Code Quality Rank: L3
Programming language: JavaScript
License: MIT License
Tags: Compression     Stream     File     Zip     Archive    
Latest version: v2.5.1

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yet another zip library for node. For unzipping, see yauzl.

Design principles:

  • Don't block the JavaScript thread. Use and provide async APIs.
  • Keep memory usage under control. Don't attempt to buffer entire files in RAM at once.
  • Prefer to open input files one at a time than all at once. This is slightly suboptimal for time performance, but avoids OS-imposed limits on the number of simultaneously open file handles.


var yazl = require("yazl");

var zipfile = new yazl.ZipFile();
zipfile.addFile("file1.txt", "file1.txt");
// (add only files, not directories)
zipfile.addFile("path/to/file.txt", "path/in/zipfile.txt");
// pipe() can be called any time after the constructor
zipfile.outputStream.pipe(fs.createWriteStream("output.zip")).on("close", function() {
// alternate apis for adding files:
zipfile.addReadStream(process.stdin, "stdin.txt");
zipfile.addBuffer(Buffer.from("hello"), "hello.txt");
// call end() after all the files have been added


Class: ZipFile

new ZipFile()

No parameters. Nothing can go wrong.

addFile(realPath, metadataPath, [options])

Adds a file from the file system at realPath into the zipfile as metadataPath. Typically metadataPath would be calculated as path.relative(root, realPath). Unzip programs would extract the file from the zipfile as metadataPath. realPath is not stored in the zipfile.

A valid metadataPath must not be blank. If a metadataPath contains "\\" characters, they will be replaced by "/" characters. After this substitution, a valid metadataPath must not start with "/" or /[A-Za-z]:\//, and must not contain ".." path segments. File paths must not end with "/", but see addEmptyDirectory(). After UTF-8 encoding, metadataPath must be at most 0xffff bytes in length.

options may be omitted or null and has the following structure and default values:

  mtime: stats.mtime,
  mode: stats.mode,
  compress: true,
  forceZip64Format: false,
  fileComment: "", // or a UTF-8 Buffer

Use mtime and/or mode to override the values that would normally be obtained by the fs.Stats for the realPath. The mode is the unix permission bits and file type. The mtime and mode are stored in the zip file in the fields "last mod file time", "last mod file date", and "external file attributes". yazl does not store group and user ids in the zip file.

If compress is true, the file data will be deflated (compression method 8). If compress is false, the file data will be stored (compression method 0).

If forceZip64Format is true, yazl will use ZIP64 format in this entry's Data Descriptor and Central Directory Record regardless of if it's required or not (this may be useful for testing.). Otherwise, yazl will use ZIP64 format where necessary.

If fileComment is a string, it will be encoded with UTF-8. If fileComment is a Buffer, it should be a UTF-8 encoded string. In UTF-8, fileComment must be at most 0xffff bytes in length. This becomes the "file comment" field in this entry's central directory file header.

Internally, fs.stat() is called immediately in the addFile function, and fs.createReadStream() is used later when the file data is actually required. Throughout adding and encoding n files with addFile(), the number of simultaneous open files is O(1), probably just 1 at a time.

addReadStream(readStream, metadataPath, [options])

Adds a file to the zip file whose content is read from readStream. See addFile() for info about the metadataPath parameter. options may be omitted or null and has the following structure and default values:

  mtime: new Date(),
  mode: 0o100664,
  compress: true,
  forceZip64Format: false,
  fileComment: "", // or a UTF-8 Buffer
  size: 12345, // example value

See addFile() for the meaning of mtime, mode, compress, forceZip64Format, and fileComment. If size is given, it will be checked against the actual number of bytes in the readStream, and an error will be emitted if there is a mismatch.

Note that yazl will .pipe() data from readStream, so be careful using .on('data'). In certain versions of node, .on('data') makes .pipe() behave incorrectly.

addBuffer(buffer, metadataPath, [options])

Adds a file to the zip file whose content is buffer. See below for info on the limitations on the size of buffer. See addFile() for info about the metadataPath parameter. options may be omitted or null and has the following structure and default values:

  mtime: new Date(),
  mode: 0o100664,
  compress: true,
  forceZip64Format: false,
  fileComment: "", // or a UTF-8 Buffer

See addFile() for the meaning of mtime, mode, compress, forceZip64Format, and fileComment.

This method has the unique property that General Purpose Bit 3 will not be used in the Local File Header. This doesn't matter for unzip implementations that conform to the Zip File Spec. However, 7-Zip 9.20 has a known bug where General Purpose Bit 3 is declared an unsupported compression method (note that it really has nothing to do with the compression method.). See issue #11. If you would like to create zip files that 7-Zip 9.20 can understand, you must use addBuffer() instead of addFile() or addReadStream() for all entries in the zip file (and addEmptyDirectory() is fine too).

