There is QuaggaJS library (open source) for reading barcodes, all done in JavaScript.
It currently supports EAN, CODE 128, CODE 39, EAN 8, UPC-A, UPC-C, and CODABAR
I suspect it will meet your needs.
Update: 2021-August
Looks like the original QuaggaJS is no longer actively maintained.
Quagga2 is active and continues the goals of the original.
(thanks @bertasoft for the tip!)