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Constants

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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

Brotli constants

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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).

Zstd constants

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Stability: 1Experimental

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,
  },
});
JavaScript

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.
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