Note that even when yazl provides the file sizes in the Local File Header, yazl never uses ZIP64 format for Local File Headers due to the size limit on buffer (see below).

Size limitation on buffer

In order to require the ZIP64 format for a local file header, the provided buffer parameter would need to exceed 0xfffffffe in length. Alternatively, the buffer parameter might not exceed 0xfffffffe in length, but zlib compression fails to compress the buffer and actually inflates the data to more than 0xfffffffe in length. Both of these scenarios are not allowed by yazl, and those are enforced by a size limit on the buffer parameter.

According to this zlib documentation, the worst case compression results in "an expansion of at most 13.5%, plus eleven bytes". Furthermore, some configurations of Node.js impose a size limit of 0x3fffffff on every Buffer object. Running this size through the worst case compression of zlib still produces a size less than 0xfffffffe bytes,

Therefore, yazl enforces that the provided buffer parameter must be at most 0x3fffffff bytes long.

addEmptyDirectory(metadataPath, [options])

Adds an entry to the zip file that indicates a directory should be created, even if no other items in the zip file are contained in the directory. This method is only required if the zip file is intended to contain an empty directory.

See addFile() for info about the metadataPath parameter. If metadataPath does not end with a "/", a "/" will be appended.

options may be omitted or null and has the following structure and default values:

  mtime: new Date(),
  mode: 040775,

See addFile() for the meaning of mtime and mode.

end([options], [finalSizeCallback])

Indicates that no more files will be added via addFile(), addReadStream(), or addBuffer(), and causes the eventual close of outputStream.

options may be omitted or null and has the following structure and default values:

  forceZip64Format: false,
  comment: "", // or a CP437 Buffer

If forceZip64Format is true, yazl will include the ZIP64 End of Central Directory Locator and ZIP64 End of Central Directory Record regardless of whether or not they are required (this may be useful for testing.). Otherwise, yazl will include these structures if necessary.

If comment is a string, it will be encoded with CP437. If comment is a Buffer, it should be a CP437 encoded string. comment must be at most 0xffff bytes in length and must not include the byte sequence [0x50,0x4b,0x05,0x06]. This becomes the ".ZIP file comment" field in the end of central directory record. Note that in practice, most zipfile readers interpret this field in UTF-8 instead of CP437. If your string uses only codepoints in the range 0x20...0x7e (printable ASCII, no whitespace except for sinlge space ' '), then UTF-8 and CP437 (and ASCII) encodings are all identical. This restriction is recommended for maxium compatibility. To use UTF-8 encoding at your own risk, pass a Buffer into this function; it will not be validated.

If specified and non-null, finalSizeCallback is given the parameters (finalSize) sometime during or after the call to end(). finalSize is of type Number and can either be -1 or the guaranteed eventual size in bytes of the output data that can be read from outputStream.

Note that finalSizeCallback is usually called well before outputStream has piped all its data; this callback does not mean that the stream is done.

If finalSize is -1, it means means the final size is too hard to guess before processing the input file data. This will happen if and only if the compress option is true on any call to addFile(), addReadStream(), or addBuffer(), or if addReadStream() is called and the optional size option is not given. In other words, clients should know whether they're going to get a -1 or a real value by looking at how they are using this library.

The call to finalSizeCallback might be delayed if yazl is still waiting for fs.Stats for an addFile() entry. If addFile() was never called, finalSizeCallback will be called during the call to end(). It is not required to start piping data from outputStream before finalSizeCallback is called. finalSizeCallback will be called only once, and only if this is the first call to end().


A readable stream that will produce the contents of the zip file. It is typical to pipe this stream to a writable stream created from fs.createWriteStream().

Internally, large amounts of file data are piped to outputStream using pipe(), which means throttling happens appropriately when this stream is piped to a slow destination.

Data becomes available in this stream soon after calling one of addFile(), addReadStream(), or addBuffer(). Clients can call pipe() on this stream at any time, such as immediately after getting a new ZipFile instance, or long after calling end().

This stream will remain open while you add entries until you end() the zip file.

As a reminder, be careful using both .on('data') and .pipe() with this stream. In certain versions of node, you cannot use both .on('data') and .pipe() successfully.


jsDate is a Date instance. Returns {date: date, time: time}, where date and time are unsigned 16-bit integers.

