How can it read a ZIP without extracting everything?

A ZIP file ends with a central directory that indexes every entry: its name, compressed and uncompressed size, method, CRC, and date. The tool reads only that index, so listing is instant even for large archives.

Is my archive uploaded anywhere?

No. The .zip is read with FileReader and parsed in your browser. Even single-file extraction is done locally with the browser's built-in DecompressionStream. Nothing is transmitted.

Which entries can it extract?

Stored (uncompressed) and standard Deflate entries — by far the most common — extract directly using the browser's native Deflate support. Rare methods like BZIP2 or LZMA inside a ZIP cannot be expanded here.

What does the compression ratio mean?

It is how much smaller the stored data is than the original: 1 minus compressed size over uncompressed size, as a percentage. Already-compressed files like JPEGs or MP4s show near 0 percent; text and code compress well.

Why does a folder show as a zero-byte entry?

ZIP stores directories as separate entries whose names end in a slash and have zero length. They mark folder structure; the actual files are listed as their own full-path entries.

ZIP File Inspector — Gera Tools

A ZIP archive keeps an index of everything inside it called the central directory, written at the end of the file. That means you can see exactly what an archive contains — names, sizes, dates, compression — without unpacking a single byte. This inspector reads that directory in your browser and lets you pull out individual files on demand.

How it works

The tool first locates the End of Central Directory (EOCD) record by scanning backward for its signature 0x06054b50. The EOCD tells it how many entries exist and where the central directory starts.

It then walks each central directory file header (signature 0x02014b50), reading the compression method, CRC-32, compressed and uncompressed sizes, the MS-DOS packed date/time, and the file name. The packed date is decoded into a normal timestamp (DOS time stores the year as an offset from 1980).

To extract one file, the tool jumps to that entry’s local file header, skips its variable-length name and extra fields to find the data, then either copies stored bytes verbatim (method 0) or expands Deflate data with the browser’s DecompressionStream('deflate-raw') (method 8).

Notes and tips

Listing is instant regardless of archive size because only the directory index is read.
The compression ratio column quickly shows which files actually compressed — useful for spotting already-compressed assets bloating an archive.
ZIP64 archives (over 4 GB or 65,535 entries) use extended fields not parsed by this lightweight reader; standard archives work fully.
Extraction is local; the file you download never round-trips through a server.