You don't know everything, and it's OK
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Creative Commons: https://j.mp/1dqsh5W
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I remember when I started as a Technical Evangelist: I wanted to know everything, and I was feeling bad when I didn't know something. I had this imposter syndrome as I thought that, as a mentor for many developers, my role was to be that go to person for all in all. Over time, I learned that it doesn't make sense and that it's OK not to know everything.
In my work, like in many others, there is tremendous knowledge to acquire. Just today, I would have to know the inside out of HTML, CSS, JavaScript, most popular libraries, new ones, browsers' features, developer tools, Firefox OS API, Firefox OS devices' specifications, and more. Even with my primary focus, Firefox OS, it's not easy: the OS is changing so quickly, and even if learning technology is part of my role, I don't have as much time as I would like. So I figured out in my previous role, always as a Technical Evangelist, that my work, and any jobs I'll have in the future, Evangelist or not, is about knowing enough to be able to make an amazing job. The second part of my work is about knowing when to say I don't know, but also where to find the information. I have access to online documentation, forums, co-workers, and many more resources. Don't get me wrong! I'm eager to learn more, and I will continue to gain knowledge, and master new subjects, but in the end, I learned that it's ok to say that I don't know. In that situation, I'll do everything to find the answer, but I don't expect myself to have an answer at first sight to anything related to my work.
You should think about this too: no matter what is your day to day work, it's ok not to know everything. You'll keep your credibility by saying, "I don't know, but I'll check for the answer", than trying to answers with information you aren't sure. You don't know everything, and it's OK...