Do I need to know every technology in the stack before I start any given job?

If I don’t, then do I need to know 75%, 50% or even just 25%?

What if I have never used any of the technology you are currently using, but I have worked with similar paradimes in the past?

Is getting a job about what you have done or what you can do? Are you a better developer if use can program in 50 languages or are an expert in one?

My belief is that I can excel at any (programming) task set before me. The question is what can I do to ensure that prospective employers can see that?

In a recent conversations with two different (junior) developers, I suggested that personal projects are the key. They provide an outlet to try new technology and are a useful tool come interview time.

For myself I have used personal projects to expand my knownledge of javascript programming (framworks). Using first angular and the react to build applications. As a proof of my concept I was able to get a job requiring significant amounts of angular development as a result.

As a final note, I want to stress that the projects I do are for my own enjoyment, they help me to learn which is something I enjoy, and the work being undertaken is something that I have a specific usecase for.

I would not suggest starting a project to get a better job, instead start work on something you know (stuff) about and want to be involved in, that is the only way you can ensure you will get something beneficial from it.