The CRAM format addresses rising DNA storage costs for short-read sequencing by aligning reads to a reference genome and encoding the resulting alignment with Huffman, subexponential, Elias gamma, rANS, gzip, and other codes. The CRAM codec is complex, and until now, there has been no JavaScript implementation.We have developed a JavaScript library, Cram-JS, that natively reads and decompresses the CRAM format on-the-fly. The library is used in the JBrowse and IGV-JS (http://igv.org/) genome browsers and can readily be used by other JavaScript applications, in the web browser or in Node.Cram-JS is written to the ES-6 standard and is available from the GitHub repository at https://github.com/GMOD/cram-js.None.