A friend of mine was recently hired as a "senior" engineer. He's a sharp guy. He went through a bootcamp. He had worked at a company for a couple years as a junior, gaining experience and learning along the way. And now a new company was offering him a job with a bump up in title (and presumably pay). He had accepted the job and as his first day approached, the feelings of imposter syndrome started to creep in. Would he be able to keep up with people around him? Would he be able to live up to the title? What if they "found him out" and called him what he felt like, an imposter?
Then he did something you should do when you have those feelings. He reached out to people who knew him, people he trusted, and people who had been through similar situations. He asked for advice. Advice on what do to about imposter syndrome and advice on how to spin up at a new job quickly. I happened to be one of the people he asked and so I wrote him a message with my thoughts. It seemed to help him, so I decided to spruce it up a little bit and give the same advice to you.
This advice assumes that you have had at least one job where you felt like you knew what you were doing when you left it. It also assumes that you objectively have the technical and soft skills necessary to do the job that you're hired for. That is, it assumes that you are not actually an imposter. (Don't try to trick people into thinking that you know more than you do to get a better job title. It won't work for very long.) And, it assumes that your new job is a good and healthy work environment. There are places where this advice will not work and my advice to you then would be to find a new place to work. With all that said, let's go.
It Will Feel Like You Don't Know Enough
The first point is that it will almost certainly feel like you know less than you did at your previous job. Because you do. You probably haven't lost any of your technical skills (you may even be sharper after brushing up for interviews and whatnot), but what you don't have now is familiarity. And unfortunately for you, familiarity is going to be strongly correlated with velocity. This can compound in a couple of way that leave you feeling useless.
You have switched from a context where you had a lot of familiarity to one where you have very little. And because you are a human with recency bias, you will compare your velocity now with your velocity when you left your last job. But really, you should compare it with your velocity when you started your last job. Did it take you two weeks to merge your first PR at the last job? If it takes you one week this time, you've cut that ramp up time in half. Regardless of actual metrics, know that it will take you less time to reach the same level of familiarity at the new company as it did at your last company because you have a whole set of experience under your belt now that you didn't then. Disclaimer: You shouldn't only look at ramp up time because this is also largely dependent on the quality of your new employer's documentation and onboarding process.
You have switched from a context where you may have been one of the people with the most familiarity to one where you likely have the least. The people around you at the new job will be familiar with the processes/patterns/technologies/code/etc in ways that you are not (yet). It will feel like everyone you work with is flying and you're trying to keep up on a unicycle. That's (probably) not because they are better developers than you, it is largely because they have been here longer and are more familiar with the requirements at this particular job. You will get there in time too.
Regardless of how you feel, try to remind yourself that they wouldn't have hired you as a senior if they didn't think you were qualified for that title. No one knows everything. And you probably know a lot more than you feel like you do. I can tell you from experience that if you make any effort at all to continue learning and growing as an engineer after you get a job, you are already on trajectory to reach the upper echelon of engineers.
How to Spin Up Like A Senior
Write down anything that doesn't make sense to you. Anything you get stuck on. Any questions you have (this includes questions about how the business works and why things are the way they are). Then you can just fire them off one after another whenever you meet with your onboarding buddy or manager or whoever, and you'll look really prepared. No one expects that you will have a high velocity right when you first start, but they will be impressed if you are actively participating in the process and pushing forward. Bonus points if you use those questions/answers to improve their existing documentation for the next person who has to onboard.
Look for quality of life improvements that you can make for other people. Tackle tech debt and refactors where you can because they are helpful for the team and they will help you get familiar with the codebase/patterns. I'm not saying take on every bit of menial work that exists, but everywhere I've ever worked has had at least a couple of things that they have been meaning to get to and no one has time for. You typically don't have too many responsibilities right when you first start, so use that time to knock a couple of things off the list. Refactor those last few legacy classes to the current pattern, fix the dark mode or accessibility bug, or whatever it is that needs to be done.
Stick to the style guide. Code style is not the place to differentiate yourself when you start. If there is a style guide at your new company, it probably took a while to get to and there is probably at least one person with very strong opinions about it. Do your best to stick to it when you start and if there is something that you have strong opinions about, you'll have a much better idea of when/where to bring it up in three to six months. If there isn't an existing style guide, see if you can head up the process to establish one and get a linter in place.
Meet as many people as you can (especially if you're remote). Ask lots of questions about them, their role at the company and how the business works. It has been incredibly helpful in my own career to figure out who does what (meaning, who I should ask about different kinds of things) and what the levers of the business are so you can understand why decisions are made and when to speak up with ideas if you have them. On top of that, people like to work with people that they like. Be personable, develop your soft skills and make some friends.
Starting a new job is basically the same as getting over a cold. Prepare to feel out of sorts for a few weeks (or months), but know that you'll get over it eventually. Get lots of sleep and drink lots of fluids. Work hard and do your best, but make sure you establish a good work/life balance from the start.
So there you go, that is my advice as it stands today. Have confidence in yourself, know that your familiarity (and velocity) will grow over time, and there are things you can do that both provide value and help you accelerate growing that familiarity. How about you? What has your experience been? What advice would you give a friend to help them get over imposter syndrome or spin up at a new job?