Hello!

We're running into issues updating from version 4.2.x to 5+ for our legacy browser builds. Technical details follow, but I think it would be most helpful if this library was clear about which browser versions it supports.

Is there a document anywhere detailing the minimum browser requirements for MQTT.js?

Through much experimentation, we've determined they are at least:

• Chrome 67
• Edge 79
• Safari (Mobile) 14
• Firefox 68
• Opera 54
• Internet Explorer: unsupported

This appears to be due to:

• MQTT.js bumping a dependency: readable-stream
• readable-stream is a browser port of a Node built in library
• Node uses BigInt constructors and literals
• as of readable-stream 4+, it embeds BigInt literals into its code
• BigInt is not capable of being transpiled or polyfilled for lower browser versions (Why?)

I feel this is worth pointing out because the whole thing is so confusing and it's wasting many developers' time. We get JS compilation issues that we aren't used to ("why does everything else transpile but not this? what code is even using this?", etc.) We get brief responses to issues on various libraries that seem like the maintainer may not fully understand the issue (even the build tool maintainers.)

Love to hear any thoughtful comments on all this!

2 possible recommendations:

• (easy/quick) update the README to mention minimum browser versions and the greater BigInt issue
• (harder) update the project to defend against this: downgrade the dependency, use something else, etc