JSZip lets a web app create, read, and edit ZIP archives in the browser. You can pass selected File objects directly into an archive, generate a Blob, and offer it as a download without uploading the files to a server. The work still uses the browser’s memory, and JSZip does not support every ZIP variant.
Create a ZIP from files selected in the browser
Install JSZip and, for the download helper shown below, FileSaver. Then pass the browser’s File objects directly to zip.file(); no FileReader conversion is needed for this basic workflow. JSZip also accepts strings, ArrayBuffers, Uint8Arrays, Promises, and, in Node.js environments, streams. See the JSZip file data documentation.
import JSZip from "jszip";
import { saveAs } from "file-saver";
async function downloadZip(fileList) {
const zip = new JSZip();
for (const file of fileList) {
zip.file(file.name, file);
}
const blob = await zip.generateAsync({ type: "blob" });
saveAs(blob, "archive.zip");
}Connect the function to a file input, for example by passing document.querySelector("input[type=file]").files after the user selects files. The sample uses each selected file’s name as its archive path. If files have duplicate names, or you want them organized under a folder, choose archive paths deliberately; JSZip supports creating folders as well as adding files. The project’s examples show the basic create-and-download pattern.
generateAsync() returns a Promise, so await it and handle rejection in the calling UI—for example, by displaying an error rather than silently failing. Its output type is required. Use blob for a browser download; choose arraybuffer or uint8array when another part of the application needs those bytes. Availability depends on runtime support, which can be checked with JSZip’s support information. The options and return behavior are documented in the generateAsync API reference.
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Choose whether to compress the contents
JSZip’s default compression method is STORE, which adds entries without compressing them. For content likely to compress, such as text, request DEFLATE when generating the archive:
const blob = await zip.generateAsync({
type: "blob",
compression: "DEFLATE"
});Compression can reduce the resulting archive size, but it costs processing time. The right choice depends on the files and the user’s device; the documentation does not establish a universal size or speed advantage. See the generation options for supported settings.
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Read and extract a ZIP selected by the user
Load the selected archive with loadAsync(), then use the returned archive’s file entries and await each entry’s asynchronous extraction method. For example:
async function readZip(file) {
const zip = await JSZip.loadAsync(file);
const entry = zip.file("notes.txt");
if (!entry) {
throw new Error("notes.txt is not in this archive");
}
return await entry.async("string");
}Here the example expects a text entry named notes.txt. For binary content, request an appropriate binary representation instead of string. Check that an expected entry exists and catch rejected Promises: the selected file may not be a valid archive, loading may fail, or the entry may be absent. JSZip’s loadAsync API and entry extraction API describe the asynchronous interfaces.
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Do not treat an archive’s filenames as trusted application paths when extracting or writing files elsewhere. Validate entries against what the application intends to accept, especially if extracted content will be sent to a server or handled by another system.
Know which ZIP archives JSZip can handle
JSZip handles classic ZIP files, but it is not a reader for every ZIP format feature. Its limitations documentation says encrypted and multi-volume archives are unsupported; loading those archives fails. ZIP64 loading also has limitations related to JavaScript number precision and 32-bit bitwise operations, so it should not be taken as a guarantee of arbitrary ZIP64-scale support. Check the project’s limitations page if your application depends on specific archive variants.
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Loading and regenerating an archive is not a byte-for-byte round trip. JSZip may discard metadata, and regenerating can add entries such as subfolders. If preserving the original archive exactly matters, do not assume that unpacking and rebuilding with JSZip will preserve it; see the documented limitations.
Plan for memory use and browser responsiveness
JSZip’s asynchronous APIs avoid the browser-freezing behavior of a synchronous operation, but they still hold the complete generated result in memory. The source files, intermediate representations, output size, browser, and device all affect memory pressure. There is no single safe archive-size ceiling established by the documentation, so test with realistic files on the devices you support rather than relying on a universal threshold. See JSZip’s memory and performance limitations.
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When handling binary data, use suitable binary representations such as Uint8Array or ArrayBuffer instead of converting it to a JavaScript string unnecessarily. JavaScript strings use UTF-16, which can add avoidable memory overhead for binary content.
Handle filenames and text encoding deliberately
JSZip’s native encoding path is UTF-8. ZIP files may not identify the encoding used for filenames, so a legacy archive with non-UTF-8 names can display mojibake unless the application knows the original encoding and supplies a custom decoder. For non-UTF-8 file contents, extract bytes and decode them with a tool configured for the content’s actual encoding; filename decoding and file-content decoding are separate concerns. See JSZip’s encoding notes.
Choose the output for the next step
| Need | Output or method | Trade-off |
|---|---|---|
| Offer the generated archive as a browser download | type: "blob", then pass the Blob to a download mechanism |
Convenient for browser downloads; generated output remains in memory. |
| Pass archive bytes to other code | type: "arraybuffer" or type: "uint8array" |
Useful for further processing; support depends on the runtime. |
| Skip compression | Default STORE |
Avoids compression work, but does not compress entry data. |
| Compress eligible content | DEFLATE |
Can reduce archive size while requiring processing time; actual results depend on the content and device. |
The Blob-to-download flow is shown in the JSZip examples. The output types and compression options are listed in the generateAsync reference; these are choices, not benchmarked performance guarantees.
Check licensing and pin the version you use
The JSZip repository states that the project is dual-licensed under MIT and GPLv3. Review the license terms against your application’s distribution and legal requirements; the project’s repository provides its license information. The documentation pages cited here do not establish a specific current release version, so pin the dependency version in your project and consult that version’s API documentation before relying on exact compatibility details.
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