FileSaver.js

FileSaver.js is the de facto library for triggering file downloads directly from client-side JavaScript. It implements an HTML5 saveAs() function that takes a Blob, File, or URL and prompts the browser to save it under a chosen filename, with no server round-trip required.

Weighing in at under 6KB with zero runtime dependencies, it abstracts away the browser-specific quirks of downloading generated content, making it a foundational building block for web apps that export CSVs, PDFs, images, and other files created entirely on the client.

What You Get

• A single saveAs(blob, filename, options) function that works across modern browsers
• Automatic feature detection that picks the best download strategy per browser
• Support for saving Blobs, File objects, and remote or same-origin URLs
• Optional automatic byte-order-mark (BOM) insertion for correct UTF-8 text encoding
• A tiny, dependency-free UMD bundle usable via npm, a bundler, or a plain script tag

Common Use Cases

• Exporting client-generated CSV or JSON data as a downloadable file
• Saving PDFs or images produced in the browser (e.g. from a canvas)
• Downloading remote files while controlling the saved filename
• Offering ‘Save’ functionality in web apps that create content entirely offline

Under The Hood

Architecture - The entire library lives in a single UMD module (src/FileSaver.js) that resolves a global scope across window, self, and global, then defines saveAs() by branching on runtime capabilities. When the a[download] attribute is supported it creates an anchor, points it at either a same-origin URL, a CORS-checked remote download, or an object URL from a Blob, and dispatches a synthetic click; on legacy IE/Edge it delegates to navigator.msSaveOrOpenBlob; and as a last resort it falls back to a FileReader that reads the blob as a data URL and opens it in a popup, with reverse-tabnabbing guards. Helper functions bom(), download(), corsEnabled(), and click() keep the branches small.

Tech Stack - It is plain ES5-style JavaScript with no runtime dependencies. The build tooling is Babel (@babel/cli with plugin-transform-modules-umd and babel-preset-minify) which transpiles the source into UMD dist bundles (dist/FileSaver.js and a minified build). Distribution is via npm and bower, and TypeScript consumers pull types from the separately published @types/file-saver package.

Code Quality - The source is compact and readable, with comments pointing at the specific browser bugs and issue numbers each code path works around. There is no automated test suite, however: the package.json test script simply echoes ‘no test specified’ and exits, so correctness relies on manual browser testing and the library’s long production track record.

API Design - The public surface is a single saveAs(blob, filename, options) function, which makes the library trivial to adopt with essentially no boilerplate. The README documents every scenario (text, URLs, canvases, File objects) with runnable snippets, and the only option, autoBom, has a sensible default. This minimal, well-documented API is the main reason it became the ecosystem standard.