipFile stays open. Serializing it any other way (for example directly through zlib.createZipArchive()) leaves it spent and unreadable.

Because source may hold an operating-system resource (a file read stream, say), a streaming entry is disposable: its Symbol.dispose and Symbol.asyncDispose methods destroy source if it has not been consumed. An entry passed to an archive is disposed by that archive (see zlib.createZipArchive()); dispose an entry directly only when it was built but never handed to one. Disposal is a no-op for non-streaming entries - in particular a file-backed entry never closes the ZipFile descriptor it borrows.

zlib.ZipEntry.createSymlink(filename, target, options?): ZipEntry

Creates a symbolic-link entry: a stored entry whose content is target and whose Unix mode type bits mark it as a symlink, so zipEntry.isSymlink is true when it is read back. Extraction tools that honor symlink entries recreate the link; treat target as untrusted (see zipEntry.name on path safety).

zlib.ZipEntry.createSync(filename, data, options?): ZipEntry

The synchronous version of zlib.ZipEntry.create().

zlib.ZipEntry.read(buffer): Iterator

Parses every entry out of buffer directly, without indexing it into a ZipBuffer. Like ZipBuffer, the yielded entries hold zero-copy views of buffer rather than copies of their content, so the same rule applies: do not mutate or reuse buffer while any of them is still in use.

true if the entry's content is stored in compressed form (any compression method, currently deflate or Zstandard); false if it is stored uncompressed.

zipEntry.content(options?): Promise

Throws an ERR_ZIP_ENTRY_TOO_LARGE error if the entry's declared size exceeds maxSize, an ERR_ZIP_ENTRY_CORRUPT error if the content fails CRC-32 verification or does not match its declared size, and an ERR_INVALID_STATE error for a streaming entry (zlib.ZipEntry.createStream()) whose content is not yet available (see that method for when a streaming entry becomes readable).

zipEntry.contentSync(options?): Buffer

The synchronous version of zipEntry.content().

zipEntry.contentIterator(options?): AsyncIterator

Unlike zipEntry.content(), this does not buffer the whole member in memory. For a file-backed entry (one returned by zipFile.get()) the compressed bytes are read from disk as the iterator is consumed and nothing is retained; the entry is valid only while its ZipFile is open.

Because streaming is the bounded-memory path for arbitrarily large members, it is not capped by zlib.getMaxZipContentSize() the way zipEntry.content() is - that default guards a single large allocation, which streaming never makes. Output is still bounded per chunk to the declared uncompressed size; pass maxSize to impose an explicit ceiling.

For an in-memory entry stored without compression, the yielded chunks are zero-copy views of the entry's retained content (see zipEntry.rawContent); do not mutate them.

The yielded chunks are provisional until the iterator completes. CRC-32 verification (and the final declared-size check) can only run once every byte has been read, so a corrupt or truncated entry is reported by the iterator throwing after the last chunk, not before the first. Each chunk is still bounded so the total never exceeds the declared size or maxSize, but a consumer that must not act on unverified bytes should buffer them (or use zipEntry.content(), which verifies before returning anything) rather than processing chunks as they arrive.

The entry's raw general-purpose bit flag.

true if the entry is a directory (its name ends with /).

true if the entry is a regular file — that is, neither a directory nor a symbolic link.

true if the entry is a symbolic link (its Unix mode type bits are S_IFLNK); its content is the link target. Always false for archives not written on a Unix-like system. When extracting, treat a symlink's target as untrusted — see zipEntry.name on path safety.

The entry's Unix mode permission bits, including the setuid, setgid, and sticky bits (the low 12 bits, 0o7777), or 0 if the archive was not written on a Unix-like system. The file-type bits are not included here; use zipEntry.isDirectory / zipEntry.isSymlink for the type.

The entry's last-modification time. When the archive carries a higher-fidelity timestamp in an extra field — an NTFS (0x000a), Info-ZIP extended (0x5455), or Info-ZIP Unix (0x5855) field, as most modern tools write — that absolute (UTC) time is used; otherwise the coarse, local-time MS-DOS date/time field (2-second resolution) is used.

Some tools store their high-fidelity timestamp only in the local file header, so on a file-backed entry (one returned by zipFile.get()) the first read of this property may perform a small synchronous positioned disk read to resolve that header. If that read fails, the value silently falls back to the central-directory data.

The entry's raw compression method: 0 for stored, 8 for deflate, 93 for Zstandard.

The entry's name, decoded from the central directory, which is treated as authoritative — a local file header that disagrees is ignored, so a mismatched-header ("ZIP-confusion") archive cannot make name disagree with what is read. The bytes are decoded from a valid Info-ZIP Unicode Path extra field (0x7075) when one is present; otherwise as UTF-8 when the language-encoding flag (general-purpose bit 11) is set or the bytes are valid UTF-8 (plenty of tools wrote UTF-8 names without ever setting the flag); and as CP437 — the historical default — only when they are not. See zipEntry.nameBuffer for the raw bytes.

