Have you considered using a shared-source license where they can inspect and build from source that they have to pay for? And where people can obtain the source

Have you considered using a shared-source license where they can inspect and build from source that they have to pay for? And where people can obtain the source freely for academic research and/or security reviews? Shared-source proprietary goes as far back as Burroughs B5000 mainframe whose customers got the source and could send in fixes/updates. Microsoft has a Shared Source program. Quite a few suppliers in embedded do it. There's also a company that sells UI software which gives the source to customers buying higher-priced version. I will warn that people might still rip off and use your code. Given it's JavaScript, I think they can do that anyway with reverse engineering. It also sounds like they could build it themselves anyway. Like most software bootstrappers or startups, you're already in a race with other players that might copy you with clean slate implementations. So, I don't know if the risk is that big a deal or not. I figured I should mention it for fairness.