Regarding ZIP64 Support

yazl automatically uses ZIP64 format to support files and archives over 2^32 - 2 bytes (~4GB) in size and to support archives with more than 2^16 - 2 (65534) files. (See the forceZip64Format option in the API above for more control over this behavior.) ZIP64 format is necessary to exceed the limits inherent in the original zip file format.

ZIP64 format is supported by most popular zipfile readers, but not by all of them. Notably, the Mac Archive Utility does not understand ZIP64 format (as of writing this), and will behave very strangely when presented with such an archive.

Output Structure

The Zip File Spec leaves a lot of flexibility up to the zip file creator. This section explains and justifies yazl's interpretation and decisions regarding this flexibility.

This section is probably not useful to yazl clients, but may be interesting to unzip implementors and zip file enthusiasts.

Disk Numbers

All values related to disk numbers are 0, because yazl has no multi-disk archive support. (The exception being the Total Number of Disks field in the ZIP64 End of Central Directory Locator, which is always 1.)

Version Made By

Always 0x033f == (3 << 8) | 63, which means UNIX (3) and made from the spec version 6.3 (63).

Note that the "UNIX" has implications in the External File Attributes.

Version Needed to Extract

Usually 20, meaning 2.0. This allows filenames and file comments to be UTF-8 encoded.

When ZIP64 format is used, some of the Version Needed to Extract values will be 45, meaning 4.5. When this happens, there may be a mix of 20 and 45 values throughout the zipfile.

General Purpose Bit Flag

Bit 11 is always set. Filenames (and file comments) are always encoded in UTF-8, even if the result is indistinguishable from ascii.

Bit 3 is usually set in the Local File Header. To support both a streaming input and streaming output api, it is impossible to know the crc32 before processing the file data. When bit 3 is set, data Descriptors are given after each file data with this information, as per the spec. But remember a complete metadata listing is still always available in the central directory record, so if unzip implementations are relying on that, like they should, none of this paragraph will matter anyway. Even so, some popular unzip implementations do not follow the spec. The Mac Archive Utility requires Data Descriptors to include the optional signature, so yazl includes the optional data descriptor signature. When bit 3 is not used, the Mac Archive Utility requires there to be no data descriptor, so yazl skips it in that case. Additionally, 7-Zip 9.20 does not seem to support bit 3 at all (see issue #11).

All other bits are unset.

Internal File Attributes

Always 0. The "apparently an ASCII or text file" bit is always unset meaning "apparently binary". This kind of determination is outside the scope of yazl, and is probably not significant in any modern unzip implementation.

External File Attributes

Always stats.mode << 16. This is apparently the convention for "version made by" = 0x03xx (UNIX).

Note that for directory entries (see addEmptyDirectory()), it is conventional to use the lower 8 bits for the MS-DOS directory attribute byte. However, the spec says this is only required if the Version Made By is DOS, so this library does not do that.

Directory Entries

When adding a metadataPath such as "parent/file.txt", yazl does not add a directory entry for "parent/", because file entries imply the need for their parent directories. Unzip clients seem to respect this style of pathing, and the zip file spec does not specify what is standard in this regard.

In order to create empty directories, use addEmptyDirectory().

Size of Local File and Central Directory Entry Metadata

The spec recommends that "The combined length of any directory record and [the file name, extra field, and comment fields] should not generally exceed 65,535 bytes". yazl makes no attempt to respect this recommendation. Instead, each of the fields is limited to 65,535 bytes due to the length of each being encoded as an unsigned 16 bit integer.

Change History

  • 2.5.1
    • Fix support for old versions of Node and add official support for Node versions 0.10, 4, 6, 8, 10. pull #49
  • 2.5.0
    • Add support for comment and fileComment. pull #44
    • Avoid new Buffer(). pull #43
  • 2.4.3
  • 2.4.2
    • Remove octal literals to make yazl compatible with strict mode. pull #28
  • 2.4.1
    • Fix Mac Archive Utility compatibility issue. issue #24
  • 2.4.0
  • 2.3.1
    • Remove .npmignore from npm package. pull #22
  • 2.3.0
    • metadataPath can have \ characters now; they will be replaced with /. issue #18
  • 2.2.2
  • 2.2.1
    • Fix Mac Archive Utility compatibility issue. issue #14
  • 2.2.0
    • Avoid using general purpose bit 3 for addBuffer() calls. issue #13
  • 2.1.3
    • Fix bug when only addBuffer() and end() are called. issue #12
  • 2.1.2
  • 2.1.1
    • Fixed stack overflow when using addBuffer() in certain ways. issue #9
  • 2.1.0
    • Added addEmptyDirectory().
    • options is now optional for addReadStream() and addBuffer().
  • 2.0.0
    • Initial release.