The name is returned verbatim: it is never normalized, and a name containing .., a leading /, a drive letter, or backslashes is neither rewritten nor rejected. A ZipFile/ZipBuffer never writes to disk, so guarding against path traversal ("Zip Slip") when extracting is the caller's responsibility.

The entry's raw name bytes, before any character decoding. Useful when the archive's names are in an encoding other than UTF-8 or CP437 and the caller wants to decode them itself.

The entry's raw (still compressed, if applicable) content when it is held in memory, or null when there is no in-memory buffer to expose - for an entry created with zlib.ZipEntry.createStream(), or a file-backed entry returned by zipFile.get(), whose bytes are read from disk on demand rather than retained. Use zipEntry.content() or zipEntry.contentIterator() to read a file-backed entry.

The entry's uncompressed size, in bytes.

The ZIP archive API is experimental. Using any part of it (this class among them) emits an experimental warning the first time; merely importing node:zlib does not.

A random-access view over the entries of a ZIP archive on disk. Only the archive's tail and central directory are read up front; member content is read from disk lazily, on demand. Writable when opened with { writable: true }: zipFile.addEntry()/zipFile.add() append the new member's data where the central directory used to be, then rewrite the central directory immediately after it; zipFile.delete() just rewrites the central directory. Both mean the file is altered as soon as the method's returned Promise fulfills. Deleted or replaced members are left behind as dead space; zipFile.compact() produces a stream with none.

These in-place edits are not crash-atomic. Rewriting the central directory happens in place, so a write that fails partway - the disk fills, the device disconnects, the process is killed - can leave the archive on disk with a partial or missing central directory, i.e. unreadable, even though the member data before it is intact. The rejected call surfaces the underlying error and the ZipFile object is left usable (its in-memory view is not discarded, so a caller can attempt recovery - for example re-writing the entries elsewhere with zipFile.compact()), but that in-memory view may no longer match the bytes on disk. Write to a copy, or compact() into a fresh file, when durability across a failure matters.

Every method has a *Sync counterpart. As with the synchronous node:fs APIs, these block the Node.js event loop and further JavaScript execution until the operation completes; use them only where synchronous execution is appropriate (for example, short-lived scripts or startup code), not in code that must stay responsive. A synchronous method throws ERR_INVALID_STATE if called while an asynchronous add(), addEntry(), delete(), or close() on the same ZipFile has not settled yet, since letting the two interleave could corrupt the archive.

import { ZipFile } from 'node:zlib';
import { Buffer } from 'node:buffer';

const zip = await ZipFile.open('archive.zip', { writable: true });
try {
  const entry = await zip.get('member.txt');
  console.log((await entry.content()).toString());
  for await (const chunk of await zip.stream('huge.bin')) {
    // Process each chunk without buffering the whole member.
  }
  await zip.add('new.txt', Buffer.from('hello'));
  await zip.delete('unwanted.txt');
} finally {
  await zip.close();
}

const { ZipFile } = require('node:zlib');

async function main() {
  const zip = await ZipFile.open('archive.zip', { writable: true });
  try {
    const entry = await zip.get('member.txt');
    console.log((await entry.content()).toString());
    for await (const chunk of await zip.stream('huge.bin')) {
      // Process each chunk without buffering the whole member.
    }
    await zip.add('new.txt', Buffer.from('hello'));
    await zip.delete('unwanted.txt');
  } finally {
    await zip.close();
  }
}
main();

zlib.ZipFile.open(filename, options?): Promise

Throws an ERR_ZIP_ARCHIVE_TOO_LARGE error if the archive's central directory is too large to buffer in memory.

zlib.ZipFile.openSync(filename, options?): ZipFile

The synchronous version of zlib.ZipFile.open().

zipFile.add(filename, data, options?): Promise

Equivalent to zipFile.addEntry(await zlib.ZipEntry.create(filename, data, options)).

zipFile.addEntry(entry): Promise

Writes entry where the central directory currently starts, then rewrites the central directory to include it, replacing any existing entry of the same name. Throws ERR_ZIP_NOT_WRITABLE if the ZipFile was not opened with { writable: true }.

The returned (same) entry is left readable: a streaming entry created with zlib.ZipEntry.createStream(), which would otherwise be spent once serialized, is promoted in place to a file-backed entry pointing at the copy just written (valid while this ZipFile is open). In-memory entries keep their own buffer unchanged.

zipFile.addEntrySync(entry): ZipEntry

The synchronous version of zipFile.addEntry(). entry must not be a pending streaming entry (one created with zlib.ZipEntry.createStream()) - there is no synchronous way to drain its asynchronous source.

zipFile.addSync(filename, data, options?): ZipEntry

The synchronous version of zipFile.add(). Equivalent to zipFile.addEntrySync(zlib.ZipEntry.createSync(filename, data, options)).