Recently, I had a need to download the preview of the OfficeJS API for testing in an offline environment. The primary (and preferred) way to access the preview beta library is to point to it on the CDN via your HTML, like this:
However, if you need to download this for use offline in testing, you can actually download it using Node Package Manager. All you need is this command:
npm install @microsoft/office-js@beta
I was able to do my testing as if I were online with the CDN, so all was good. Happy coding! 
